Chapter –1

Classification of Meteorology

Meteorology is defined as a branch of physics dealing with lower atmosphere

(Atmosphere is deep blanket of gases surrounding the earth) with particular emphasis to

the individual phenomenon. In other words it is concerned with the study of the

characteristic and behavior of the atmosphere. It explains and analyses the changes of

individual weather elements such as air pressure, temperature and humidity that are

brought about due to the effect of insolation on the earth’s surface. (Insolation refers to

radiation from the sun received by earth’s surface).

Agro metrology is a science investigating the meteorologic, climatologic

and hydrologic conditions, which are significant for agriculture owing to their interaction

with the objects and processes of agricultural production. In nutshell, it is a science

dealing with climatological conditions, which is directly related to agriculture.

Climatology

Climatology refers to the study of weather patterns over time and space. It

concerns with the integration of day-to-day weather over a period of time. It refers to the

average conditions of the weather.

Development of Agricultural Meteorology

Climatology is made up of two Greek words, klima + logos; Klima means slope of

the earth, and logos means a discourse or study. In brief, climatology may be defined as

the scientific study of climate. Climatology is simultaneously an old and a new science.

Superstition served to interpret atmospheric mysteries such as rain, wind and lightening.

In the early civilization, Gods were often assigned to the climatic elements, Indians still

hold ceremonial worships/dances to Gods to produce rains at times of drought.

The Greek philosophers showed a great interest in meteorological science. In fact

the word “Meteorology” is of Greek origin, means the study on things about meteors and

optical phenomena. In fact, the word “Meteorology has been borrowed from Aristotle’s

1

“Meteorologica” dated about 350 BC. The period of weather tradition and superstitions

in the development of meteorology lasted until the beginning of the 17 th Century when

the invention of instruments for scientific analysis of weather phenomenon was made. In

1593, Galileo constructed a thermometer and in 1643, his student Toricelli discovered the

principles of mercurial Barometer. The climatological map was published by British

astronomer “Edmund Hally’ in 1686. By 1800, dependable weather observations were

made in Europe and USA. An International Meteorological Organization had been

established in 1878. The World Meteorological Organization (WMO) took its present

form in 1951. It serves as a specialized agency to carryout the worldwide exchange of

meteorological information with the head quarters in Geneva, Switzerland.

The India Meteorological Department (IMD) was established in the year 1875.

The division of Agricultural Meteorology was started by the IMD in 1932 to meet the

needs of agriculture and researchers. The IMD has brought out many useful publications

on rainfall. The Rainfall Atlas of India was published based on the rainfall data from

1901 to 1950. In addition to rendering advice from time to time, the IMD began to offer

regular weather service and farmers weather bulletins from 1945. The bulletins are

broadcast daily in 20 regional languages in all the All India Radio stations on expected

weather conditions during the next 36 hrs. Weather report is also broadcasted through

television. At present 8000 rain gauge stations and 52 principal types of Agro met

observatories are available in our country.

Development of Agricultural Meteorology:

Distance form East to West: 2933kmDistance form North to South: 32144km

1. Distance from the sea

Divisions of Meteorology:

1.Dynamic meteorology:

It deals with the forces that create and maintain motion and the latest

transformations associated therewith.

2.Physical meterorolgy:

2

It deals with pure physical nature such as radiation, heat, evaporation,

condensation, precipitation, ice accretion (continuous coherence) and optical acoustical

and electrical phenomena.

3.Climatalogy:

Statistical meteorology which determines the statistical relations, mean value

normals, frequencies, variation distribution etc.

4.Synoptic meteorology:

Its purpose is the analysis and forecasting of the weather phenomena. Thus

synoptic meteorology comprises dynamic as well as physical meteorology and to a lesser

extent climatology in order to obtain a synopsis of the state of atmosphere.

5.Aeronatical meteorology:

It deals with application of meteorology to the problems of aviation.

6.Maritime meteorology:

It is related to marine navigation.

7.Agricultural meteorology:

It deals with application of meteorology to agriculture, soil conservation etc.

8.Hydrometerology:

It is concerned with meteorological problems relating to water supply, flood

control, irrigation etc.

9.Medical meteorology:

It deals with the influence of weather and climate on the human body.

10.Aerology:

It is a branch of meteorology that is concerned with the conditions of the free

atmosphere on the basis of direct observations.

Meteors and its classification:

Meteors are defined as any atmospheric phenomenon, having a luminous

appearance that travels through space as aerolites, fireballs, stars etc.

Classification:

1.Aerial meteors: Wind, Tornado

3

2.Hydro or Aqueous meteors: Rain, hail, snow and dew

3.Litho meteors: Dust and smoke

4.Luminous meteors: Rainbow and halos (circle of light and sound luminous body

around sun or moon)

5.Igneuous meteors: Lightening and shooting stars.

2.Division of Climate

Eg. 1. Canadian wheat is of better quality than Egyptial w

Eg. 1. Canadian wheat is of better quality than Eqyptial wheat.

Eg. Multon - January Temp. 540F

1. Equator: An imaginary circle around the earth, equally distant at all points from

both the North Pole and the South Pole. It divides the earth’s surface into the

northern hemisphere and the southern hemisphere.

Equinoxes refer to the time of the year at which the sun crosses the equator and

day and night are equal.

Equinoxes:

Events such as leafing, flowering, fruiting, leaf shedding, migration of birds,

occurrence of insects etc provide indications of the coming season.

IMPORTANCE AND SCOPE OF AGRICULTURAL METEOROLOGY

Climatic factors alone affect the yield of crops to an extent of about 40%. In

India the success of agriculture depends mainly on monsoon rains. Agricultural

Meteorology is mainly concerned with microclimatology in which the influence of the

shallow layer of atmosphere immediately above the surface is studied. Successful crop

production depends not only upon the total seasonal rainfall but also on the proper

distribution. The study of agricultural meteorology helps the farmers to know when the

monsoon rain begins, its distribution etc. Apart from this the farmer will be able to know

about the weather abnormalities and their destructive effect on crops.

4

The major weather abnormalities and destructive meteorological phenomena

which affects crops adversely and the scope of controlling and minimizing their bad

effects are as follows:

1. Flood

2. Drought

3. Cyclonic stroms and depressions

4. Cold waves and frost

5. Thunder strom, Hail storm and Dust strom

6. Heat waves

7. Excessive (or) defective insolation

8. High winds

1. FLOOD

This is the most serious weather abnormality caused by sudden heavy rains. The

flood will damage big dams, buildings, and standing crops and cause so many other

havocs. The following precautionary measures can be taken up.

a. Afforstation: Growing trees like casuarinas, Eucalyptus, Cashew etc; in

catchments areas (where rain occurs)

b. Construction of dams and anicuts where the flood havoc is often met with.

c. Growing flood resistant varieties like PTB 15 rice varieties in flood areas.

2.DROUGHT

This is caused by continuous failure of monsoon rains and growing of crops in the

field is a failure.

The overcome this different. a. Moisture conservation methods like mulching

(covering the soil with stray or stubbles) to prevent evaporation loss can be adopted. b.

Growing drought resistant crops like (mulch cholam, Varagu, Thennai. c. Economic use

of water.

5

3.CYCLONIC STROMS AND DEPRESSIONS

Caused by the depressions and as result heavy wind with a high speed cause

severe damage to the crops, trees, buildings etc.

The direction, speed and the on set of cyclone can be forecasted and the standing

crops, stored food grains can be saved. Sowing and harvesting can be adjusted.

4.COLD WAVES AND FROST

This occurs only in hilly regions. Due to sudden fall in temperature the

atmospheric moisture is crystallized and the crystals fall over the trees, houses, fruits and

other crops.

Preventive measurements:

a. The fruits and vegetable as can be covered by polethyne bags.

b. Sowing time can be adjusted.

c. If possible irrigational smoking can be attempted.

5.Thunder strom, hail strom and dust strom:

Usually occurs before the on set and after the withdrawal of monsoon. Largest

known hail is 5” in diameter and weighs about1/2 kg. Only precautionary are possible

and usually seen in north India.

6.Heat waves:

In hot summer heat waves cause injuries to the crop. Difficult to control the

adverse effects.

Shade giving trees can be grown in the fields frequent irrigations can keep down

the bad effects of heat waves.

6

7. Excessive and Deffective insulation:

Insolation in a function of solar inclination of solar rays, length of the day and

transparency of atmosphere. In black soil the temperature goes even up to 1670 F. It thin

layer of chalk is spread on the surface of the soil helps to keep the soil layer cool and

conserve moisture.

8.High winds:

Very high winds some times break Orchard tree. Branches and damages standing

crops. This can be controlled by growing trees like casuarinas as wind breaks in the

direction of wind.

Beside the above control measures for whether abnormalities, the study of

agricultural meteorology helps the farmer in so many ways.

1. To obtain better yield by adjusting time of sowing and selecting proper

varieties.

2. To forecast pest and disease occurrence due to climatic factors and to suggest

control measures early blight disease of potato Japan blast disease in paddy,

Britain- late blight in potato, USA- Rust drain of wheat and barley.

3. To help the farmers in forecasting the weather for day-to-day operations. E.g.

Weeding and spraying can be avoided during rainy days.

4. To help and choose the proper time of warming as well aw west coast for

paddy crop. Ammonia sulphate has to be applied only at proper time.

5. To help the farmers in selecting suitable season when new crops are

introduced E.g. Potato. When newly introduced in plains.

6. For better utilization of farm by adjusting farm, the oventel operations such as

painting repairs of implements etc.

7. For the construction of farm building setting of windmills etc. And to carry

out them in sunny weather.

8. For the study of phenology.

PHENOLOGY:

It indicates the coming season. It is a science, which deals with the reoccurrence

of important phases of animals and vegetable life in relation to climate during the year.

7

Events such as leafing, flowering, fruiting, leaf shedding, migration of brids,

occurrence of insects etc provide indications of the coming season.

The observations made in mango on flowering by the Indian Meteorological

Department, shows the following. The flowering takes place by 15 th December in

Madras, Andhra while in northern state it is as late as 15th of March.

Seasons:

1. Spring – January to March - Fresh leaves formed in trees.

2. Summer - April to June – Flowering and fruiting takes place.

3. Autumn - July – September.

4. Winter – October – December.

The sequence of flowering obeys Hopkins Bioclimatic law, to which the time of

flowering develops upon the latitude, longitude and altitude. According to the law,

1. For every degree of latitude north or south of equator, flowering is retarded by 4

calendar days.

2. For every 50 of longitude for East or West on land areas flowering is advanced by

4 calendar days.

3. For each 400’ increase in altitude flowering is retarded by 4 calendar days.

Boyle’s Law:

The Volume of a given quantity of air varies inversely as the pressure upon it,

provided the temperature remains constant.

Charle’s Law:

The volume of a given quantity of air varies directly as the absolute temperature,

provided the pressure remains constant.

8

(Chapter –2)

WEATHER AND CLIMATE

Weather:

The state of the atmosphere with respect to wind, temperature, cloudiness

relative humidity, pressure etc., at a given time for a given place. This deals the physical

conditions of the atmosphere for a smaller area for a shorter time. E.g. Max. Temperature

of Navalur Kuttappattu on 02.04.2001 is 37.50 C

Weather and Climate

Weather and climate are the important factors determining the success or failure

of agriculture. Weather influences agricultural operations from sowing of a crop to the

harvest and depends on the mercy of the weather particularly rain fed agriculture. In

India every year there is a considerable damage by floods in one part of the country and a

severe drought causing famines in another part. The total annual pre harvest losses for

the various crops are estimated from 10 to 100 per cent; while, the post harvest losses are

estimated to range between 5 and 15 per cent. Hence, study of weather element is

essential.

Weather is the condition of atmosphere at a given time. It is the day to day

interplay of temperature, humidity, pressure, rainfall etc.

The weather conditions of Coimbatore on a particular day at a particular time may

not be the same as that of Annamalainagar weather.

The state of atm. Over the period of time is known as climate. It is the synthesis

of these various elements of the weather. The word climate refers to the mean or normal

conditions over a long period such as 20-30 years or more, where as the cold weather

refers to the mean or normal conditions over a long period such as 20-30 years or more.

Where as the word weather refers to more or less instantaneous conditions in the atm. Or

the trend of there conditions over a relatively short period of time.

9

Weather Climate1. Occurrence of the weather elements at a time

Expressions of many such occurrences as the flow together in time. Average values of huge elements for a seasonal time is long period (for 30 years)

2. Differentiation of climate

Integration of weather

3.Concerned with how all the weather elements act as a given time

Concerned with how they affect the environment which is turn affects all the organisms.

FACTROS CONTROLLING THE CLIMATE:

The most important climate elements are temp erature precipitation, humidity,

wind velocity, duration of monsoons and cloudiness, etc. Their normal periodic and a

periodic variation and their extreme values etc.

Because of the intimate relation between climate and vegetation, climates and

vegetation, climates, are classified according to the type of plants grown or cultivated soil

such as tropical, forest climate, desert climate, pine forest climate, tundra climate etc.

The climatic elements are the results of interaction of number of factors such as

1. Latitude- distance from the north or south of equator.

2. Altitude

3. Distance from the sea

4. Topography

5. Soil type

6. Vegetation.

1. Latitude:

Latitude is the main factor in determining the climatic zones such as torrid,

temperate and polar zones. It is found that the quality of grains is better in higher

latitudes than lower latitudes.

E.g.1. Canadian wheat is of better quality than Egyptial wheat.

2. Italian rice is superior than Indian rice.

10

The latitude of a place in question for in it depends on the angle of incidence of the

incoming radiations from the sun. The length of the day and night, the length of the

seasons, the amount of incoming radiations etc.

2. Altitude:

The elevation of a place, the metrological elements vary rapidly with height above

the sea level.

Height above the sea level has a profound influence on a climate. Vitiating the

effects of increased the effects of increased latitudes. The important effects of altitudes

are.

1. As the height increases the pressure is decreased the barometer reading in

difference heights are as follows:

30” at sea level

29” at 830 feet.

15” at 18,500 feet.

a. Rainfall is more in mountainous region.

b. Besides the mountains cause rainy areas and rain shadow areas.

E.g. Rainy western cost.

150” –250” Rain shadow area Eastern sides

25” and low

c. The situations of hills in the rainy areas also causes changes in rainfall as shown

below.

Place Situation Annual rainfallDecca 100” miles from kharif hills 78”Bogra 60” 92”Mymensight 30” 110”Sylhet 20” 150”

11

d. Zone of maximum precipitation:

This vary from place. In Java the maximum rainfall occurs at 3300 ft.

altitude. In Western Ghats maximum rainfall is at 5000’ altitude. In Alphs 7000’

altitude. Above this the rainfall is lower.

e. Division of Climate:

The Himalayan Mountains provides variation in climate.

Eg. Multon - January Temp. 540 F

Shangai - January Temp. 380 F

Though both the places are situated on the same paraller of 320N latitude.

3.Distance from the Sea:

The difference between marine and continental climate can be classified as

follow:

Marine Continental

1.Rainfall More and well distributed Less and ill distributed

2.Temperature Variation is less (200F Variation is more (Northern

Variation at Cochin) India-600F variation)

3.Land Sea Breeze Sea breeze regular and No sea breeze

on both direction and at

particular time

4.Topography (Relief):

The frost occurrence will be mostly in the valleys rather than the hills.

Besides these, soils and vegetation as physical factors also affect climate to a

smaller extent.

Thick forest areas with more vegetation will be cooler than the desert because the

forest trees and by the surrounding environment becomes cooler. Where ever black soil

type is predominant there will be more absorption of heat and hot climate will exist

E.g. Tirunelveli and Ramnad District.

12

The latitude is the line draw in East West direction to divide the globe into

various climatic zones. Solstice is deferred as either time at which the sun is farthest

North or West or South of equator.

Showing Annual variation in the altitude

Solstice:

The time when the sun reaches its maximum distance from the equator (summer

solstice when it touches tropic of cancer on 21st June and winter solstice when it touches

tropic of caprion on 21st December.

13

661/2

231/2

0

‘0’ to 23 ½ 0 L = Tropical region

23 to 66 ½ 0 L = Temperate region

Above 66 ½ 0 = Polar region.

1.Tropical Region:

It is the region of suns movement and hence it will be hot in nature.

The Equatorial Belt:

The sun is north of the equator during the northern summer and south of the

equator during the southern summer. At the equator the sun is in Zenith at both

equinoxes. About 230 N and S the sun reaches Zenith only at the time of the solstices.

Thus near the equator the sun is in zenith twice a year and there will be maximum of

incoming radiation in spring and autumn. The length of the day varies but little

throughout the year and the sun is high in the sky every day. The annual variation in the

temperature is therefore very small. But the diurnal variation in temperature will be

relatively large because the length of the day varies but little.

Zenith:

Zenith is defined as the time at which part of the sun is directly overhead.

Equinoxes:

Equinoxes refer to the time of the year at which the sun crosses the equator and

day and night are equal.

2.Subtropical climate:

This is also characterized by high temperature alternating with low temperature in

winter.

3.TemperateRegion:

The temperate climate is disfigured by low temperature all though the year. It has

got moderate temperature with well-distributed rainfall, humidity etc. This is the ideal

climate region for successful crop production.

14

Here the sun does not reach the Zenith in mid summer. The days are long and the

sun is high in the sky in summer and in winter the days are short and the sun is low in the

sky with the result that the incoming radiation varies considerably through out the year.

As a result the annual variation in temperature tends to increase from the equator towards

the poles.

4. POLAR REGGION:

Since it is far away from the suns influence this region will be extremely chill or

cold through out the year.

Here the sun is below the horizon day and night in mid winter, and above the

horizon day and night in mid summer. At the poles there is no diurnal variation in the

incoming radiation and the daily variation in temperature vanishes. On the other hand the

difference between the incoming radiation in winter and summer has increased to a

maximum, with the result that the annual variation in temperature increases.

Horizon:

Horizon refers to as line at which earth of sea or sky seems to meet.

CLASSIFICATION OF CLIMATE:

Mr. Koppen has classified the climate into eleven principal types and are as

follows:

1.Tropical rainforest climate:

It occupies the major portions of the equatorial belt. Along the west coast the belt

is relatively narrow and along the east coast it spreads put 260 N and S because of the

monsoons and the on land trade winds which give warm weather and rainfall most of the

year.

This climate is characterized by

a. High temperature coldest weather above180 C (64.40 F) annual variation in

temperature less than 60C (110F).

b. Sufficient rainfall to maintain tropical forest, either rain at all seasons, two

rain maxima or one long rain period and one short and dry season with at least

6 cm rainfall.

c. Vegetation of the megatherm type, which require high constant temperature,

abundant precipitation and high relative humidity.

15

2. Tropical –Savanna Climate:

This zone surrounds the tropical rain forest. They have a dry period caused by the

migration of the doldrums and the climate is characterized by

a. High temperature, coldest much above 180 C annual variation in temperature

less than 120 C.

b. Relatively abundant rainfall in summer and dry winter, with at least one

month with less than 6cm rainfall.

c. Vegetation related to the tropical rain forest, but because of the winter dryness

the forests are replaced by open land with trees.

3.Steppes

The steppes continue for into the interior continent where the dryness is in part due to

the large distance from the coast and lack of moisture bearing winds. The equatorial

part and eastern part of the steppe region has light summer rainfall chiefly because of

summer showers, and the portion indicated by WR (winter rainfall) has dry summer

and slight winter rainfall. The steppe climate is characterized by

a. Temperature varying within wide limits.

b. Lack of rainfall, evaporation-exceeding precipitation most of the rain at rare

intervals and the amount varying considerably.

c. Vegetation adapted to high temperature large temperature variation and long

day periods.

4.Deserts:

Here the descending air in the subtropical anticyclones causes extreme. The

deserts are characterized by:

a. High summer temperature, large diurnal variation and moderate annual

variation temperature.

b. Cloudiness sky, extreme dryness, dust and sand stroms, rain s squalls at

rare intervals.

c. Very sparse vegetation of steppe type.

5. Warm climate with dry winter:

16

Adjacent to saranas. Winds are mainly monsoon type, dry winter and wet

summer.

a. Mean temperature of the coldest month below 180C but above –30C mean

temperature of warmest month over 100C.

b. Dry winter and wet summer at least 10 times as much as rainfall in the west

month of summer as in the driest month of winter.

6. Warm Climate with dry summer:

Under the pole ward part of the subtropical anticyclones where because of the

annual migration of these anticyclones, the prevailing westerlies give rain in winter.

a. This zone is characterized by temperature as in climatic zone – 5.

b. Dry summer and moist winter with at least month of winter as in the driest

month of summer having less than 3 cm of rainfall.

c. Vegetation of the mesothermal type adapted to dry and warm summers and

moderately cold and wet winters. The summer is frequently too dry and

whether is too cold for the vegetation. As a result most plants blossom in

spring and autumn where there are sufficient heat or moisture.

7. Humid temperate climate:

They are under the influence of moisture throughout the year with a high

temperature in winter and sufficient rainfall in all seasons.

a. Temperature as in climate zone (5 and 6).

b. No appreciable annual variation in rainfall.

c. Vegetation of mesothermal type adopted for high moisture throughout the

year (ever greens).

8. Cold climate with moist winter:

Coincides with sub polar belts of pine forests.

a. Mean temperature of coldest month less than warmest month above 100C.

b. Rains all through the year on the coast mostly in winter inland mostly in

summer.

c. Vegetation – is thermal type, which required short summer and long winter

and needs snow cover for protection during the long and cold winter (E.g.

Pine and fir).

17

9.Cold Climate with Dry Winter

In high latitudes because of the low winter temperature and the great distance

from moisture bearing winds, the rain during winter is very small other characteristics are

similar to zone. (8).

10.Tundra Climate:

In the northern most part of the continent. The mean temperature of the

warmest winter is below 100C. Subsoil is frozen throughout the year and there are

no forests.

1. Ice climate:

The polar cap of /snow and ice with mean temperature of the warmest month is

below 00C (32.50)

1. Tropical rain forest

2. Savanna

3. Steppe

4. Desert

5. Warm summer rain

6. Warm winter rain

7. Temperate rain all seasons

8. Cold moist climate

9. Cold winter dry climate

18

10. Tundra

11. Ice.

If we travel along the west coast from the equator towards North Pole we

pass the climatic zone in the following order. 1. Tropical forest (1) 2. Savana 3.

Steppe 4. With summer rain desert 5. Steppe 6. With winter rain warm climate

with wet winter 7. Warm climate with rain in all season 8. Cold climate with

moist winter 9. Tundra 10. Ice.

Along the east cost towards North Pole 1. With zone of tropical forest 7. Merging

gradually into were climate with rain in all season 8. Cold climate with moist winter

tundra (10) and ice (11).

Thomthwaite establishes five climatic provinces that correspond closely to natural

plant covers.

Climatic province Type of Vegetation T.E. Index

Wet Rainforest =128

Humid Forest 64 – 127

Sub humid Grass land 32 – 63

Semi arid Steppe 16 – 31

Arid Desert <16

The following are some of the most important elements of weather, which

in different combinations make up the climate of particular place or areas.

Weather parameters/Weather elements

19

1. Solar radiation

2. Temperature

3. Air pressure

4. Wind velocity and wind direction

5. Moisture (humidity)

6. Cloudiness (Sunshine hours)

7. Precipitation (Rainfall)

All these are highly variable and constitute the weather / climate. A change in one of

the elements generally brings about changes in the others.

Factors affecting weather and climate: (Climatic controls)

1. Latitude:

The distance from the equator either south or north, largely create variations in the

climate. Based on latitude the climate has been classified as (i) Tropical, (ii)

Subtropical, (iii) Temperate and (iv) Polar.

The tropical climate s characterized by high temperature throughout the

year. Subtropical is also characterized by high temperature alternating with low

temperature in winter. The temperate climate has low temperature throughout the

year. The polar climate is noted for its very low temperature throughout the year.

2. Altitude: (Elevation):

The height from the mean sea level creates variation in climate. Even in the

tropical regions, the high mountains have temperate climate. The temperature

decreases by 0.60 C for every 100 m from the sea level . Generally there is a decrease

in pressure and increase in precipitation and wind velocity. The above factors alter

the kind of vegetation, soil types and the crop production.

3. Precipitation:

20

The quantity and distribution of rainfall decides the nature of vegetation and the

nature of the cultivated crops. The crop region are classified on the basis of

average rainfall which are as follows:

Rainfall (mm) Name of the climatic region

Less than 500 Arid

500-750 Semi Arid

750-1000 Sub humid

More than 1000 Humid

4.Soil Type:

Soil is a product of climate action on rocks as modified by landscape and

vegetation over a long period of time. The colour of the soil surface affects the

absorption, storage and re-radiation of heat. White colour reflects while the black

absorbs more radiation. Due to differential absorption of heat energy, variations in

temperature are created at different places. In black soil areas the climate is hot while in

red soil areas it is comparatively cooler due to lesser heat absorption.

5.Nearness to large water bodies: (Nearness to sea)

The presence of large water bodies like lakes and sea affect the climate of the

surrounding areas. E.g., Islands and coastal areas. The movement of air from earth

surface and from water bodies to earth modifies the climate. The extreme variation in

temperature during summer and winter is minimized in coastal areas and Islands.

6.Topograpy: (Relief)

The surface of landscape (leveled or uneven surface areas) produces marked

changes in the climate. This involves the altitude of the place. Steepness of the slope

and exposure of the slope to light and wind.

7.Vegetation:

Kind of vegetation characterizes the nature of climate. Thick vegetation is found

in tropical regions where temperature and precipitation are high. General types of

vegetations present in a region indicate the nature of climate of that region.

8.Others factors are

i) Semi permanent high and low pressure systems.

21

ii) Winds and air masses.

iii) Atmospheric disturbances or storms.

iv) Oceans currents.

v) Mountain barriers.

(Chapter –3)

ATMOSPHERE

The word atmosphere derives from the Greek word “Atmos” which means vapour

and “Sphaira” which means sphere. It is used now to denote the gaseous sphere

surrounding the earth.

Stratification and Composition of Atmosphere

The atmosphere is a mechanical mixture of many gases, not a chemical

compound. In addition, it contains water vapor volume and huge number of solid

particles, called aerosols. Some of the gases (N, O, Ar, CO2) may be regarded as

permanent atmospheric components that remain in fixed proportions to the total gas

volume. Other constituents vary in quantity from place to place and from time to time. If

the suspended particles, water vapour and other variable gases were excluded from the

atmospheres, we would find that the dry air is very stable all over the earth up to an

altitude of about 80 kilometers.

1.Composition of Atmosphere

Principal gases comprising dry air in the lower atmosphere.

Constituent Percent by volume

Nitrogen (N2) 78.08

Oxygen (O2) 20.94

*Argon (Ar) 0.93

Carbon dioxide (Co2) 0.03

*Neon (Ne) 0.0018

22

*Helium (He) 0.0005

Ozone (O3 ) 0.00006

Hydrogen (H2) 0.00005

*Krypton (Kr) Trace

*Xenon (Xe) Trace

Methane (Me) Trace

Inert chemically never found in any chemical compounds.

As shown in the table, two gases, nitrogen and oxygen, make up about 99 per cent

of the clean, dry air. The remaining gases are mostly inert and constitute about 1 per cent

of the atmosphere generally homogenous and it is called as homosphere. At higher

altitudes, the chemical constituents of air changes considerably. The layer is known as

the heterosphere.

1.NITROGEN:

It is chemically inactive and an important plant nutrient, but it has to be fixed in

the soil to make it available to the plant. The fixation of nitrogen in the soil is carried out

by the following agencies.

i. a. Symbiotic root nodule leguminous bacteria – Rhizobium group.

b. Symbiotic root nodule leguminous bacteria – on Casuarinas.

c. Symbiotic leaf nodule bacteria on Pavetta and Dioscorea.

d. Symbiotic root nodule - actinomycetes in Mystic and Almus.

ii. Nitrogen fixation by free living:

a. Azotobactor and clostridium group of bacteria.

b. Photosynthetic and chemo synthetic – Sulphur bacteria.

c. Free living east cells of fungi.

d. Blue green algac.

The above agencies are known as biological agencies, which fixes the

atmospheric nitrogen in the soil.

iii. Lightning and powerful electrical charges are released and ‘N’ and ‘H’

in elements present in the atmosphere forms NH2 dissolved and

23

brought down by rain and water as NH3 . About 2 to 20 ibs of nitrogen

is added to the soil/ac.

iv. By means of artificial methods of manufacture of Amo.sulphate

(NH2SO4) – ‘N’ and ‘H’ in the atmosphere – An hydrogen’s Amo + S

– Amm. Sulphate. By electrical are method.

2.OXYGEN:

It has got considerable importance in plant and animal life. It plays an important

role in respiration, bacterial activity in soil oxidation and absorption of plant retrients and

several soil forming or weathering activities in the soil, which improve plant food

availability.

3.CARBONDIOXIDE:

It plays an active part in photosynthetic activities.

4. ARGON:

It is used extensively in electric lamp bulbs because of its inertness. It is also

used in florescent tubes. It flows with blue light.

5. NEON:

Neon is used to fill florescent tubes. It flows with distinctive orange red colour.

6.HELIUM:

It is the second highest element with a density of 0.177 gms per liter (Hydrogen

0,08988 gms /liter). It is used to inflate balloons because it will not burn.

7.KRUPTON:

This glows with brilliant green and yellow colour.

8.XENON:

It is chemically inert and glows with a blue green colour.

Besides these the atmosphere also contains small quantities of ozone (O3 ),

Methane (CH4), Nitrous oxide (N2O), Sulphur dioxide (SO2) and traces of Iodine,

NaCl,NH3 Carbon monoxide etc.,

The amount of CO2 in the atmosphere is not quite constant. The vegetable World

continuously consumes CO2 , which again is produced by the animal World, through

burning of fuels, volcanic action and various process of decay in the soil. But the oceans

24

by dissolving the excess of CO2, so effectively regulate it that the amount of CO2 in t he

atmosphere remains almost constant.

Ozone, which is present in the lower atmosphere, has a maximum in the upper

atmosphere between 10 and 25 km (30000 and 80,000) where it amount varies

considerably.

Apart from this the composition of the atmosphere is remarkably constant all over

the earths surface.

WATER VAPOUR:

The air also contains variable of water vapour. The water vapour present in the

atmosphere varies up to 4% by volume as in tropical humid climate. Most of the vapour

is found in the lower part pf the atmosphere. The maximum amount of water vapour that

the air can absorb depends entirely on the temperature of the air, the higher the

temperature of the air the more water vapour it can hold. The air is saturated with

moisture when this maximum amount is reached, when air is cooled below its saturation

temperature condensation takes place, water droplets formed or at low temperature ice

crystals formed. Small water drop lets and ice crystals are kept afloat in the air by the

ascending air currents and under special circumstances the water droplets and ice crystals

coalesce and form large drops or snow flakes which are precipitated from the clouds

when they become too large to be kept afloat.

SOLD PARTICLES OF ATMOSPHERE:

The air also contains a variable amount of impurities such as dust, soot, salts,

fungal spores, bacteria and pollen (both organic and in organic)

Over a city it is estimated to contain, 1,00,000 0articles per cc. A cigarette puff

sends about 400 crores of dust particles.

The main source of dust is the arid regions such as deserts and steppes. The

minute dust particles are readily distributed throughout the lower atmosphere and carried

for from the source. The industrial regions forest fires and volcanoes constitute the main

source of soot. Through the action of winds, spray is whirled up from the oceans, and

when it evaporation the salt remains in the air in the form of minute particles.

The presence of dust particles in the atmosphere is important since when the air is

cooled to its saturative temperature, condensation takes place on certain active nuclei.

25

The salt particles from the oceans are most active as condensation nuclei on which the

water vapour condenses to form fog or rain. They are the cause for twilight.

Layered structure of the Atmosphere:

During the international Geophysical year (1957-62), important discoveries were

made about the atmosphere and many new facts came to light. The earth’s atmosphere

consists of zones or layers arranged like spherical shells according to altitude and

temperature variations above the earth’s surface.

According to Peterson, the atmosphere is divided into the following more

significant spheres.

1.Troposhere

2. Stratosphere

3. Mesosphere (also called Ozonosphere)

4. Ionosphere

5. Exosphere

1.Troposphere: It contains about 75 per cent of the total gaseous mass of the

atmosphere. It has been derived from the Greek word ‘trops’ meaning “mixing” or

turbulence. The average height of this lowermost layer of the atmosphere is placed at

about 14 km above sea level. Under normal conditions, the height of the troposphere at

the poles is about 8 kilometers, while at the equator it is about 16 kilometers.

Troposphere is marked by turbulence and eddies. It is also called co9nnective

region. Various types of clouds, thunderstorms as well as cyclones and anticyclones

occur in this sphere because of the concentration of almost all the water vapour and

aerosols in it. Wind velocities increase with height and attain maximum at the top. The

most important feature is decrease in temperature with increasing elevation up to 14km.

Tropopause is a shallow layer separating troposphere from the next thermal layer of the

atmosphere i.e., stratosphere. Tropopause (Greek word) means where the mixing stops.

The temperature remains constant throughout the tropopause. The height of the

tropopause is about 1to 2 km.

2.Stratosphere: The stratosphere begins at the tropospause, which forms its lower

boundary. The lower stratosphere is isothermal in character (16-30 kilometers). There is

26

a gradual temperature increase with height beyond 20 km i.e., upper stratosphere

(temperature inversion). No visible weather phenomena occur above tropopause.

3.Mesosphere or Ozonosphere: There is maximum concentration of Ozone between 30

to 60 kilometers above the surface of the earth. Because of the concentration of ozone in

this layer it is called the ozonosphere. It is a warm layer because of selective absorption

of ultra violet radiation by ozone. In fact, it acts as a filter for ultra violet radiation from

the sun.

In this layer the temperature increases with height @ 50 C/km. The maximum

temperature recorded in the ozonosphere is higher than that at the earth’s surface.

Because of the preponderance if chemical processes, this sphere is sometimes called as

chemo sphere.

4. Ionosphere: Ionosphere, according to Peterson, lies beyond the ozonosphere at a

height of about 60 km above the earth’s surface. At this level the ionization atmosphere

begins to occur. Above ozonosphere, the temperature falls again reaching a minimum of

about 1000C at a height 80 km. Above earth’s surface. Beyond this level the temperature

increases again due to the absorption of short wave solar radiator by the atmos of O & N

in this ionosphere.

Layers of Ionosphere

D Layer : 60-89km.

E Layer : 90-130 km.

E1 Sporadic Layer: 110 km.

E2 Layer : 150 km.

F1 Layer

F2 Layer : 150- 380 kms.

G Layer : 400km and above.

5.Exosphere: The outer most layer of earth’s atmosphere is known as the exosphere,

which lies between 400 and 1000 kilometers. At such great height density of atmos in

the atmosphere is extremely low. Hydrogen and helium gases predominate in the outer

most regions. Kinetic temperature may reach 55680 Celsius.

Modern Views Regarding the Structure of Atmosphere

On the basis of composition, the atmosphere is divided into two broad spheres.

27

i) Homospheres: Means zone of homogenous composition height – up to 88

kilometers.

The proportions of the component gases of the sphere are uniform at different levels.

Sub-divided into

a. Troposphere - very shallow transition layer tropopause

b. Stratosphere - Stratopause

c. Mesosphere - Mesopause

Heterosphere: The atmosphere above the homosphere is not uniform in composition.

Different layers of the atmosphere in this part differ from one another in their chemical

and physical properties. In this sphere gases are said to be arranged into the following

four roughly spherical shells, each of which has its own distinctive composition.

1. Nitrogen layer – 200 km above earths surface molecular N.

2. Oxygen layer – Average ht. 1120km – atomic oxygen.

3. Helium layer - Average ht. 3520km.

4. Hydrogen layer – these layer are arranged according to the

weight of the gases.

Lapse rate: The rate of decrease of temperature with increase in height at a given place

and time is called Lapse rate. The normal lapse rate is 6.50 C per km increase in height.

28

(Chapter –4)

SOLAR RADIATION AND LIGHT

The sun is primary source of heat to the earth and its atmosphere. The heat

received from other celestial bodies as well as the interior of the earth from the sun is

about 1, 49,000,000 (1.49 * 108) kilo0meters. The diameter of the sun measures roughly

about 13,82,400(1.38*106) kilometers. The surface temperature of the sun is estimated

between 55000C and 61000C. (Or 57620K). The interior temperature ranges from 8*106 to

40*1006 0K.

Solar radiation provides more than 99.9 percent of the energy that heats the earth

and does not change appreciably from year to year and varies only with latitude and

season. Undoubtedly, the radiant energy from the sun is the most important control of

our weather and climate. The most astonishing fact about the incoming solar radiation

(insolation) that strikes the earth’s surface is that it is equal to about 23-billon

horsepower. Actually it is this amount of energy received from the sun that acts as the

driving force for all the atmospheric as well as biological processes on the earth.

Besides, all other sources of energy found on earth such as coal, oil and wood etc., are

nothing but converted from of solar energy.

All matter (not at the absolute zero temperature) what ever their temperature

sends out energy into the surrounding space in the form of electromagnetic waves and the

propagation of this energy as well as the energy it self is called “Radiation”.

(If we assume that the sun is perfectly black, the temperature it should have in

order for the flux at the outer limits of the earths atmosphere to equal the solar constant

and this is know as the “Effective temperature of the sun” and is equal to 5760 0K.)

A black body at the temperature of the sun will radiate upward 99% of its energy

between the wavelengths 0.15 and 4. About ½ of the radiation will be in the and the

region of the spectrum between 0.38 to 0.77 and the reminder in the invisible ultraviolet

and infrared regions.

The word ‘ insolation’ is contraction of “incoming solar radiation”. Radiant

energy from the sun that strikes the earth is called insolation.

29

LIGHT

Solar radiation consists of a bundle of rays of radiant energy of different

wavelengths. The sun emits radiant energy in the from of electromagnetic waves. The

visible portion of the solar spectrum appears as light. Light travels with a speed of

2,97,600 km/sec. It takes 8 minutes and 20 second to reach the earth. Light is the total

effect of the combination of the seven different colours, namely violet, indigo, blue,

green, yellow, orange and red (VIBGYOR). The waves that produce the effect of red

colour are the longest and those producing the violet are the shortest Waves

shorter than the violet are called ultraviolet rays, while those longer than the red are

known as infra red rays. The ultra violet waves form only 6 per cent of the insolation, but

have strong photochemical effects on some substances. The infrared rays, even though

invisible, form 43 per cent of the insolation. They are largely absorbed by water vapour

that is concentrated in the lower atmosphere.

DEFLECTION OF SOLAR RADIATION:

The incoming solar radiation suffers deflection as follows:

1. Absorption by ozone layer in the upper atmosphere (about 5%).

2. Scattering by dry air.

3. Absorption, scattering and diffuse reflection by suspended solid particles and

4. Absorption and scattering by water vapour.

REFLECTION OF SOLAR RADIATION BY EARTH’S SURFACE AND BY

CLOUDS. ALBEDO OF EARTH:

The surface of the earth is a poor reflector of solar radiation.

1. Fresh snow reflects 80 –85% of incoming radiation.

2. Old snow – 40%

3. Grsdd reflected – 20 to 44%

4. Rock – 12 to 15%

5. Dry earth – 14%

30

6. Wet earth – 8 to 9%.

No radiation is reflected be a smooth water surface when the sun is with 400 of the

Zenith.

7.Cloud reflects 78%.

The “Alpedo of the earth” is a quantity used to measure the total reflecting power

of the earth and atmosphere. It is defined as the fraction of the incoming solar radiation

returned to space by scattering and reflection in the atmosphere and by reflection at

clouds and at the earth surface. It represents the unused fraction of the incoming solar

energy; the part that is absorbed neither in the atmosphere marine in the earth.

TRANSFER OF HEAT:

The atmosphere is a poor absorber and the earths surface is good absorber of

incoming radiation, and the atmosphere receives most of the heat energy via the earths

surface. The heat received in one place may be transported to other places by

1.Conduction

2.Radiation

3.Turbulence

4.Aadvection

TURBULANCE:

The wind is never a steady current. It consists of a succession of gusts and lulls of

short period (Gust=sudden blast of wind) (Lull= to become calm). This irregular motion

is called Turbulence is made up of number of small eddies that travel with general air

current, super imposed on it. These eddies carry heat, moisture, dust etc, with them as

they travel from one place to other. The turbulence transfer of heat is most effective in

the vicinity of earths surface is distributed through air column, through mixing of

neighboring air masses.

Advection or large-scale air currents:

These are mainly horizontal currents and so heat is transported from one place to

another mainly through horizontal currents and hence only in horizontal direction where

as turbulence and convective currents transport heat along the vertical.

31

VERTICAL MIXING:

In vertical mixing of heat the air is subject to pressure changes as it moves up or

down through the atmosphere (in turbulence).

In the atmosphere the ascending air will be cooled adiabatic ally and the

descending air will be heated adiabatic ally. The result of turbulence mixing along the

vertical is to create dry adiabatic lapse rate if the air is unsaturated and a moist adiabatic

lapse rate if the air saturated.

Vertical mixing will tent to decrease the temperature and increase moisture

content in the upper portion of the mixed layer and increase the temperature and decrease

the moisture content in the lower portion. This will decrease the relative humidity near

the earth’s surface and increase in the upper surface.

HORIZONTAL MIXING:

The horizontal mixing takes place at constant pressure and no adiabatic change

involved. Two different air masses of different temperature either of which is saturated

might become saturated after complete horizontal mixing.

CONVECTION:

The instability is created in the lower layer of the atmosphere either through the

diurnal heating of the earths surface by the sun or through heating of the air when it travel

towards warmer regions. Gustiness, curmulus clouds, showers and thunderstorms squalls

are directly caused by instability.

As soon as the temperature lapse rate near the earth exceeds the dry slightest

disturbance will upset the stratification. Air from earth’s surface rises and air from

higher levels sinks to replace the ascending masses. This process of overturning of

unstable air is called “Convection”. If the rising currents reach the condensation level,

clouds will form. The descending air surrounding the rising masses will be heated

adiabatic ally the R.H. will be lowered and the sky will be broken clouds of the cumulus

type. The weather phenomena that convection will produce depend on the depth of the

unstable layer, the height of the condensation level and the distribution of temperature

aloft.

Radiation: Radiation is the process of transmission of energy by Electro magnetic waves

and is the means by which energy emitted by the sun reaches the earth.

32

Conduction: Conduction is the process of heat transfer through matter by molecular

activity. In this process heat is transferred from one part of a body to another or between

two objects touching each other. Conduction occurs through molecular movement.

Convection: Convection is the process of transfer of heat, through movement of a mass

or substance from one place to another. Convection is possible only in gases or fluids,

for they alone have internal mass motions. In solid substance this type of heat transfer is

impossible.

Heat Budget or Radiation balance: Of the total solar radiation reaching the outer limit

of the atmosphere, about 25 percent is reflected by clouds and 7 percent of scattered

reflects 2 percent pf radiation to the space. About 19% of solar radiation is absorbed by

gases and water vapour in the atmosphere. About 47 % is absorbed by the earth. Out of

which 23 percent is absorbed by the earth from scattering of clouds and atmosphere. And

24 percent is received directly from the sun. Thus approximately two-thirds of the total

radiation is effective in heating the earth.

Fig

The total energy coming to the earth over a considerable period of time is equal to

the total outward losses. In order to maintain the terrestrial heat balance, the 66 percent

of solar radiation gained must be balanced by the same amount of energy radiated back to

space in the form of long-wave terrestrial radiation (transferred by conduction and

convection). In this way the overall heat budget of the earth is balanced. If this were not

so, the earth would soon become either very hot or very cold. Actually there is a

deficit of heat at higher latitudes and surplus in low latitudes.

Albedo: It is the capacity of any surface to reflect the incoming radiation (light) or it is

Is the radio of incoming radiation to the outgoing radiation. The total reflectivity is

known as earth’s albedo. Averge albedo value for earth is 34%.

33

Latent heat: Normally, when heat is given to a substance, its temperature rises.

However, the heat which changes the physical state of a substance but not raise its

temperature is called latent heat of that substance to change its state without change of

temperature. The latent heat is used up in over coming the force of attraction between the

molecules of the substances.

Sensible heat flux: same as enthalpy and this is the product of heat capacity times the

Kelvin temperature, at constant pressure for a perfect gas. This is used in meteorology in

contrast of air is referred to as sensible heat.

Sensible heat advection: The process in which warm dry air passing over a field

supplies energy for transpiration.

Solar Constant: It is the amount of solar energy incident on a unit area at right angle to

the suns rays at the earth’s mean distance per unit in the absence of atmosphere. Solar

constant is 2 cal /cm2/ minute. The sun is the source of more than 99 per cent of the

thermal energy required for the physical processes taking place in the earth atmosphere

system. Every minute, the sun radiates approximately 56*1026calories of energy.

In terms of the energy per unit area incident on a spherical shell with a radius of

1.5*1013 cm (the mean distance of the earth from the sun) and concentric with the sun,

this energy is equal to

56x1026 cal. Min-1

S = ------------------------- = 2.0langely min-1.

4 (1.5x1013 cm) 2

(Langley =gram calories cm-2).

Solar constant = 2.0 gram calories cm-1min-1

34

The solar constant (S) is a true constant, but fluctuates by as much as 3.5 percent

abut its mean value, depending upon the distance of earth from the sun

Solar constant is defined as the rate at which solar radiation is received outside the

earth’s atmosphere on a surface perpendicular to the sun’s rays when the earth is at an

average distance from the sun.

The Smithsonian Institute, USA has come to the conclusion that the standard

value of solar constant is 1.94g cal. Cm-2 min-1.

Since there is fluctuation in the amount of radiant energy emitted by the sun due

to periodic disturbances on the solar surface, the amount of solar constant , therefore,

registers a slight increase or decrease. However, this hardly exceeds 2-3%.

The amount of insolation received on any date place on the earth is governed by

i) The solar constant which depends on (a) energy out put of the sun and (b)

distance from the earth to sun.

ii) Transparency of the atmosphere.

iii) Duration of the daily sunlight period.

iv) Angle at which the sun’s rays strike the earth.

The distance between the earth and the sun varies between 94.5 million miles

(157.5m km) at aphelion (July 1st) and 91.5 million miles at perihelion (January1st). The

amount of radiation received is seven percent greater at perihelion than at aphelion. This

is a consequence of the inverse square law, which states, in effect, that the radiation

received on any unit area decreases in proportion to the square to the distance to the

sources.

1

Intensity ------

d 2

(Aphelion – The point farthest from the sun in the orbit of a planet.

Perihelion - The point nearest from the sun in the orbit of a planet)

Transparency of the atmosphere has a more important bearing upon the amount of

insolation, which reaches the earth’s surface. The areas having dust, clouds, water

vapour and cloudiness or polluted air will receive less direct insolation. The transparency

35

of atmosphere depends on the latitude of a place. At middle and high latitudes the sun’s

rays must pass through thicker layers of reflecting/scattering material and it is not so at

tropical latitudes.

EFFECT Of LIGHT ON PLSNTD

Solar radiation is the primary source of electromagnetic spectrum having different

wavelength. Different type lf radiation is shown below. (Wavelength in micron)

1. Cosmic rays 10-7 to 10-4 micron

2. Gamma rays 10-4 to 10-3 micron

3. X rays 10-3 to 10-1 micron

4. U.V.1 to 390 micron

5. Visible 390-760 micron

6. Infrared 760-106 micron

7. Radio wave 106-1013micron

Visible solar radiation id called as light. The shorter wavelength in the solar

spectrum is harmful to the plants when exposed to excessive amounts. The atmosphere,

however, absorbs almost all the shorter wavelengths. The infra radiation has thermal

effect on plants by supplying necessary energy for evaporation of water from the plants.

The visible portion of the solar spectrum is the light with wavelength ranging

from 0.4 to 0.7. Light is one of the important climatic factors for many vital functions of

the plant. It is essential for the synthesis of the most important pigment i.e., chlorophyll.

The chlorophyll absorbs the radiant energy and converts into potential energy of

carbohydrates (photosynthesis). The carbohydrate thus format is the connecting link

between solar energy and living World. In addition, it regulates the important

physiological functions like transpiration.

Effect of light on plant can be studied under four headings (i) Light intensity (ii)

Quality of light (iii) Duration of light and (iv) direction of light.

1.Light Intensity: The intensity of light is measured by a standard unit called candle.

The amount of light received at a distance of one meter from a standard candle is known

36

as “Metre Candle or Lux”. The light intensity at one foot from a standard candle is called

“foor candle” or 10.764 luxes and the instrument used is called as “Lux meter”. About

one per cent of the light energy is converted into biochemical energy. Very low light

intensity reduces the rate of photosynthesis and may even results in result in the closing

of the stomata detrimental to plants in many ways. This results in reduced plant growth.

Very high light intermities are it increases the rate of respiration. It causes rapid loss of

water, i.e., it increases the transpiration rate of water from the plants resulting in closure

of stomata. The most harmful effect of high intensity light is that it oxidizes the cell

contents, which is termed as “Solarisation”. This oxidation is different from respiration

and is called as “Photo oxidation”.

Under natural conditions light intensity varies greatly and plants shoe marked

response to changes of light intensities. Based on the response to light intensities the

plants are classified as follows:

i) Sciophytes: (Shade loving plants) The plants that grow better under partially

shaded (low light) conditions e.g., betel vines, buckwheat, turmeric etc.,

ii) Heliophytes: (sun loving Plants) Many species of plants produce maximum

dry matter under high light intensities when the moisture is available at the

optimum level, e.g. maize, sorghum, rice etc. Except under glass house or

shaded conditions, intensity of light cannot be controlled.

2.Quality of Light: When a bean of white light is passed through a prism, it is dispersed

into different colours with their wavelengths partied. This is called the visible part of the

solar spectrum. The different colours and their wavelength are as follows:

Violet & Indigo 400-435nm

Blue 435-490nm

Green 490-574nm

Yellow 574-594nm

Orange 594-626nm

Red 626-750nm

Visible rays 390-760 mill micron //nm

1

Micron = ------------- meter or 10-6m

37

1000000

1

= ---------- Mm 10-3mm

1000

Milli micron: 10-9 m-nanometer

The Principal wavelengths absorbed and used in photosynthesis are in the violet-

blue and the orange-red regions. Among this, red light is the most favorable light for

growth followed b violet-blue. Ultra violet and shorter wavelengths kill bacteria and

many fungi.

3.Duration of light: The duration of light has greater influence than the intensity. It has

a considerable importance in the selection of crop varieties. The response of plants to the

relative length of the day and night is known as photoperiodism. The plants are classified

based on the extent of response of day length as follows.

i) Long day plants: The plants which develop and produce normally when the

photoperiod is greater than the critical minimum (greater than 12 hours) e.g. cereals,

potato, sugar beet, wheat, barley etc.

ii) Short day plants: The plants which develop normally when the photoperiod is less

than the critical maximum (less than 12 hours) e.g. tobacco, soybean, millets, maize,

sugarcane, etc.

iii) Indeterminate or day neutral plants: Those plants which are not affected by

photo period, e.g., Tomato, Cotton, Sweet potato, pineapple etc.,

The photo periodism influences the plant characters such as floral initiation and

development, bulb and rhizome production etc. If a long day plant is grown

during periods of short days the growth of internodes are shortened and flowering

is delayed till the long days come in the season. Similarly when short day plants

are subjected to long day periods, there will be abnormal vegetative growth and

there may not be any floral initiation. (CO 38 rice). But now a days many crops

do have photo-insensitive varieties.

4.Direction of light: The direction of sunlight has a greater effect on the orientation of

roots shoots and leaves. In temperate regions, the southern slopes show better growth of

plants than the northern slopes due to higher contribution of sunlight in the southern side.

38

Orientation of leaves: The changes of position or orientation of organs of plants caused

by light is usually called as “Phototropism”. For example, the leaves are oriented at right

angles to incidence of light to receive maximum radiation.

Photomorphogenesis: Changes in the morphology of plants due to light is known as

photomorphogenesis. This is due to ultra violet and violet rays of the sun.

Duration of daily sunlight period (Length of day)

Fig:

39

(Chapter –5)

SUN AND EARTH

FACTS ABOUT EARTH:

1.Superficial area: 19,69,50,000 sq. miles

2. Land surface: 5,75,10,000 sq. miles

3. Water surface: 13,94,40,4000 sq. miles.

a. Earth make one complete revolution on its axis in 23 hours and 56

minutes.

b. Earth rotates round the sun in 365 1/4 days.

c. Earth revolves in its orbit round the sun at a speed of 6,66,000 m.p.h.

d. Earth rotates on its axis at an equatorial speed of 1000 m.p.h.

e. The earth is closest to the sun on January 1 at about 91,342,000 miles and

farthest away on July 2nd 94,454,000 miles.

The sun is a star with a surface temperature of about 60000C radiates into space.

On a surface exposed normal to the Sun’s rays at the mean distance from the sun, energy

of 1.94 gm cal/cm2 per minute is received on an average. This energy amount of 1.94 gm

cal/ minute is called the Solar Constant.

The mean intensity of the solar radiation received on January 1st 2nd at the

boundary of the atmosphere is 2.007 and 1.877gm cal/cm2/minute respectively.

The inclination of the earth 66033’ against the plane of the orbit. The inclination

is the main reason for the season. Only at the time of equinox (March 21, September 23)

does the dividing line of the lighted and dark half of the earth parallel and pass through

the poles. Between March 21 and September 23 to the north pole is tilted towards the sun

and June, September 23 to March 21 the South Pole is tilted towards the sun.

The sun is fixed in its place but rotates on its axis once in 25 1/3 days. The path

taken by the earth round the sun is called the “Ecliptic”. The orbit of earth round the sun

is roughly circular, with only a slight eccentricity (in a conic section).

40

The sun’s ray strikes the surface of the earth perpendicularly near the equator and

with greater obliquity as the place moves from the equator to the poles. As the

Obliquity increases, the surface over.

Which the rays spread out is increased and the isolation received by unit surface

decreases.

The vertical rays of the sun at noonday fall directly overhead at the equator on

March 21st and this is called ‘Vernal equinox’. The vertical rays continue to move

northern to the tropic of cancer and are overhead there on June 21st and this date is known

as “summer solstice”(in Northern hemisphere). Afterwards the rays return to the equator

on September 21st and this date is known as ”Autumnal equinox”. Then it reaches the

tropic of Capricorn on December 21st and this date is known as “Winter solstice” (In

Northern hemisphere). The summer and winter solstices will be reverse in the southern

hemisphere. At equinox days and will night are of equal length throughout the World. In

summer solstice the day will be longer whereas in winter solstice the day will be shorter

than night. The Northern pole will be in daylight for the full 24 hours on summer solstice

and will be dark for full 24 hours on winter solstice of Northern Hemisphere.

Angle of the sunrays: The effect of varying angle at which the sunrays strike the earth

can be seen daily the march of the sun across the sky. At solar noon the intensity of

insolation is the greatest but in the morning and evening hours when the sun is at low

angle, the amount of insolation is also small.

Fig:

41

At equator the angle of incidence varies from 23 1/20 North of the zenith to 23

½ 0 South of the zenith. The intensity of solar radiation ranges from 92% on June 21st and

December 21st to 100% on March 21st and September 23rd. The range is only 8%.

At 450N latitude the angle of incidence varies from 211/20 South of zenith to

681/20 South of only 211/20 above the horizon. The variation in intensity due only to the

change in the angle of incidence is from 93% of maximum on June 21st to 98% on

December 21st.

42

(Chapter –6)

AIR TEMPERATURE

Temperature refers to the degree of hotness or coldness of a substance or a thing.

Temperature provides a measure of the intensity of heat energy. (Scale of temperature

and relationship between scales – study practical record).

TEMPERATURE OF AIR:

Importance of temperature:

Temperature is necessary for the weathering of soils, promoting bacterial activity,

sterilization of soils, killing of weeds, pests and disease, for drying grains and maturity of

crops. Every living organism, plant, or animals or insects requires optimum temperature

for carrying out the basic biochemical activities for survival. Excessive temperature is

harmful for germination growth, flowering and maturity of fruits. It increases the

transpiration from plants and evaporation from the soil and necessitates frequent

irrigation.

It is the most important phenomenon of solar energy. In climatology the word

temperature denotes “Shade” temperature to avoid the influence of direct rays and sun.

It is measured by means of thermometers. Day temperature at any given time is meant

the temperature of the air measured under standardized condition and with certain

recognized precautions against errors introduced by radiation from the sun or other

heated body.

TEMPERATURE VARIATIONS:

1.Diurnal variation:

The difference between the maximum and minimum temperature on a day is

called diurnal range. It is smaller in the wet season than in the dry season and smaller in

coastal areas in the interior place.

The amount of the daily range of variation varies widely many factors like

cloudiness and humidity of the air, nature of earth’s surface, the vertical lapse rate of

temperature, wind, elevation and latitude.

43

1.CLOUDINESS:

Cloudiness influences the penetration of insolation to earth’s surface by day and

retardation of net loss of heat by terrestrial radiation at night.

2.HUMIDITY OF THE AIR:

There is only very small diurnal variation of temperature over the ocean; on land,

after heavy rains where soil is moist and water stands on the surface, temperature ranges

are less than during dry weather, because of the humidity of the air. The average range of

temperature increases with distance from water sources.

3 & 4.AIR WITH STEEP LAPSE-RATE:

Heating during the day is accompanied by deep convection where by energy

absorbed by air near the earths surface is distributed through a thick layer of air.

Similarly at night steep lapse rates are often accompanied winds and turbulent mixing

that keep the lower layers warmer than in conditions of still stable air with steep lapse-

rate shows small diurnal variation. Diurnal ranges of temperature are usually smaller at

in the station than nearly valleys.

Daily range of temperature increase with latitude up to subtropical latitude.

Maximum daily ranges have been recorded in subtropical deserts where clear air &dry

land surface prevails. But in the same latitude along foggy coasts parallel by cool ocean

current, sea, breaze chop off maximum temperature, fog interferes with terrestrial

radiation at night and hence daily range are the lowest in the World. In middle latitudes,

daily ranges very less with latitude than with distance from the sea. In high altitudes

diurnal range decrease again, owing to the lessened effectiveness of the daily successive

of the sunlight and darkness.

2.Annual Variation:

The difference between the temperatures in a year is annual variation. The

temperature is more in May and June and lesser in November, December in Tamilnadu.

MEAN DAILY TEMPERATURE:

It is the mean of 24 readings taken at hourly intervals as in the self-recording

instruments, like thermograph. But in other thermometers these are usually taken in the

morning (8 a.m.), afternoon (2 p.m.) and evening (6 p.m.).

44

MEAN MONTHLY TEMPERATURE:

It is the average of the total daily mean temperature for the month divided by the

number of days in a month.

MEAN ANNUAL TEMPERATURE:

It is the average of the 12 months temperature total of monthly means divided

by 12.

MEAN ANNUAL RANGE:

The difference between the warmest and coldest months is he means annual

range. Only mean temperatures are usually quoted in describing climate.

The centigrade scale labels the temperature of boiling point of water under 1 atm.

of pressure 1000C and the freezing point of water as 00C. The forenheit scale labels the

same temperature as 2120F and 320F respectively. The numerical relation between the

two scales is then

0 C = 100 0F-32 180

SENSIBLE TEMPERATURE:

The temperature recorded by the thermometer does not always agree with the

sensations of heat felt by the human body. The sensation of the heat depends upon air

movement and humidity. 800F in the equatorial zone is more uncomfortable than 100 in

the desert because of humidity.

Vertical distribution of temperature (Altitude)

As a general rule throughout the troposphere, the temperature decreases with

elevation. The rate of decrease with altitude is not uniform; it varies with time of the day,

season and location. The average decrease is approximately 0.650C/100m. (6.50C/km).

This is known as normal lapse rate or vertical temperature gradient.

Temperature Inversion

Although normally, the lower several miles of atmosphere show a decrease in

temperature with increasing altitude when the solder air lies below warmer air and closer

to earth’s surface the normal lapse rate is reversed and this is called temperature

inversion.

45

Horizontal distribution of temperature (Latitude)

The lines connecting places, which have same air temperature, are called

isotherms. Thus, all the points on a map through which any one isotherm passes have

identical average temperature for the period indicated. There is general decrease from

equator to poles (increase in latitude).

Factors affecting temperature

Elevation of a place

Soil type

Nearness to water body

Presence of hill or mountain

Location of the earth (co-ordinate)

Anthrophic

Seasonal variations

Temperature (Diurnal, mean and range) vary according to the season. The main

factors contributing to seasonal variations are: -

1. The angle of inclination of solar rays, which decides the intensity of radiation.

2. Distance between earth and sun

3. The movement of seasonal winds which contributes to rain and precipitation.

Effect of temperature on Plant growth / Crop Productivity

Air temperature is the most important weather parameter, which affects the plant

life. The growth of higher plants is restricted to a temperature between 0 to 600C and the

optimum i.e., 100C to 400C. Beyond these limits, plants are damaged severely and even

get killed. The maximum production of dry matter occurs when the temperature ranges

from 20 and 300C.

As already seen the temperature of a place is largely determined by latitude and altitude.

Based on the above the vegetations are classified as tropical (rain forest, desert,

grassland), temperature (Grassland, deciduous forest), taiga ( coniferous forest), tundra

(lowshrubly growth, lichen) and polar. Some investigators have classified the vegetation

of the world into four classes based on the prevailing temperature conditions. The four

classes are

46

1. Mega herms- Equatorial and tropical rain forests

2. Mesotherms- tropical and sub tropical, tropical deciduous forests

3. Microtherms – temperate and high altitude, alpine vegetation and mixed

coniferous forests and

4. Hekistotherms - artic and alpine regions

High night temperature favors growth of shoots and leaves and it also affects

plant metabolism. On the other hand low night temperature injure the plants. Tender

leaves and flowers are very sensitive to low temperature and frost.

Temperature is of paramount importance life because of the following factors: -

1. Temperature governs the physical and chemical processes within the plants,

which in turn control biological reactions, that take place within the plants.

2. The diffusion rate of gases and liquids change with temperature.

3. Solubility of different substances is depending upon temperature.

4. The rate of reactions varies with variations in temperature.

5. Equilibrium of various systems and compounds is a function of temperature and

6. Temperature affects the stability of the enzyme system.

Every plant has its own minimum, optimum and maximum temperature limits for

its normal growth and reproduction. The vital physiological activities of a plant stop

both at below the minimum level ad at above the maximum level, whereas

physiological activities will be at its maximum at optimum temperature levels. These

levels of temperature are known as cardinal temperature points.

Cardinal temperature for the germination of some important crops (Bierhyzen, 1973)

47

S.No Plant Cardinal Temperature 0C

Minimum Optimum Maximum

1 Rice 10-12 30-32 36-38

2 Sorghum 8-10 32-35 40

3 Maize 8-10 32-35 40-44

4 Wheat 3-4.5 25 30-32

5 Barley 3-4.5 20 38-40

6 Sugar beat 4-5 25 28-30

7 Tobacco 13-14 28 35

8 Carrot 4-5 8 25

9 Peas 12 32-34 40

10 Oats 4-5 25 28-30

11 Lentil 4-5 30 36

In General

Cool season crops 0-15 25-31 31-37

Hot season crops 15-18 31-37 44-50

Apart from yield reductions, many visible injuries on the plants are seen due to

very high temperature.

Cold injury: (Low Air Temperature and Plant Injury)

1.Chilling injury: Plants, which are adapted to hot climate, exposed to low temperature

for sometime, are found to be severely injured. Some effects of chilling are development

of chlorotic condition (Yellowing)

Example: Chlorotic bands in the leaves of sugarcane, sorghum and maize in winter

months when the night temperature is below 200C.

Based on the reaction to chillness, plants can be divided into five categories.

i) Plants killed by exposure to temperature in the range of 0.5 to 5.0 0Cfor 60

hours. Rice, cotton, cowpea.

ii) Plants injured by the above condition but recovered after being placed in

favorable conditions-Sudan grass, Spanish and Valencia peanut

iii) Plant not likely to suffer serious injury-Corn, sorghum and pumpkin.

48

iv) Plants injured by prolonged chillness-Buck wheat and soybean.

v) Plants not injured by prolonged chillness-Sunflower, tomato and Potato.

In temperate climate two types of injures occur because of low temperature. They

are delayed growth and sterility.

Example: In Japan, rice yields decreases due to insufficient grain maturation

caused by low temperatures during the ripening period. Flowering is delayed by low

temperatures at a certain stage before heading.

Rice yield decreases due to sterility of spike lets of caused by low temperature at

the booting stage or at an thesis. The observed injuries may be stoppage of anther

development, Pollen unrippeness, Partial or no dehiscence, Pollen grains remaining in

anther loculi, Little or no shedding of Pollen grains on stigma and Failure of

germination of pollen on stigma

2. Freezing injury: Plant parts or entire plant may be killed or damaged beyond

repair as a result of actual freezing of tissues. Ice crystals are formed first in the

intercellular spaces and then within the cells. Ice, within the cells, causes more

injury by mechanical damage on the structure of the protoplasm and plasma

membrane.

Freezing of water in intercellular spaces results in withdrawal of water from the

cell sap due to dehydration and causes death of cells. E.g., Frost damage in

potato, tea, etc.,

3. Suffocation: In temperate regions, usually during the winter season, the ice or snow

forms a thick cover on the soil surface. As a result the entry of O2 is prevented and

plants suffer for want of O2. Ice coming in contact with the roots prevents the diffusion

ofDCO2 outside the root zone. This prevents the respiratory activities of roots leading to

accumulation of harmful substances.

4.Heaving: This is a kind of injury caused by lifting up of the plants along with soil

from its normal position. This type of injury is common in temperate regions. The

presence of ice crystals increases the volume of soil. This causes mechanical lifting of

the soil.

49

Effect of high Temperatures

Cells of most plant species get killed when the temperature ranges from 50 to 60

C. This point of temperature is called Thermal death point.

But it varies with

1.the species

2.the age of tissue and

3.the time of exposure to high temperature

It is reported that most plant cells are killed at a temperature of 45 to 55 C.

Some plants tissues withstand a temperature of up to 105C.

The aquatic plants and shade loving plants are killed at comparatively, lower

temperature (40C); where as, for xerophytes it is 50C.

High temperature results in desiccation of the plants and disturbs the balance

between photosynthesis and respiration. Higher temperature increases the respiration

leading to rapid depletion of reserve food in plants resulting in growth stunted due to

incipient or starvation.

Heat injuries:

i. Sun clad: Injury caused by high temperature on the sides of bark is known as sun

clad, this is nothing but exposure of barks of the stems to high temperature during

daytime and low temperature during nighttime.

ii. Stem girdle: It is another injury associated with high temperature. High

temperature at the soil surface scorches the stems at ground level. This type of

injury is very common in young seedlings of cotton in sandy soil where the after

noon soil temperature exceeds 60C to 65C. The stem girdle injury is first

noticed through a discolored band a few millimeters wide. This is followed by

shrinkage of the tissues, which have been discoloured. The stem girdle causes the

death of the plant by destroying the conductive and cambial tissues or by the

establishment of pathogens in the injury. As direct effects on crop plants high

temperature causes sterility in flowers. The general effects of excessive heat are

defoliation, pre-mature dropping of fruits. In extreme cases, death of the plants

may also occur.

50

Temperature aberrations

Heat Wave: A region is considered to be in the grip of moderate heat wave when it

recorded maximum temperature exceeds the normal by 5 to 8C. Heat wave is common

in UP (54%Probablity) in the month of June. Incidences are maximum in western UP.

Persistence is 5-6 days particularly more in June.

Effect of Heat wave: Already dealt in effect of temperature crop growth. Thermal death

point affects photosynthesis and respiration. Increased respiration depletion of reserve

food, sun clad, stem girdle.

Soil Temperature: In many cases soil temperature is more important to plant life than air

temperature. It influences the germination of seeds and root activities. It influences the

soil-borne diseases like seeding blight, root rot etc. The decomposition of organic matter

will be higher in higher soil temperature with necessary moisture. It controls the nutrient

availability. In tropics high temperature of soil causes regeneration of potato tubers. It

also affects nodulation in legumes.

Cold Waves: A region is said to be in the grip of a moderate cold wave when it recorded

minimum temperature falls short of the normal by 6C to 8 C and severe cold wave is

prevailed when the minimum temperature short falls up to 8C. Generally experienced

from Nov. to March.

Severe clod wave generally prevail form Jan. to March common in

U.P.WesternU.P-1day, Eastern UP-2-7days.

Degree-days: At a given location, the period between planting and harvesting is not a

specific number of calendar days but rather a summation of energy units, which may be

represented as degree-days.

A degree-day for a given crop is defined as a day on which the mean daily

temp is one degree above the zero temps. (That is the minimum temp. for

growth) of the plant.

Zero temp: Spring wheat 32-400F (depending on variety)

Oat: 430F; Field Corn: 54-570F Sweet Corn: 500F,

Potatoes: 450F, peas: 400F cotton 620F-640F

51

The period required for achieving maturity is also a function of the length of day

is photoperiod. Crop, planted in early in the spring requires more calendar days to

mature than the same crop planted later.

Growing Degree days (GDD): GDD or accumulated day degrees is also called as

Effective Heat Unit. This is an arithmetic accumulation of daily mean temperature

above certain threshold temperature. This is computed by using the formula suggested by

IWATA (1984).

Maximum temperature + Minimum temperature

Gdd = ---------------------------------------------------------- --- base temperature

2

52

(Chapter –7)

SOIL TEMPERATURE

The surface of the soil is exposed to the direct radiation and movement. It gains

heat during the daytime and losses some arts during the night to the atmosphere.

Diurnal variation of soil temperature

- As depth of soil increases – temperature increases up to 20 cm/ remains

unchanged beyond 30 cm.

- Surface temperature is doubled in the afternoon compared to morning due to

insolation.

- Variation but morning and afternoon temperature beyond 30cm depth is

negligible.

- Variations beyond 30 cm is only seasonal changes / variation.

Effect on crop growth

In many instances soil temperatures is of greater importance to plant life than air

temperature. For example, beach and oak trees can withstand air temperature of –250C

but roots of these trees cannot tolerate even up to –160C. It influences the soil borne

disease like seedling blight, root rot etc and decomposition of organic matter. Input =

storage + output. Storage causes changes below surface. Conduction of heat, to lower

layer depends on thermal properties of the soil like, specific heat, thermal conductivity

and thermal diffusivity of the soil. It influences the germination of seeds and root

activities. It also influences the soil borne, diseases like seedling blight, root rot etc., and

decomposition of organic matter. Greater the soil temperature higher will be the

decomposition of organic matter. It controls the nutrient availability. In the tropics high

temperature of soil causes degeneration of potato tubers. It affects nodulation in

legumes.

53

Cardinal Temperature – Temperature of vital activities

Cool season crops Hot season crops

Minimum 0-50C 15-180C

Optimum 25-310C 31-370C

Maximum 31-370C 44-500C

54

(Chapter –8)

ATMOSPHERIC PRESSURE

Pressure is defied as the force acting over any surface. Atmospheric pressure 8is

the weight of the air, which lies vertically above a unit area, centered at a point and

expressed by the height of mercury in ‘inches’ or ‘millimeters’. Pressure mainly affects

temperature and precipitation. The weight of the air presses down the earth with the

pressure of1.034 g/cm2 . The weight of air mass is over 56 trillion tons. (56x1014ton).

Weight of 1sq. Inch column of air from sea level to top of the atmosphere weighs

nearly 15 1b. This weight is balanced by column of mercury 29.93 inches or 760 mm tall

having the same cross sectional area. This is the pressure at sea level at latitude 450 .

Another unit of measurement millibar is widely adopted by national weather service of

the world. (Millibar = 1000 dynes / cm2). Dyne is an unit of force approximately equal

to the weight of a milligram. Sea level pressure under this system is 1013.2 m.bars (mb).

One tenth of an inch of mercury is approximately equal to 3.4 mb.

UNITS OF MEASUREMENT:

Upto the year 1914 the unit of measurement of pressure was in inches or in m.m.

At sea level the atmospheric pressure is 30” or 76” cm or 760 mm. At a temperature of

2730A.

In the year 1914 a scientist by name Bjehkres derived a new unit called the

“millibar” (mb). Normal pressure at sea level is roughly 300 or 760 mm. Which

corresponds to 1013 MB.

The conversion from units of length to unit of pressure is as follows.

Suppose the Hg column at M.S.L. is 76cm it is then multiplied by the density of

mercury (13.595) and mass of Hg column is found out.

76 x 13.595 = 1033.22 gm.

The acceleration of gravity (normal) in CGS units is 980.665. Multiplying the

mass by gravitational force i.e. 1033.22 x 980.665 we obtain the pressure in CGS units

55

(centimeter gram second) is 10,13,250 dynes/sq.cm. For convenient sake it is taken as

10,00,000 dynes called a ‘bar”.

One thousandth of a bar is called a “millibar”.

A millibar = 1000 dynes/cm2

Approximately 760 mm = 1000 m.b.

or 1 mb = 0.76 mm.

33 m.b. = 0,76 x 33

= 25.08 mm or

= 196.

The observed pressure is reduced to 320F or 00C at M.S.L. at 450 Latitude as the

standard to facilitate the comparison of pressure of different stations.

FACTORS AFFECTING THE ATMOSPHERIC AIR PRESSURE:

Pressure varies according to latitude measures to sea etc., the variation is used in the

determination of

1. Height of places as hills

2. Use in airplanes by mean Altimeter.

Before the start the altimeter is set to a definite reading. The factors affecting

atmospheric pressure are 1.Altitude 2. Latitude 3. Temperature 4. Nearness to the sea.

1.ALTITUDE:

It is the relative height of place above M.S.L. The pressure decreases for every

increases of the altitude. For every 900’ 25 mm or 33 mb pressure is decreased.

2.LATITUDE:

When the latitude increases the pressure will increase.

TEMPERATURE:

When the temperature increases the pressure will decrease.

NEARNESS TO THE SEA:

Places near to the sea are offer subjected to cyclones due to low pressure.

***

56

ISOBARS

The isobars are the lines connecting different places of same pressure one chart or map of

country of world. The lines can be drawn after reducing the readings to M.S.L. Such

lines or curves arte called ‘Isobars’. These lines are drawn every 5th of a millibar.

ISALLOBAR:

The change in atmospheric pressure during 3 hours preceding the observation is

called “barometric tendency”. When the tendencies have been plotted on the map the

lines connecting the points are called “Isallobars” they represent the pressure changes as

that of isobars but are drawn for each millibar.

Usually the tropical regions ate low-pressure belt due to high temperature in and

around the equatorial line. The temperature regions ate high-pressure belt (areas).

Beyond temperate belt, the pressure diminishes regularly in south but irregularly in

North. (Alaska and Ice lands have high pressure).

SIGNIFICANCE OF PRESSURE:

The cyclones are formed by the pressure. Whenever the atmospheric pressure of

a place drops from the normal conditions, depression occurs and cyclone may be formed.

The barometer reading is the best indication of the possible occurrence of cyclone

or storm as well as rain in area.

EFFECTS OF TEMPERATURE ON PRESSURE:

A column of air 1 sq. inch in cross sectional area extending from sea level to the

top of atmosphere weight approximately 14.7 1bs. This weight is balanced by a column

of mercury 29.92” or 760 mm tall having the same cross sectional area at an latitude of

450 at sea level. The density of weight of given volume of air vary with temperature.

Thus when air is heated, it expands and becomes less dense, so that column of warm,

light air weighed less than a column of cold, heavy air both having the same height and

cross sectional area.

Changes in temperature produces changes in air density which set up vertical and

horizontal movement region air is heated it expands and over flows aloft to adjacent

region when temperature are lower. As a result of this horizontal transfer, the weight of

the air is reduced in the warm region and increased in the adjacent cooler regions. Hence

region with high temperature are likely to have lower air pressures than other regions

57

where tempt. is not so high. In other words. High temperature tends to produce low sea

level pressure while low temperature isconclusive to high sea level pressure

Fig: (damed laws)

- - - - - - - - - Surface & equal pressure

- - - - - - - - - - - - - -

- - - - - - - - - - - - - - -

- - - - - - - - - - - - - -

- - - - - - - - - - - - - - - -

high - - - - - - - - - - - - - - -low

Pressure / / / / / / / / / / / / / pressure

There is a rapid decrease in air weight or pressure with increasing altitude. The

lower layers of atmosphere are densest because the weight of all layers above which rests

up on them. For the first few thousand feet above the sea level the rate of pressure

decrease, is in neighborhood of 1”or 34 mb of pressure for each 900 to 1000’.

Types of pressure systems of the world

Pressure system differs greatly in both size and duration. Pressure System are of two

types

i. High

ii. Low pressure system

Centers of low pressure are called as depression, cyclones or lows. Prolonged

low pressure, centers are called troughs. The equatorial belt of low pressure is called

doldrums (50 N & 50 S of Equator). It is because

i) Sun falling vertically all round the year

ii) Water vaporization being high

iii) Rising of air

The doldrums belt is spread over Amazon, Congo, Passion and Guinea belt etc.,

The centers if high pressures are called anticyclones or highs. An elongated high

pressure is called as ridge. Near 300N and 300S the pressure is always high because

58

i) intensive hot air from the equator descends down in this belt and ii) polar air from

the sub-polar belts also descents here.

Factors controlling pressure: pressure never remains constant – changes with temp.,

altitude water vapor content and rotations of the earth.

i) Temperature: Hot air expands and exerts low pressure. Cold air contracts

and exerts high pressure the equator has a low pressure due to prevalence of

high temperature but poles have a high pressure due to prevalence of high

temperature but poles have a high pressure.

ii) Altitude: At sea level, the air column exerts its full pressure, but when we go

up, we leave a portion of the air, which cannot exert pressure. At sea level

high pressure and at high altitude low pressure. For every 10 m of altitude,

the pressure gets reduced by 1 mb.

iii) Water vapour: Moist air of high temperature exerts less pressure. When

compared to moist air of low temperature. Because water vapour content is

lighter in cold area than air, which is dry.

iv) Rotation of the earth: Due to rotation of earth the pressure at 60 – 650N and

S becomes low. The airs to escape from these belts which move towards the

horse latitude (30 – 350 N&S) these belts absorb air from the sub-polar belts

making the pressure high.

v) Seasonal variation: Pressure system changes according to the season.

Season changes according to the position of the sun. When the sun mo9ves to

the tropic of cancer, pressure belts move to the North by 50 away from their

normal. When sun moves to tropic of Capricorn, the pressure belt also moves

south and sight by 50 away from their original position. This is known as

“Swing of pressure belts”.

Sea breeze and land breeze due to seasons

During summer horse latitudes receive the direct sunrays and an area of low

pressure increases over the continent masses and they enlarge a small high-pressure

center over the continents. But surrounding seas have a vast high-pressure area in

summer the wind blows from sea (high pressure) towards the lands. (Low pressure)

59

In winter season, a major area of high pressure covers the landmasses. The sea

areas are comparatively at low pressure. So winds start moving form the land

towards the sea.

Diurnal variations

To find out the mean daily change in air pressure, the average of hourly-observed

pressure for a long period of time is calculated. The mean value of the daily pressure

is free from the temporary effect of atmospheric disturbances. There is a definite

rhythm in the rise and fall of mercury. Insolational heating and radiational cooling

are the principal reasons for diurnal variations of air pressure. In other words,

pressure changes are mainly dir to the expansion and contraction of the air.

Seasonal or annual variation

This is clearly the effect of annual variation in the amount of insolation received

in a particular region. Annual pressure variation in the tropical region in larger than other

region of the world. The equatorial regions record the smallest amount of variation in

their seasonal pressures, because there is practically no variation in the amount of

insolation received at the equator throughout the year

High pressure - cold season

Low pressure - warm season

Isobars: these are lines connecting places having the same atmospheric pressure at a

given elevation. Pressure distributive charts are constructed for sea level and for no of

constant pressure surfaces in the atmosphere.

700 mb – at 10,000 ft.

500 mb – at 18,000 ft.

In sea level pressure chart all pressures at different elevation arte reduced to pressure

receiving to sea level.

Pressure gradient: rapid change in pressure in a direction at right angles to the isobars.

The rate if change in atmospheric pressure between two points at the same elevation is

called the pressure gradient of isobaric slope. It is proportional to the difference in

pressure, which causes the horizontal movement air.

60

Storm: A marked atmosphere disturbance characterized by a strong wind, usually

accompanied by rain, snow, sleet (rain that freezes as it falls-mixture of rain with snow or

hail) or hail and often thunder and lighting.

Thunder Strom: A storm invariably produced by a cumulonimbus cloud and always

accompanied by thunder, usually attended by strong wind, gusts, heavy rain and

sometimes hails. It is usually of short duration, seldom over 2 hour.

- Vertical motion is having many weather modifications.

- Upward motion results due to expansion that it gets cooled and eventual

condensation.

- Cumulonimbus cloud types are closely related to the strength of the vertical

motion.

- A thunderstorm is as the name implies a storm accompanied by thunder and

therefore lightning. As Benjamin Franklin demonstrated in 1750 lightn9ng

discharges giant electrical sprakes.

- Cumulonimbus clouds therefore are great electrical generators. The cloud

produce ‘+’ and ‘-‘ value charges by charged poles.

- The lower part of the cloud is negatively charged and upper part is positively

charged.

Hall: Precipitation in the form of balls or irregular lumps of ice.

Hail Strom: Small round pieces of ice hail that sometimes fall during thunder storms

(frozen rain drops, hail storms).

- Hails may be sometimes greater in size than a large marble.

- It falls from cumulonimbus clouds.

- Hails are destructive to crops – mechanical damage, structures etc.

Hurricane: A violent tropical cyclone with wind speed of 73 or more miles per hour or

134 and more km/h usually accompanied by torrential (very heavy fall) rain, originating

usually in West Indian regions.

Tornado: Tornado – Spanish word – Torn as means, “to turn”

- The smallest vortex (whirlpool, whirl or powerful eddy of air, whirl wind – a

whirling mass of water forming a vacuum at its center, into which anything

caught in the motion are drawn).

61

- Eddy - current of air, water, etc., moving against the main current and worth

circular motion.

- But most powerful one.

- The intense rotation is confined normally to diameter of kilometer or less.

- But its wind speed can reach even 300 km/h.

Water spouts

- The tornado occasionally forms over water.

- Because of high moisture content of the sir, the funnels are heavily laddened with

water drops, so they look somewhat like a stream of waterspouts.

Dust Devil: A whirlwind that frequently forms on very hot days especially over desert is

the dust devil. Normally there ate no clouds associated with it.

Cyclone (A system of winds blowing around the center of low barometric pressure)

means closed circulation about a low-pressure center, which is anti clockwise in the

Northern Hemisphere.

- Cyclonic whirls are the “Storms” of middle latitude.

- In the temperate latitude they produce much of the winter precipitation.

- Around the low-pressure centers.

- Air circulates anti clockwise direction in Northern Hemisphere.

- The air is heterogeneous in relation to temperature and moisture.

Anticyclone: (The atmospheric pressure distribution in which there is a high central

pressure relative to the surroundings) Circulation clockwise in northern hemisphere and

anti clockwise in Southern hemisphere.

- This circulation subside whirling @ 10-15 cm/sec. And fair weather generally

Prevail.

- The air masses are homogenous with respect to temperature and moisture.

Typhoon: Any violent tropical cyclone originating in the western pacific especially in

the South China Sea

Plant growth: Resultant of all the environmental factors-climatic, physiographic,

edaphic and biotic.

For a particular field - it is primarily a function of climate with temperature and

height – being the most important factors.

62

- Close relati9onship exists between plant phenology and both latitude and altitude.

63

(Chapter –9)

Wind

Wind is defined as the moving air of atmosphere parallel to earth’s surface air in

horizontal motation. All other masses of air in motion (vertical) should be called as air

currents. Wind is an invisible weather element but the effect of wind can be Sean from

the movement of tree branches, dust particles and by feeling. The pattern and intensity of

wind is affected by various factors.

Advantages of wind:

1. Fresh wind is useful for renewing the environment.

2. Wind is useful for effecting pollination in the crops.

3. It is useful for cleaning for agricultural produces.

4. It is used as a force in certain machines such as windmills, winnowing

machines etc.

Effect of Wind on crops

1. Increases transpiration under normal condition with increasing wind velocity.

Layers of humid air adjacent to plant leaf surfaces are removed by wind and

become mixed with dry air above. This keeps RH low and increases transpiration

rate. There is a greater increase in cuticular transpiration than stomatal

transpiration witch cause moisture stress in plants.

2. Wind increases the rate if Photosynthesis. Wind increases turbulence in

atmosphere thus raising the supply of Co2 to the plants and thereby increasing the

rate of photosynthesis. However, the increase is only up to a certain wind speed.

3. When the wind is hot it accelerates the drying of the plants by replacing humid air

by dry air in the intercellular spaces. At the time of cell expansion, the hot dry

wind affects the maturing cell and that result in dwarfing of plants.

64

4. Much damage is caused by hot dry winds at or near the time of flowering. The

internal water balance is upset, resulting in poor seed setting. Another form of

injury is “blossom injury” caused by evaporation of secretions in the stigma.

5. Interfere pollination by insects. But mild wind will favor pollination by wine.

6. Deplete soil moisture.

7. Due to mechanical effect of wind the growth pattern and shape of trees ate

changed lopsidal growth.

8. Uprooting of plants: Crops and trees with shallow roots are uprooted.

9. Cause fruit drops in plants. Example-citrus fruit drop. Fruits and nuts are

stripped off from trees.

10. Soil erosion: When the plant cover is not thick, strong winds remove the dry soil

exposing their roots and killing them. The eroded material from one place is

deposited in another place causing hazard to small plants in that place. The

deposited materials reduce the aeration around the roots and plants.

11. Salt deposition by wind: Wind from sea carries salts as spray on coastal area and

makes it impossible to grow crops which are sensitive to excess salts.

Disadvantages of wind:

1. High-speed wind accelerates the drying of moisture from the soil and also it

increases the rate of transpiration in plants thereby necessitating frequent

irrigation.

2. High-speed wind results in lodjing of many crops such as Banana, Sugarcane

and other fruit trees.

3. Heavy wind will affect the fruit set and also the available fruits to fall or to be

withered.

4. Heavy wind also results in soil erosion.

Causes for the formation of the wind:

1. Due to variation in the atmospheric temperature, pressure etc., i.e. when the

atmospheric temperature is very high the pressure will decrease

correspondingly. Due to fall of the atmospheric pressure the air moves from

high-pressure area to low-pressure area.

2. Due to deflection of atmosphere air over the earth surface while it revolves.

65

Wind force

The following are the wind forces and they are the factors affecting the wind

motion.

1.Pressure force:

The forces that move the air depend primarily at the distribution of pressure. Let

us consider a vertical cross section through a cube of air with horizontal and vertical

faces.

Since the atmospheric pressure decreases with elevation the pressure “P1” on the

lower face of the cube is greater a force that of ‘P2’ on the top face. This force is

counteracted by the weight of air with in the cube or the gravity force. Usually there is

balance between the two forces so that no vertical motion results. Rarely there will be in

balance and vertical acceleration results and convective currents are created.

Large wind systems are mainly horizontal currents. The pressure also varies in

the horizontal direction and the pressure on the vertical force will exceed the other force

and the difference in pressure is equivalent to a force to drive the cube horizontally from

high to low pressure.

2. Pressure gradient force and Isobars:

Suppose when we observe the atmospheric pressure in large number of places in a

horizontal surface and plot the pressures on a map and draw curves through the points

66

that have identical pressure gradient may be defined as the decrease in pressure/unit

distance in the direction in which the pressure decreases most rapidly.

The rate and direction of change of pressure as indicated by isobaric lines is

refereed as pressure gradient or barometric slopes. Two important factors that exist

between pressure gradient and winds are:

1. The direction of airflows is from regions of greater to those of less density

i.e. from high to low pressure, which may be represented by a line drawn

at night at night angles to the isobars.

995 millibar

Pressure 1000 millibar

Gradient

Wind 1005 millibar

1010 millibar

High

The pressure gradient is:

1. Is every where perpendicular to the isobars

2. Points from high to low values of pressure

3. And is inversely proportional to the distance between the isobars, the more

crowed the isobars the stronger is the pressure gradient.

3.Horizontal deflection force due to earth rotation:

Surface winds do not flow directly down the barometric slope (right angles to the

isobars) but instead are deflected into oblique courses. Thus a west wind in the

northern hemisphere becomes northwestern wind. The cause for the deviation of

wind from the gradient direction is the deflective force of the earth’s rotation plus

friction. This causes all winds to be turned to the right in the northwest to the left to

the S.H. (Ballot’s Law). This deflective force is called the “Coriolis” force. It is a

resultant effect of the two motions.

67

1. Rotational movement of the earth.

2. The movement of the body relative to the surface of the earth.

This deflective force of the earth is minimum near the equator and it increases with

latitude and is maximum near the equator and it increases with latitude and is maximum

at the poles. Therefore air moves rather directly across the isobars in low latitudes and is

greatly deflected in the Polar Regions. This deflective force also increases with the wind

velocity. The Coriolis force is directly proportional to the moving mass of air and its

velocity. It acts at right angles to the direction of the motion and has no influence on

influence on the velocity of the wind.

The broken arrows shows the direction of the pressure gradient and the solid

arrows shows the direction of wind due to ‘Corolis’ force. Friction it’s next factor, which

affects the wind motion. It modifies the effects of gravity and deflection.

68

Friction prevents the winds from attaining velocities and also from

blowing parallel with the isobars.

CENTRIFUGAL FORCE:

The amount of deflection due to this force is dependent on the velocity of the

wind. More the velocity greater will be the outward force and hence greater will be the

deflection produced. Therefore in the northern hemisphere the rotational deviation is to

the right and therefore the centrifugal force will enhance this deflection. This force is

negligible near the surface of the wind is low. If the path of the wind is curvilinear than it

will be subjected to centrifugal force.

PRESSURE BELTS:

These are the regions of the high and low pressure formed on the earth as a result of

1. The differences in the rate of insolation

2. Differences in the rate of absorption of heat by water and the different types of

earths surfaces and

3. The rotation of the earth.

There are two types of pressure belts namely High and Low pressure belts.

* Earth where the velocity of

LOW PRESSURE BELTS:

The areas of low pressure are usually called as depressions, cyclones or simply

low. A depression or cyclone may then be defined as an area within which the pressure is

low relative to the surroundings and the wind circulation around a cyclone is counter

clockwise with a slight drift towards the left of the isobars.

The equatorial strip and the polar zones are low-pressure belts. As a result of

intense heat at the equator, the air rise to the upper layers, producing a belt of low density

and pressure of air and the lower layers near the surface of the earth called the doldrums.

The air in the Polar Regions is swung to the temperate regions by the rotation of the

earth. The atmosphere above the Polar Regions is of low density and pressure and these

are called “Polar calms”.

HIGH PRESSURE BELTS:

69

The areas of high pressure relative to the surroundings are called high-pressure belts or

anticyclone. The wind circulation is clockwise around an anticyclone with a drift away

from the center.

Air currents at the upper layers from both the equator and the poles meet at

latitudes 300 to 350 N and S called the horse latitudes and produce a belt of high pressure.

From these horse latitudes, winds blow towards the equator and the poles. These should

take northerly and southerly courses but are deflected by the rotation of the earth. Thus

in the northern hemisphere N.E. wind blows towards the equator and S.W. winds towards

the poles.

In the southern hemisphere S.E. blows towards the equator and N.W. winds

towards the poles.

Tropes of pressure systems

A trough of low pressure is an elongated area of relatively low pressure, which

extends from the center of a cyclone. The trough may have ‘U’ shaped ‘V’ shaped

isobars. The wind circulation around a trough is essentially of the cyclone type. A

wedge of high pressure is an elongated area of high pressure that extends from the center

of an anticyclone, and the wind circulation is anticyclonic.

A col is the saddle-backed region between two anticyclones and two cyclones.

Model of pressure distribution and prevailing winds at the

Surface uniform earth

70

Around the equator there is a region of almost uniform pressure in which the

winds are light and variable and this belt is called the “doldrums”. The winds converge

from the both the hemisphere into the doldrums. This convergence results in ascending

air currents, adiabatic cooling, condensation and precipitation. The doldrums are

therefore characterized by frequent showers, thunderstorms and heavy rainfall. Further

away, from the equator are belt of high pressure with easterly winds on their equatorial

sides and westerly winds on the pole ward side. These belts of high pressure are called

the subtropical anticyclones. The winds on this equatorial side are called the subtropical

anticyclones. The winds on this equatorial side are called Trade winds. They blow

mainly from the east and have a component towards the equator; on the pole ward side

the winds have a pole ward component. The subtropical anticyclones are regions of

descending air currents, low R.H. almost clear sky and deficit of rainfall most deserts are

found in the region.

In the central portion of the subtropical anticyclone the winds are light and

referred to by seamen as the “Horse Latitude”. The wind on the pole ward side of the

high pressure are called prevailing westerlies. They increase in strength as the latitude

increases.

WIND SYSTEMS

There are three types of wind systems namely:

1. Primary wind system

2. Secondary wind system and

3. Special a type wind system. The primary and secondary wind systems consist of

Trade winds and monsoons respectively and special type consists of land and sea

breezes.

71

1.Trade winds

Trade winds are the winds of primary wind systems that blow from subtropical

centers towards the equatorial side low between 30 and 350 and the winds on the

equatorial side are called “Trade winds”. They are the most regular winds. Their

steadiness has earned their name trade winds. They blow with greater strength and

constantly in winter than in summer. They are regular and steady over the oceans. They

blow away from the landmasses over continents.

When the equatorial region gets heated, the air sizes from the surface and passes

to the upper layers. The pressure of the atmosphere near the surface decreases in due

coarse. Air moves towards this low-pressure area from both north and south and this

phenomenon continues right through the year.

Winds Lower Layer

Winds in upper Layer

The resulting winds takes the same course or track and is hence called “Trade

winds”. As the hot air arise to the upper layers over the equator, the pressure is raised

there in due coarse and the surplus air moves northwards and southwards in the lower

layer. The movement is towards the equator form the north and south in the lower layers

and from the equator towards north and south in the upper layers. The latter are called

“antitrade” winds.

Relationship of wind and pressure

- Earth rotates from west to east along with atmosphere. Atmosphere is fixed to

earth by gravitational equilibrium.

- Wind therefore moves in addition to rotation.

- Horizontal motion is greater than vertical motion.

72

- Wind takes several days to cross the ocean but up and down movement is only in

few minutes.

73

Seasonal and Local winds

The monsoons are the most important among seasonal winds. In this system, the

direction of the winds changes seasonally. They are experienced over parts of North

America and much of South Asia, including the Indian subcontinent. These winds are

primarily a result of differential heating of land and sea. In summer, southern Asia

develops a low pressure and airflows landwards from the Indian Ocean. This is known as

the summer monsoon. In winter, the pressure over land is higher than over the sea and

consequently the air starts flowing from land to sea. This is called the winter monsoon.

The modern theories consider theories the monsoon a result of the shift in the pressure

and wind belts.

According to the dynamic theory, monsoons are a result of the pole ward shift of

the Inter Tropical Convergence (ITC) under the influence of the vertical sun during the

summer season. During summer in the northern hemisphere, in the months of May and

June the sun shines vertically over the Tropic of Cancer and the ITC shifts north of the

equator. The ITC is the convergence zone of the trade winds bowing from northeast in

the northern hemisphere and from the southeast in the southern hemisphere. As ITC

Shilts northern of the equator, the southeast tread winds start blowing north of the equator

to reach the ITC, and as they cross the equator, their direction is altered due to the

influence of the coriollis force, i.e., they are deflected towards their right and thus it gives

rise to the formation of a belt of equatorial westerilies blowing between the equator and

the ITC. These westerlies in the months of May and June blow from the equator towards

the ITC from the southwest to the northeast and they are called the southwest monsoon.

During the winter season the ITC again moves southwards and the areas north of

the equator, which experienced the equatorial westerlies during the summer season, now

come under the influence of the northeast trade winds. These northeasterly winds are

caked the northeast monsoons.

74

During this very season the ITC shifts south of the equator and the northeast

trades blowing towards the ITC, get deflected upon crossing he equator southward.

Here they give rise to the equatorial westerlies blowing from the northwest to the

southeast, replacing the trade winds of the southern hemisphere between the ITC and the

equator. Thus the areas situated in the tropical zone come under the influence of the

trade winds during the respective winter and the equatorial westerlies during the

respective summer season. Thus the direction of the winds is reversed seasonally and it

makes up the monsoon system of these regions.

In certain regions, local winds are generated as a result of the influence of the

local terrain. One example of this is the simple system of land and sea breeze

experienced in coastal areas. Due to differential heating, the air moves from sea to land

during the day and from land to sea at night. Mountain and valley winds also follow

daily alternation of direction. During the day air moves up along the valley slopes, as

the slopes are very hot. When the slopes cool at night air moves valley wards.

THUNDER STROMS:

They are the local phenomena very prevalent in tropics during the post and pre

monsoon periods. Thunder stroms forms cumulonimbus type clouds in excessively

unstable air. The formation of thunder strom is as follows:

When the rain drops from larger tan 4 mm in diameter they will fall with a

velocity of exceeding 8 m/sec. And the drops break up into smaller drops, which fall less

rapidly.

If the ascending currents in the Cumulonimbus exceed 8 m/sec the large raindrops

will split up into smaller drops and will be carried upward. The ascending current in a

cumulonimbus is not steady, it consists of succession of gusts and lulls and the drops may

rise and fall grow and breakup repeatedly. Each time a drop breaks up into smaller

droops the – and the + electricity will be separated the air taking up –ve charge, drops

+ve charge. By repeat splitting up of drops, enormous electric charges are made

available for the thunderstorms. Since the air ascends much more rapidly than it drops

75

that breaks up, it follows that the positive charge is accumulated in the part of the cloud

where the ascending current is strongest and the rest of the cloud becomes negative or

neutral.

A fully developed thunder stroms is an accompanied by strong gusts, heavy rain

or hail (shells o clear ice) and lightning and thunder. The precipitation, which

commences as a sudden heavy down pour, changes into continuous rain and gradually

decreases.

The passage of a thunder strom is frequently accompanied by strong gusts, which

may cause complete loss of control of an air graft.

TORNADOES

A tornado is a circular whirl of great intensity and small horizontal extent, in

which the wind velocity is usually super hemi cane force. The horizontal diameter of the

tornado varies from a few feet up to a mile. The wind velocity sometimes exceeds 200

m.p.h. The pressure in the center of tornado is much lower than in the immediate

surroundings and this together with high winds, produces destructive effects. The air in

the center is rising rapidly and the whirl is accompanied by heavy rain or hail and thunder

and lighting.

The tornadoes are short lived, usually not lasting more than an hour or two.

Tornado frequently occurs in Mississippi valley. (Water spronts are called as tornadoes

formed at the sea).

***

CYCLONES

A cyclone may be defined as a region of low pressure surrounded by closed

isobars. It is also defined as an area within which the pressure is low relative to the

surroundings. The wind circulative around a cyclonic region is in anticlockwise

direction.

Prior to 1918, a cyclone was thought of merely as a region of low pressure usually

associated with bad weather. In 1918, J.Bjerkness found that a cyclone normally consists

of two air masses separated from one another by a front. A tongue of warm air, usually

form where the pressure is lowest. This tongue of warm air is called the “warm sector”.

76

The warm sector is surrounded by cold air of polar origin, the warm air of the tropical

region.

Thus the cyclone is built up of two air masses of widely different temperature and

life history. The front that runs through the cyclone was called the polar front. Along the

right hand side of the front, warm air replaces colder air, which is called “Warm Front”.

The cold air replaces warm air and called as “Cold Front”.

Formation Of A Cyclone

As a result of sudden fall in pressure, the place becomes the center of depression or the

low-pressure area. Because o low pressure a sort of in balance in wind system occurs.

As a result the wind from the surrounding high-pressure area begins to blow very fast

towards the depression. The velocity of the wind blow depends upon the intensity of

depression. The forcible wind blow from all the directions of surrounding high-pressure

area towards the low-pressure area till it get saturated with pressure. At the same time

heavy rain will also occur. The direction of the wind in a cyclone is from upward to

downward and in an anticlockwise direction. The wind velocity will be around from 100

to 250 mph. After the cyclone, the sky becomes clear, the wind velocity decreased and

the pressure becomes normal.

THE CYCLONE MODEL

Fig:

77

The middle portion of the diagram represents the cyclone. The lower portion

represents a vertical cross section running from west to east to the south of center and the

upper portion represents a similar cross section to the north of center.

In the lower portion of this diagram the cold air in advance of the warm front

forms a wedge under the warm air above. The warm air being lighter than pressure and

cools adiabatic ally and results in condensation of water vapor. This the cloud system in

the middle portion of the diagram rests on wedge of the cold air. At the warm front

surface the clouds in the high and thin (chirus and ciro stratus). As we go down the

slope, the clouds become denser (altostratus) and merge gradually into the rain cloud

(mimbo stratus). In the rear end of cold front the weldge of cold air pushes under the

warm air and a clouds system results upper portion. Warm air does not come to the

earth’s surface and present aloft.

DEVELOPMENT OF CYCLONE:

If the wave forming on a major air mass boundary is dynamically unstable, its

amplitude will increase as it travels along the front and the associated low -pressure

system will deepen.

A – Represents initial undisturbed condition of the front. Here a westerly current

of warm air next to easterly current of cold air and air streams parallel to the front and

hence stationary.

78

B – shows a developing frontal wave with a converging flow pattern and a small

precipitation area to the north of the warm front.

C- the wave development now has progressed (wave unstable) sufficient far and

called as “young wave cyclone”. The cyclone commences as a slight wave on a almost

stationary front, its amplitude increases and after12 to 24 hours. The wave has developed

to cyclone model. (Wave shorter than 100 km are stable and wave length between 100

km to 3000 km are unstable) only the unstable wave develops into cyclone.

D and E – indicate the occluded stage of cyclone. The amplitude continuous to

increase, the cold front dissolves and cyclone develops into a large whirl of more or less

homogeneous air. The greatest intensity, the minimum central pressure and highest

vertical extent are observed.

The cyclone attains a maximum intensity 12 to 24 hours after the occlusion

process has commenced. When the front begins to dissolve the energy supply decreases

and the strom feeds on the kinetic energy already created. This energy gradually

dissipated through friction and winds around the cyclone decrease.

Over takes the warm front and an occluded front results. As the occlusion process

continuous the occuluded front dissolves and

TROPICAL CYCLONES

They are small cyclonic whirls, having circular isobars and very strong winds

circulating in a counter clockwise direction in the northern hemisphere and in a clockwise

direction in the s9oythern hemisphere. The tropical cyclones are called “Cyclones” in

India, “Huricanes” in West Indies and “Typhons” in East Asia. They originate in

doldrums over the oceans between 60 and 200 N and 200 S, and travel in the direction of

trade winds. The wind is light and variable in center (the eye) of the tropical cyclone

around which thee is a whirl of hurricane winds and torrential rainfall often accompanied

by thunder stroms. The horizontal diameter of this cyclone varies from few miles up to

several hundred miles. The wind velocity exceeds 100 miles per hour.

79

ANTI CYCLONES

It is an area of high pressure surrounded by closed isobars. The winds blow

around the anticyclone in a clockwise direction in the northern hemisphere and in a

counter clockwise direction in southern hemisphere. In the center winds are light and

variable and are moderate in anticyclones except on the outers skirts. The wind has an

outward drift from the central part of the anticyclone and is compensated for by

descending air at higher levels the decending motion dissolves the high medium clouds.

The anticyclone is therefore of a region of stable and fair weather.

Cyclones and Anticyclones

Cyclones and anticyclones are two special pressure and wind systems. A cyclone

is a system of very low pressure in the center surrounded by increasing high pressure

outwards. In a cyclone, the winds blow in a circular manner in a clockwise direction in

the southern hemisphere and in an anticlockwise direction in the northern hemisphere. It

is believed that most cyclones in the temperate regions occur due to the coming close and

imperfect mixing of two masses of air of contrasting temperature and humidity

conditions. Cyclones of this type are also known as wave cyclones. On the other hand

cyclones in tropical areas result from the intense heating up of air in some regions

causing great loss to life and property in coastal areas. These tropical depressions are

known as cyclones in the Indian Ocean, Hurricanes in the West Indies, typhoons in the

China Sea and willy-willies in northwest Australia.

Anticyclones, which are the centers of high pressure, are the opposite of cyclones

in all respect.

Tornadoes are very strong tropical of a small size. They are specially feared in

some parts of southeastern United States. Sometimes, when they occur over sea, the

funnel-shaped cloud formed by the whirling motion of the wind descends to the surface

and draws up the water forming a column of water know as a ‘waterspout’.

80

The jet Stream

The jet stream is a system of upper-air westerlies. It givers rise to slowly moving

upper-air waves. In the upper-air waves are some narrow zones in which wind velocities

of up to 250 knots are observed in some air stream is believed to affect the onset and

retreat of monsoons in India. Jet streams develop over areas of steep pressure gradient.

Measurement of wind speed

In 1805 Admiral Francis Beaufort introduced a wind force (speed) scale, which

was based upon the response of certain objects to the wind. In applying Beaufort scale

the extent to which smoke is carried horizontally or to which trees bend before the wind

is used as an index of speed. At sea, the condition of waves, swell and spray in addition

to the response of sails and masts is the basis for wind speed estimates.

Table: The Beaufort scale of wind force with velocity equivalents

Beaufort

Number

Beaufort Descriptive

Term Land Criteria

Velocity,

miles/hour

0 Calm Calm, smoke rises vertically Less than 1

1 Light air Direction shown by smoke drift, not by wind vans 1 to 3

2 Light breeze Wind felt on face; leaves rustle; ordinary vane moved by

wind

4 to 7

3 Gentle breeze Leaves and small twigs in motion; wind extenda light

flag.

3 to 12

4 Moderate breeze Raises dust and loose paper, small, branches moved 13 to 18

5 Fresh breeze Small trees in leaf being to away. Created wavelets form

on inland waters

19 to 24

6 Strong breeze Large branches in motion; whistling in telegraph wires;

timbrellas used with difficulty

25 to 31

7 Moderate gale Whole trees in motion; some difficulty walking against

wind

32 to 38

81

8 Fresh gale Breaks twigs off trees; progress generally impeded 39 to 46

9 Strong gale Slight structural damage occurs (chimney pots and slate

removed)

47 to 54

10 Whole gale Trees uprooted; considerable structural damage occurs;

seldom experienced inland

55 to63

11 Strom Very rarely experienced; accompanied by widespread

damage

63 to 75

12 Hurricane Above 75

From Trewartha. An introduction to Climate. McGraw-Hill, N.Y., 1954.

In modern method wind vane and anemometers are used for measuring the direction and

wind speed.

MONSOON

Monsoon is defined as a periodic wind system occurring in many parts of the

world. This periodic wind system recurs every year in the same period. Such periodic

wind system is caused due to fluctuation or change in temperature and pressure over a

large area.

In winter when the land is cold and the surface pressures are high, an outflow of

air towards the ocean takes place that may reinforce or weaken currents set up by the

planetary atmospheric circulation. Similarly in summer the land is warm, surface

pressure are lowered, and a tendency for an inflow of air from the ocean to land takes

place. Again this gradient is superimposed upon the general circulation. These seasonal

land and sea breeze is called the ‘monsoons’. They affect all continents. The position

and intensity of the subtropical high-pressure cells on both hemispheres, hence dynamic

effects, are distinctly linked with purely thermal effects of continents and (oceans). The

Classical Indian and East Asiatic monsoon are composite effects of both the effects. In a

typical monsoon climate during the winter (land) monsoon, the prevailing wind is

offshore; precipitation, cloudiness and humidity show minima. During the summer (sea)

monsoon, the prevailing wind is onshore; precipitation, cloudiness and humidity show

maxima.

82

Our Indian subcontinent has got two well-defined monsoons.

They are

1.South West Monsoon –June – September

2.North East Monsoon – October – December

India is naturally well adapted with certain factors, which attribute for the

formation of monsoons.

They are: 1. The subcontinent is surrounded by the Terrestrial surface in the Northern

side and sea surface the southern side.2.The northern boundaries and western boundaries

are strongly guarded by hills and mountains which help to confine the monsoon wind

within our subcontinent.

During summer the continents of land area get hotter than the sea while the

conditions will be reverse in winter. Thus the continents behave as cold centers in winter

and hot centers in summer. The oceans behave as hot centers in summer. The wind

movements on account of these differences occur for some period in one direction and for

new period in the opposite direction, which are called monsoon winds.

I.SOUTH WEST MONSOON:

From March onwards the sun moves towards the north from the equatorial line to

the tropic of cancer heating up the continent while the Indian oceans in the south gets

cooled. Thus during March to June summer temperature rises and becomes a low-

pressure area (north India) and the Indian Ocean in the south India serves as a Center of

high pressure. The South East trade wind crosses the equator, when it is deflected by the

rotation of the earth and becomes South West Monsoon wind. It gets charged with

moisture when it passes over the Indian Ocean. Clouds are consequently formed, thunder

stroms develop and monsoon bursts into rain on reaching the west cost of India by the

end of May, June (beginning).

The South West Monsoon enters India both from Arabian sea and Bay of Bengal.

The Arabian Sea branch is more important for South India. It appears in the west cost in

the month of May, June and spreads northwards and northeastwards, precipitating rains

83

over a large part of Tamil Nadu from June to September. The west coast districts of

Malabar and south Canara get very heavy rains, and also the Nilgris. Regions on the

leeward side of the Western Ghats like Coimbator gets very light rains and these are

called rain shadow regions. The Bay of Bengal branch benefits the east coast and the

northern circa gets fairly heavy rain. The southern coastal district also gets fair rain.

West Bengal, Orrisa, Assam and Bihar receive a considerable rainfall.

In Sahyadri mountain on the west coast and the Himalayan ranges in the N.E, also

gets rain. Hence the area, which receives heavy rains, are the windward side of the

Sahyadri ranges (Karkan ragion), the hills of Assam and the Himalayan ranges. It is from

these watersheds the major rivers like Ganga, Yamuna and Bramaputra originate intensity

of S.W. Monsoon and the distribution of rainfall are controlled by a series of depressions

develop in the Bay of Bengal and travel in a northwesterly direction across the country.

As a result heavy rainfall occurs along the tracts 3 to 4 depression /month occur during

the monsoon.

SOUTH WEST MONSOON:

Diagram:

After June 21st the sun beings to move south ward and crosses the equator on

September 23.

84

The South West Monsoon is followed by North East Monsoon towards the end of

September. The North East Monsoon is also known as the retreating monsoon. This is

an example of transfer of directing of wind with the migration of the sun southward. In

the cold season September to December, central Asia becomes the cold center of high

pressure. The doldrums becomes the hot center of low pressure and draw the air from

central Asia. The wind coming from Central Asia passes Tibet, India and the Indian

Ocean to the Southern hemisphere. The North East monsoon is a dry wind system.

However when the currents pass over and across the Bay of Bengal and get deflected

South Westerly, they carry humid air and strike the coastal areas of TamilNadu.

Cyclones develop at the head of Bay of Bengal, which cross over Peninsular India.

Madras, Bengal and Burma are affected frequently by such cyclones. Almost the entire

Andhra and Madras State get a fair amount of rainfall. The parts of southern districts of

Madras not benefited b south West Monsoon good rain in North East monsoon.

85

(Chapter –10)

ATMOSPHERIC HUMIDITY (MOISTURE)

Moisture present in the atmosphere plays a significant role in weather and climate

of a region. There are three major components in the atmospheric moisture.

i. Humidity

ii. Precipitation

iii. Evaporation

Humidity: The terminology related to humidity and concerned with gaseous form of

water i.e., water vapour, several expression of the amount of water vapour in the air are

used.

1. Absolute humidity: It denotes the actual mass of water vapour in given volume of air.

It may be expressed as the number of grams of water vapour in cubic meter of moist

air or mass of water vapour per unit volume of air.

2. Specific humidity: It is defined as the moisture content of moist air as determined by

the ratio of the mass of water vapour to the mass of moist air in which the mass of

water vapour id contained.

3. Relative humidity: Relative humidity is a common parameter for expressing water

vapour content of the air. It is the percentage of water vapour present in the air in

comparison with saturated condition at a given temperature and pressure. The R.H.

can be expressed as

100r

RH = ------

rw

Where “r” is the mixing ratio of moist air at pressure (p) and temperature and “rw” is

the saturation-mixing ratio at same temperature and pressure.

4. Mixing ratio: The mass of water vapour per unit mass of dry air is a convenient

parameter to express the relative composition of the mixture. It is defined as the

ratio of the mass of water vapour to the mass of dry air with which the water vapour

is associated.

5. Den point: The temperature at which saturation occurs in given mass of air. The

86

dew point temperature is often compared with the temperature of free air and also

used to predict the occurrence of fog, dew, frost or precipitation.

6. Vapour pressure: This is the amount of partial pressure created by water vapour in the

air expressed in the units of millibar (or) inches of mercury.

7. Vapour pressure deficit (VPD): It is the difference between saturated vapour pressures

and actual vapour pressure. Express as bar Pascal. When the VPD is up to 1.5 Kpa the

air is said to be humid and over and above 2.5 Kpa it is drier. It gives the rough

estimate of drying power of air similar to RH. Rate of evaporation and transpiration

are indicated by the magnitude of VPD

8. Saturation point: When air contains all the vapour it can hole at that temperature air

said to be saturated at the temperature reached saturation point.

FACTORS AFFECTING HUMIDITY OF THE AIR:

1. Temperature:

If the temperature of the atmospheric air is more, the water vapour present will be

less. But at the same time the high temperature will increase the capacity of the

atmospheric air to absorb more water from the earth surface.

2. Nearness of the place to the seacoast:

The places near the seacoast are supposed to be cooler due to high deposition of

water in vapour form in the atmospheric air from sea.

3. Climate:

Based on the various climatologically a factor such as temperature rainfall etc., a

particular place is divided into various climatic periods like summer winter etc.

Summer period is marked by high temperature, low rainfall and low humidity.

Rainy period is marked winter season is also marked by low temperature, but not with

frequent rain and high humidity.

IMPORTANCE OF HUMIDITY:

It decides the dampness or dryness of the atmospheric air. Humidity has got the

same effect as that of rain in deciding the water needs of the crops. The high humidity

has also got some adverse effect on the crop growth. There will be high incidence of pest

and diseased under high humidity.

87

The rate of evaporation and transportations entirely depends upon the saturated

condition of the atmospheric air with water vapour.

MEASUREMENT OF HUMIDITY:

The amount of vapour (water) in the atmospheric air is measured by gravimetric

method, and also by using wet and dry bulb thermometers, An man Psyschrometer

Hygrograph etc.

Effect of Relative Humidity on crops

RH directly influences the water relation of the plant and indirectly affects 1) Leaf

growth, 2) Photosynthesis 3) Pollination 4) Uptake and translocation of nutrients 5)

Occurrence of pest and diseases 6) Economic yield of crop

Water relation: RH affects the transpiration by modifying the vapor pressure gradient.

In dry region RH will be low which causes severe water deficit in plants and reduce the

leaf water potential, plants become dry and wilt. High RH lowers the ET.

1. Leaf growth not only depends on photosynthesis and biochemical presses but

also depend on physical process of cell enlargement. Cell enlargement occurs as

a result of turgor pressure developed within the cell. Turgor pressure is high

under high RH due to less transpiration. Thus, leaf enlargement is high in humid

region. E.g., cotton 40% RH recorded increase the growth rate compared to 25 or

65%RH.

2. Photosynthesis: RH indirectly affects photosynthesis. When RH is reduced

transpiration increases causes water deficit in plants. Water deficit causes partial

or full closed of stomata and increases mesophyll resistant blocking the entry of

CO2 thereby Photosynthesis is affected.

3. Pollination: Moderately low air humidity is favorable for seed set in many crops

provided in soil moisture supply is adequate. E.g., Seed set was higher in wheat

at 60% RH compared to 80 % RH. When water availability in soil is not limiting,

88

due to increase pollen germination. When RH is increased pollen may not

disturbed from the anther. Low RH causes pollen sterility.

4. Uptake and translocation of nutrient: High RH decreases the transpiration,

which affects uptake of nutrients and causes deficiency. Uptake of P, K and Ca

was higher at high RH the 60%. Increased the RH increases P uptake. RH 60%

is effective for most of the crop growth by better nutrient uptake.

5. Pest and disease incidence: It increases with increased RH. Higher RH favors

easy germination will be ore under high RH.

6. Crop Yield: Very high or low RH is not ideal. In maize low yield due to high

RH. Pest and disease incidence at maturity stage. Low RH is useful. 60-80%

RH is ideal for most of the crop.

Diurnal variation in RH: The mean maximum RH occurs in the early morning hours and

minimum in the early afternoon. The RH has its maximum at equator and decreases

towards the poles up to 300 N and S due to subsiding and diver sing air masses. From

About 300 to poles the RH increase the result of decreasing temperature. This trend is

known as Diurnal variation in RH.

Effect of relative humidity on Plant Growth

Increase in RH-decreases the temp. This phenomenon increases heat load of the

leaves. Since transpiration is reduced-not much heat energy used. Excessive heat due to

closure of stomata entry of CO2 is reduced. Reduction in transpiration reduces the rate of

food translocation and uptake of nutrients.

Very high RH is beneficial to - Maize, Sorghum, Sugarcane, (C4Plants)

Harmful to - Sunflower, Tobacco.

Affect water requirement of crops: For almost all the crops it is always safe to have a

moderate R.H. of above 40%. 60-80 % conducive for growth and development of plants.

89

(Chapter –11)

CLOUDS AND PRECIPITATION

Clouds are condensed moisisting of droplets of water and ice crystals. The nuclei

of those droplets are dust particles.

Near the surface these drops forms as fogs and in the free atmosphere, they form

clouds.

Clouds has been defined as a visible aggregation o minute water droplets and / or

ice particles in the air, usually above the general ground level.

FORMATION OF CLOUDS

Clouds are formed by condensation of moisture n the air by cooling.

1. It is due to direct cooling as they come in contact with cold surface.

2. By mixing of hot and cold air.

3. By expansion.

When a current of air rises upwards due to increased temperature it goes up, expands

and gets cooled. If the cooling continues till the saturation point is reached, the water

vapour condenses and forms clouds. The condensation takes particles individually are

very small and suspended in the air.

Only when the droplets coalesce to from a drop of sufficient weight, to overcome

the resistance of air, they fall as rain.

Clouds are considered essential and accurate tools for weather forecasting. Every

feature of air masses (discontinuity, subsidence, instability and stability) is reflected by

the shape, amount and structure of clouds.

Classification of clouds

Though confusion apparently arise from the number of kinds or species, the

genera seems reasonably clear-but, if we are able to recognize their main characteristics.

Clouds are usually classified according to their height and appearance. For

convenience we list them in descending order. High clouds, middle clouds and low

clouds. Since for one do not fit in any of these categories. But fortunately their

particular characteristics make them easily, identifiable as vertical development clouds.

90

Genera with common distinguishing characteristics main sum division of a family. We

must exercise some caution in relying on height data. There is some seasonal as well as

latitudinal variation and there is some overlapping from time to time. However, the

appearances of clouds are quite distinctive for each height category.

The main cloud genera are defined and described in the international cloud atlas

of the WMO genera1957. That can be listed according to their heights as under.

A. High (mean heights 5 to 13 km) (Mean lower level 20000 ft)

Clouds

i) Cirrus (ci) men height 9900 m.

ii) Cirrocumulus (cc) 8300 m.

iv) Cirrostratus (Cs) 6500 m.

B. Middle (Mean height 2 to 7 km) (6500 to 20000’)

Clouds

i) Altostratus (As) 4300 m.

ii) Altocumulus (Ac) 4300 m.

C. Low (mean heights 0 to 2 km) (Close to earth’s surface to 6500’)

Clouds

i) Nimbostratus (Ns) 2000 m.

ii) Stratocumuls (Sc) 500m.

iii) Stratus (St) 900-1200 m.

D. Vertical clouds

i) Cumulus (Cu) 1500-2000 m.

ii) Cumulonimbus (Cn) 3000-5000 m.

Clouds with vertical development

1.Cirrus: Detached clouds in the form of white, delicate filaments or white or mostly

white patches of narrow bands. Those clouds have a fibrous (hair like) appearance or a

delicate silk) appearance or both. All the cirrus or cirro-type clouds are composed of ice

91

crystals. Cirrus clouds have brilliant colours of sunset sunrise. These clouds do not give

precipitation.

2.Cirro-Status: Transparent which cloud v

3.Cirro-cumulus: Thin, white flakes, sheet or layer of cloud without shading.

Composed of very small elements in the form of grains, ripples etc. This type of cloud is

not common and is often connected with cirrus or cirrostratus. When arranged

uniformly, it forms a “Mackerel sky”. Mackerel – Fish has Greenish blue stripped back

and silvery white belly.

4.Alto-stratus: A uniform sheet cloud of “Grayish or bluish cloud frequently showing a

fibrous appearance, totally or partly covering the sky, and having parts this enough to

reveal the sun at least Waverly as through ground glass. Altostratus does not show halo

phenomena. This type of clouds a may cover all or large portions of the sky.

Precipitation may fall either as fine drizzle or snow.

5. Alto-Cumulus: “white or grey, or both white and grey, patch, sheet or layer of cloud.

They have devel shedding on their under-surfaces. Sometimes referred to as “sheep

clouds” or “ Woolpack clouds”.

6.Nimbo- Stratus: “Grey cloud layer, often dark, the appearances of which is rendered

diffuse by more or less continuously falling rain or snow. Which in most cases reaches

the ground. It is thick enough throughout to blot out the sun. It is a rain, snow or sleet

cloud. It is never accompanied by lightening, thunder or hail. Streaks of water (rain) or

snow falling from these clouds but not reaching the ground are called “Virga”. Wisps or

streaks of water or ice particles falling from base of a cloud but evaporating completely

before reaching the ground. Wisps=bundle as of straw.

7. Strato-Cumulus: “Grey or whitish or both grey and whitish patch, sheet or layer of

cloud which almost always has dark parts, composed of tessellation’s, rounded masses,

rolls, etc.

8. Stratus: Generally grey cloud layer with a fairly uniform base, which may give

drizzle, ice prisms or snow grains, sky may be completely covered by this type of cloud.

Sun is visible through this cloud.

92

9. Cumulus: “Detached clouds, generally dense and with sharp outlines, develop

vertically in the form of rising mounds, domes of towers, of which the bulging upper

parts often resembles a cauliflower. Cumulus is generally found in the dry time over land

areas. They dissipate at night. They produce only light precipitation

10.Cumulonimubs: “Heavy and dense cloud, with a considerable vertical extent in the

form of a mountain or huge towers. This type of cloud is associated with heavy rainfall,

thunder, lightening, hail or tornadoes. This type of clouds is easily recognized by the fall

of a real shower and sudden darkening of the sky.

Clouds formation: Air contains moisture – this is extremely important to the formation

of clouds.

- Clouds are formed around microscopic particles such as dust, smoke, salt crystals

& other materials that are present in the atmosphere.

- These materials are called “Cloud condensation Nucleus” (CCN)

- Without these no cloud formation will take place.

- Certain special types known as “ice nucleus” on which cloud droplets freeze or

ice crystals form directly for water vapour.

- Generally condensation nuclei are present in plenty in air

- But there is scarcity for special ice forming nuclei.

- Generally clouds are made up of billions of these tiny water droplets of ice

crystals or combination of both.

There are two rain forming process viz,

1. Warm

2. Cold Rain process

Warm rains: it refers rainfall process n the tropics.

- Rains occur when the temp is above 00C never colder than 00C.

- When larger droplets collide and absorb smaller cloud droplets.

- They grow larger and larger & become raindrops.

- This process is known as “Coalescence”

Cold rain process

- Occurs when the cloud temperature is colder than 00C.

- Clouds are usually with ice crystals and liquid water droplets.

93

- These crystals grow rapidly drawing moisture from the surrounding cloud

droplets until their weight causes them to fall.

- Falling ice crystals may melt and join with smaller liquid cloud droplets resulting

in raindrops. If ice crystals do not melt, they may grow into large snowflakes and

reach the ground as snow.

Conditions favorable for the occurrence of precipitation

i) The cloud dimension (vertical –7 km horizontal 60-

70km)

ii) The lifetime of the cloud (at least 2-3 hrs.)

iii) The size and concentration of cloud droplets & ice

particles.

iv) RH should be 75%

v) Wind velocity 20km.

vi) Cloud seeding

Cloud Seeding: It is the process by which the conditions of the cloud (dimension, life

time and size) are modified by supplying with suitable nuclei us at proper time and place.

For accelerating the warm rain process seeding with very large nuclei such as salt crystals

can be used. In the case of cold rain process, seeding with ice nuclei such as silver iodide

are used to make good the deficiency in the clouds.

94

(Chapter –12)

EVAPORATION AND TRANSPIRATION

Hydrologic cycle

Hydrologic cycle involves four major steps viz, evaporation, transpiration,

condensation and precipitation. Through the cycle has neither a beginning nor and end,

the concept of cycle begins with the water of the oceans, since it covers nearly ¾ of the

earth’s surface. Radiation from the sun evaporates the water vapor from the oceans into

the atmosphere. The water vapour rises and collects to form clouds. Under certain

conditions, the cloud moisture condenses and falls back to the earth as rain, snow, hail

etc., precipitation reaching the earth’s surface may be intercepted by vegetation, o enter

into the soil, may flow as run off or may evaporate. Evaporation may be from the surface

of the ground of from free water surface. Transpiration may be from plants.

Evaporation: The change of state of water from solid and liquid to the vapour and its

diffusion into the atmosphere is referred to as evaporation. In agricultural Meteorology

Evaporation is defined as the maximum possible loss of moisture form a wet, horizontal,

flat surface exposed to weather parameters, which exist in the vicinity of plants.

Factors affecting Evaporation

1. Those affecting water supply at the evaporating surface. i.e., soil and plants

including soil storage capacity, rainfall and irrigation and

2. Those affecting energy supply to the evaporation surface like solar radiation.

Transpiration: Most of the water absorbed by plants is lost to the atmosphere. This loss

of water from living plants is called transpiration. It can be stomatal, cuticular or

lenticular.

Factors affecting Transpiration: Light, Humidity, Temperature, wind, root/shoot ratio,

availability of water to plants, Leaf characteristics.

Evapotranspiration (ET): As noted earlier, it is a combined losses of water through

evaporation from the soil and transpiration from the plants.

95

Potential Evapotranspiration (PET): Is defined as the amount of water which will be

lost from an extensive water surface or soil completely covered with vegetation where

there is about moisture in the soil at all times

Evapotranspiration is also called water use (WU) or consumptive use (CU). The

Evapotranspiration (ET) are climate and management practices.

Evaporation

- One of the four components of the endless hydrogical cycle (Evaporation-

Transpiration-Condensation-Precipitation).

- Most of the water vapor comes from ocean.

- It is also important in agriculture as it affects.

- Soil Conditions.

- Plant growth-crops.

- Water storage-dams.

Evaporation depends upon:

- Temperature of the water surface

- Vapour pressure of the air

The pressure exerted by the water vapour in the air is known as “vapour pressure”.

Evaporation is more when there in greater pressure difference between vapour pressure

and saturation vapour pressure.

- Wind movement (Removes moisture) – Evaporation increases with wind velocity.

- Salinity – presence of dissolved minerals salts reduce Evaporation from sea is 5%

less than pure water.

Factors, which affect ET from plant & Soils, are:

i) Those affecting water supply

- Soil storage capacity.- Rainfall.- Irrigation

ii) Those affecting energy supply

1. Light: Stomata open I light and close in the dark.2. Temperature: Humidity/ vapour pressure function of temperature

96

3. Relative Humidity: Less humidity higher temperature. Increases difference – incurred. Decrease temperature increase vapour pressure – reducing the saturation deficit

Wind: Saturated unit is replaced by dry air around the plant – increased temperature cooling effect on leaves vapour pressure different decreases

Plant characters:a) Root shoot – ratiob) Leaf characteristicsc) More LAI – Transpiration highd) Thick cuticle – epidermal hair – less transpiration.

When R/S ratio is more or equal then Transpiration will be more.PR-Evaporation forms a free water surface.AE - Actual Evaporation.AE is always less than PE

FACTORS AFFECTING EVAPORATION

Climatic Factors

1. Solar radiation2. Relative Humidity3. Temperature4. Wind

Soil Factors:1. Soil texture – a. Sady soil, b. Clay soil.

2. Available soil moisture

3. Soil salinity

4. Hydraulic conductivity

Plant characters:

1. Plant morphology

a. Leaf sizeb. Thickness of the cuticlec. Stomata

2.Type of plant

Other factors1. Ploughed unploughed field2. Plant population and row pattern3. Plant cover

97

ET and Crop production:1. Working out ET or PET will be useful in scheduling the irrigation. (IW/CPE ratio

method)2. ET can also help in demarcating the drought prone areas. These will form the

base for developing suitable soil and crop management practices, crop varieties, water conservation techniques, cropping pattern and ways to improve productivity of rain fed crops.

3. Water Use Efficiency can be worked out.

Condensation: The physical process by which a vapour becomes a liquid or solid-opposite of evaporation.

98

(Chapter –13)

PRECIPITATIONIt is defined as water in liquid solid form falling on the earth surface.

Types of precipitation

Conventional precipitation, Orographic precipitation, Frontal precipitation.

Conventional precipitation: This type of precipitation occurs due to convection over

turning of moist air. Conventional precipitation results in heavy showery rainfall. The

major forms of precipitation associated with this type are rains or snow showers, hail or

snow pellets.

Convention: Upward movement of relatively warm air.

Hail: Ice pellets along with rain – Updrafts of cumulonimbus clouds leads to hail.

Orographic precipitation: Precipitation resulting from raising and cooling of air

masses when they are blocked by a topographical barrier (mountains). The barriers are

important factor in increasing the rainfall on windward slopes. E.g.: Cherrapunjii in

Assam – 10000 mm of rainfall – Here the mountain barriers lie across the paths of

moisture bearing winds.

Frontal precipitation: Produced when airs current converge and rise. Most

precipitation results from condensation and sublimation. This type occurs mainly in

middle latitude.

Hydrological cycle: four major steps are involved.

Evaporation, transportation, condensation and precipitation.

Evaporation: The primary source of water vapour in the atmosphere is the moisture

evaporated from the Ocean (99%) and lands a small extent from transpiration.

Transportation: Humid tropical air masses, which become cool as they travel pole

ward, were carrying huge quantity of water vapour.

Condensation: Warm air raises and water vapour is condensed.

Precipitation: The condensed water vapour float through the air in the form of clouds

through the barrier of adiabatic cooling. Extensive air masses fall below the dew point.

99

Water particle increase in size until they are too heavy to float and then they fall as rain

or snow or other forms of precipitation.

Forms of precipitation

Rain fall, drizzle snow, sleet, hail or hail storm.

Rainfall: it forms are one kind of precipitation received through the cycle know as

hydrological cycle. It is a never-ending cycle between Ocean, atmosphere and land.

Drizzle: Minute droplets of water having a diameter less than half a millimeter or 0.02

inches. The intensity is very light and the fine droplets of water hardly reach the ground.

It falls continuously from low stratus type of clouds.

Snow: Failure of Indian monsoon during 1960’s. It is formed by crystallization of water

vapour at temperature below freezing point through the process of sublimation. A snow

cover is poor conductor and keeps the soil temperature higher. Much useful in

agriculture in region where the winter are severe. It prevents soil freezing and protects

roots of the plant.

Hail of Hailstorm: Hail are composed of hard pellets of ice or ice and snow. It ranges

from small peas to large cricket ball size. Hails rarely occur in tropic or high latitude. It

causes heavy damage to crops, buildings and to glass houses.

Sleet: It is the precipitation in the form of small particles or pellets of clear ice. Sleet are

formed either due to melting of hail or due to freezing of raindrops when it passes

through the cold air mass. Sleet occurs when there is a strong temperature inversion

above the surface.

Glaze: When the rain is composed of super cooled drops, which froze rapidly upon

striking solid surface. This forms a coasting of ice on trees, wire and other objects. Such

deposits are called glaze. Its occurrence is popularly called ass ice – straw. This

damages trees and wires by breaking due to over weight. Some times deposits of >5cm

thickness has been observed on tree twigs.

Forms of Condensation: Dew, frost and Fog.

Dew: It is condensed moisture deposited on cold objects. It has two roles.

a) Passive role – It delays raise I temperature

b) Active role – Dew is absorbed by the planes and enters in dynamic liquid

cycle. It is much useful in arid region for crops.

100

Frost: When the dew point is below 00C moisture passes directly from gaseous to solid

state. The frost occurs in low places like vallies. The cold air drains along the slope into

low places creating temperature inversion. It affects the plantation crops in higher

elevation.

Fog: It may be defined as microscope falling of small drop of water condensed and

suspended in the air at the surface of earth reducing horizontal visibility. The blend of

smoke and fog is called smog.

Rainfall: Monsoons of India

Rainy day: A day, receiving a rainfall of 2.5 mm or more.

Effective rainfall: The amount of rain received which is sufficient to meet Et or

consumptive use, is called effective rainfall.

ER=Rainfall – (deep percolation losses + runoff losses)

Isohyte: Imaginary line connecting the places having similar rainfall is called as Isohyte.

Instruments used:

Rainfall – Ordinary rain gauge, Self-recording rain gauge.

Dew: Dew gauge.

Monsoon: The trade winds the changed direction due to local factors likes topography,

Ocean etc, during the year, are called monsoon. India has well developed regular

monsoon system. 85 to 90% of total rainfall is from both the monsoons. The seasonal

wind of the Indian Ocean and southern Asia blowing from the southwest in summer and

from Northeast in winter monsoon blows. Commonly marked by heavy rains.

SWM: June - Sep. Onset of SWM – First week of June.

Withdrawal of monsoon – End of September.

India – 73% of total rainfall is from SWM.

Tamil Nadu – 32% of total rainfall is from SWM.

Formation of SWM.

During summer, central Asia and arid zones of India get heated up resulting in low

pressure. High pressure develops in the Indian Ocean on Ocean in South. Hence wind

moves from high-pressure area of India Ocean to towards lower pressure area of India.

The South East winds start from south of equator, while crossing the equator, the are

101

caught up suddenly in the air circulation over India and deflected as South –West winds.

They reach south India (Kerala) around first week of June every year (30 km/hour). In

months time they over run almost entire country. These monsoon winds over India has

two branches

1. Bay of Bengal branch moves to Assam

2. The Arabian Sea branch moves northward to Kerala coast.

NEM or Returning monsoon :October –December.

Onset in Tamil Nadu: End of September or First week or October.

With drawal: End of December.

India – 13% of total rainfall is from NEM.

Tamil Nadu: - 47% of total rainfall is from NEM.

Formation of NEM.

In winter, the large masses in China and Russia cools, because of movement of

Sun towards south of equator, resulting in high-pressure area. Lower pressure areas are

developed over Indian Ocean due to high temperature. The air masses move from high

pressure to low pressure area (Indian Ocean). These winds are cool and dry. While

reaching India, they are abstracted by Himalaya and deflected to east. The northeast

winds subsequently deflected to Southwest, they become warm. As they move across

over Bay of Bengal, it absorbs large quantity of moisture and are warm. As they strike

the cool land surface of south coastal Andhra Pradesh and Tamil Nadu coast, the air

masses are cooled and rainfall occurs.

Tamil Nadu:

SWM dominated areas: Nilgiris, Salem, Dharmapuri.

NEM dominated areas: Southern districts.

Both the monsoons: Kanyakumari

Effect of rainfall on crop production

Rainfall is the primary source of water to earth surface. India is a monsoon

country. Nearly 73% of rainfall is received during SWM and 13% during NEM season.

In Tamil Nadu 47% received in NEM and 32% from the SWM.

102

The economic importance of rainfall can be well appreciated, when the extend of

its contribution towards food production in India is seen. Out of 142.5 million ha. Under

arable land in India, rain fed area accounts 65.5% in India, where as in Tamil Nadu it

more than 90% of millers and pulses, 75% of total oil seeds, 70% cotton, 82% of maize,

61% of rice and 35% of wheat are grown under rain fed condition.

I. Effect of amount of rainfall on crop production

Selection of crop varieties and cropping system depend on the quantity of

rainfall. The rainfall limits the choice of crops. E.g.: North Western India

Seasonal rainfall 300-400mm – only pearl millet based system is practiced.

North Eastern India – Seasonal rainfall 600-700 mm –rice based cropping

system is practiced. Semi arid regions of Peninsular India –seasonal rainfall

of 300 –500 mm – only maize/ground nut/ sorghum are grown.

Generally yield levels are determined by the amount of rainfall above the

basic minimum: Under rain fed condition minimum 250mm of rainfall is

necessary for grain crops.

Rainfall may also to too excess of the optimum and cause yield reduction.

Prolonged rainfall for 4-5 months caused poor drainage – reduce the growth

and yield of crops drastically. E.g.: Germinations of crops like wheat,

gingelly, mustard, safflower are very much affected, if stagnation of water for

even two or three days. Heavy ort excess rainfall results in run off losses –

which removes the top fertile soils, plant nutrients leached out of root zones

and crops are adversely affected under anaerobic condition created by excess

rainfall.

Very low rainfall / drought causes severe moisture stress in different growth

stages resulted in poor growth, yield attributed and reduces the yield

drastically, even failure of crops under sever moisture stress conditions. The

effect is much more when there is moisture stress at critical stage of crop

growth.

103

II. Intensity of rainfall on crop production

High intensity results in run off and soil loss resulted poor soil fertility and

productivity. Further it causes degradation of land become unsuitable for cultivation.

High intensity at the time of flowering resulted in poor seed set.

III. Distribution of rainfall on crop production

The amount of rainfall received at periodical intervals like weeks, month, season

etc. Indicates the distribution. It is more important than total rainfall. E.g.: Coimbatore

– Annual rainfall – 640mm; London with same quantity of rainfall, two or three crops are

grown under rain fed condition because of well distribution of rainfall.

Number of dry spells and wet spells: The success of crops in rain fed condition depends

on the number of wet and dry spells.

Dry spell: It is number of continuous rainless days. A dry spell of > 10-14days for

alfisol and >15-20 days for vertisol is critical for the crops.

Wet spell: It is a number of continuous days of rainfall.

IV. Effect of rainfall abberation on crops: There are four different rainfall

aberrations

1.Early or late on-set of monsoon: The early onset of about 15 days before the normal

onset has no harmful effect but late on set of monsoon will affect the crops in the

following ways.

a. Late sowing reduces the length of growing season, there by reduces the crop

yield.

b. In late wowing, there is higher incidence of pest and diseases.

2. There may be prolonged dry spells during the cropping period.

a. In the case of early drought the germination of crops will be much affected results

in poor germination, yield and failure of crops.

b. Mid late season drought causes poor crop growth resulted n poor yield and failure

of crops.

3.Uneven distribution of rainfall in space and time or spatial or temporal variation

104

Flood or drought drastically reduce crop growth and yield due to either excess

moisture or severe moisture stress.

4. Early withdrawal of monsoon or extended monsoon or continuous for longer period:

Early withdrawal resulted in complete failure of crops.

5. Extended rainfall beyond the season will affect the grin setting in poor quality grains

but extended monsoon helps for raising second crop under rain fed condition. Extended

rainfall also affect the harvest of crops, thrashing and drying the crops in time.

Types of rainfall

1. Unimodel rainfall

a. South West monsoon dominant – Single cropping – North India, Niligirs, Salem

and Dharmapuri.

b. NEM dominant rainfall – Single cropping – Southern districts of Tamil Nadu.

2. Bimodel rainfall: (Kanyakumari) – Rainfall will come in two season, double cropping

is possible. E.g.: Kanyakumari, Niligiris.

Types of Precipitation: There are three types of precipitation particularly for

rainfall.

1. Conventional precipitation – due to convection in the form of turning of moist air.

Heavy and showery precipitation is most likely, Rain, snow showers, hail and snow

pellets.

2. Orographic precipitation: Precipitation resulting from raising and cooling of air masses

when they are blocked by a topographic barriers (mountains). Barrier is important factors

in increasing the rainfall on win ward slopes. Highest annual Rain falls –where the

mountain barriers lie across the paths of moisture bearing winds. (Chirrapunji) – indirect

effect – force moist air upward, they hinder the passage of low pressure areas and also

promotes convection (due to differential heating along the slopes.)

105

3. Frontal cyclonic precipitation: Fronts form as the result of confluent motion between

contrasting air masses. It may happen according to type low-pressure systems and its

stage of development.

Precipitation characteristics

a. Rainfall intensity

b. Aerial extent of rainstorm

c. Frequency of rainstorms

Monsoons of India

Monsoon represents one of the phenomena in the category of secondary

circulation of the atmosphere. The term monsoon is derived from an Arabic word

‘Mausim’ or from Malayan word ‘monsin’ which means ‘season’. The word monsoon is

applied to such a circulation, which reverse its direction every six months i.e. from

summer to winter and vice-versa.

Economic importance of Monsoon

The economic significance of monsoon is enormous, because a population of

more than 2000 million lives, i.e., roughly about half the world’s population (54 per

cent), depends on the monsoon rains for their crops. Moreover, a large percentage of

total population in the monsoon region derives its income from agriculture. In India

monsoon mean life-giving rains. Rice is their major crop, which provides food for

millions of people; hence monsoon rains are so essential for its growth. Failure of

monsoon rains cause loss of food crops. Erratic behavior of monsoon cause disastrous

floods in some parts of the country while I other parts there is severe drought.

During the hot, dry season (April-May) when temperatures rise rapidly and

pressures over land decrease, the warm and moist air form over the adjacent seas starts

blowing, towards the above-mentioned low-pressure center. However, in the beginning

the maritime air masses are drawn only from a short distance. But by the end of May or

the first week of June, when the low pressure has fully developed, the pressure – gradient

is steepned so that even the trade winds from southern hemisphere are drawn towards the

thermal low positioned in north-western region of the sub-continent. The southerly

trades on crossing the equator are deflected to their right in accordance with Ferrell’s

106

Law. Now, the originally southeast trade winds become southwesterly blowing towards

northeast.

Winter Monsoon

A secondary high-pressure system develops over Kashmir and the Punjab. The

high-pressure area controls the prevailing wind direction over the rest of the

subcontinent. Contrary to the pressure condition over land, there are low-pressure centers

formed over the India Ocean, the Arabian Sea, and northern part of Australia. In the cool

season, therefore, there is pressure gradient from land to sea as a result of which winds

begin to move from land to sea. These are the northeast or winter monsoons of northern

hemisphere.

The southern part of Indian Peninsula receives rainfall from north eat monsoon

currents. These currents while traveling over the Bay of Bengal pick up moisture from

warm ocean surface. The amount of winter rainfall on the eastern side of the peninsula is

much heavier than that on the other side. It is also known as retreating monsoon.

Flood: High degree of runoff is known as flood. Runoff is that portion of precipitation

that returns of the oceans and other water bodies over the land surface of through the soil

and water table. May be direct return of rainfall or the flow form melted snow and ice

fields – which have temporarily stored water.

Flood differs from simple runoff only in degree. Distinction between the two

depends upon how affect surface features. River floods result whenever the channel

capacity is exceeded by the runoff due to excessive runoff of rainfall or snowmelt. But,

the channel capacity may also be affected by barriers of flow, sudden change of direction

of stream, reduced gradient, siltation of the streambed, or sudden release of water due to

broken dam.

Factors affecting run off

1. The amount and intensity of precipitation

2. Temperature

3. Characters of the soil

4. Vegetative cover of the area

107

5. Slope of the land.

When rain occurs the proportion of runoff will depend on capacity of the soil and

vegetation to absorb. Plants retain some rainfall on their external structures and slow the

velocity of raindrops. They also detain water in its horizontal movement. Plants improve

soil structure and their roots provide channels to move water to greater depths. The high

humus content of soils with dense grass cover enhances absorption; for it acts something

like a sponge porous soils absorb more water by infiltration than dense clays. Impervious

sub-soil redness the amount of water that can be stored.

Climatic causes of flood

The predisposition of a climate to storms producing excessive precipitation is the

fundamental basis of the flood. In some climates flood-producing storms occur

irregularly; in others they follow a seasonal pattern.

Two types storms causing flood are

a. Violent thundershowers, which is of short duration and produces a flash flood.

b. Prolonged wide spread rain which through sheer quantity of water, creates

extensive flooding over entire watersheds.

Damages due to flood

1. Loss of human life.

2. Loss of field crops – may vary according to the duration and intensity of

flooding.

3. Loss of cattle wealth.

4. Loss of soil.

5. Loss of properties.

Not all floods are “ bad” for centuries agricultural areas in the lower – Nile flood

plain and Mesopotamia depends on annual river flooding and the accompanying deposits

of fertile silt. What is gained in this way in the lowlands must be lost at higher levels in

the watershed.

108

Management of flood

1. Conserve water in the soil where it falls by increasing porosity of the soil and

growing vegetations i.e., reduce runoff.

2. Increase the capacity of channels (rivers) to carry excess water direct to the ocean

or to the water bodies for storage.

3. Avoid silting of water course by conserving soil by adopting sol conservation

techniques such as by vegetative barriers, counter bunding, contour cultivation

allowing grassy water ways etc.,

Management of crops affected by flood

Too much of water may be just as harmful to plants as too little. The most

injurious aspects of flooding or too much of water are lack of aeration in oxygen supply.

In wet soil nitrification suffers which causes yellowing and sticky appearance of plants.

Management practices

1. Drain away excess water as early as possible.

2. Give a foliar spraying of nutrients especially nitrogen for immediate relief. (Rice:

1.0% urea + 0.5% Zn So4).

3. Spray fungicides to protect the crop from fungal diseases, which are common

under high moisture condition.

109

(Chapter –14)

WEATHER ABERRATIONS

Weather aberrations and their effect on Agriculture

Dry spells: The interval between the end of a seven day wet spell, beginning with the

onset of effective monsoon and another rainy day with 5 e mm of rain (Where “e” is the

average daily evaporation) or the commencement of another seven day rainy spell with

four of these as rainy days (Satisfying the third criterion) and with a total rain of 5 e mm

or more during this spell is called the first dry spell. If the duration of this dry spell

exceeded certain value, depending on the crop-soil complex of the region, this dry spell

was called a critical dry spell.

Drought free week: The weekly rainfall exceeds 20 mm the week is set to be drought

free week.

Critical Dry Spell (CDS)

CDS is defined as the duration between the end of a wet spell and the start of

another wet spell during which a 50% depletion of available occurs in the top 50 cm soil

layer.

It is calculated by

AMD

CDS = --------

ET

Where, CDS in day

AMD = 50% of the available soil moisture in the top 50cm soil layer, expressed in

terms of depth (mm)

ET = Average maximum daily ET of a crop (mm/day)

Criteria for forecasting rainfall characteristics (like onset of effective monsoon)

(Ashok Raj, 1979.)

110

1. The first days rain in the 7-day spell signifying the onset of effective monsoon

should not be less than “e” mm where “e” mm was the average daily evaporation.

2. The total rain during the 7-day spell should not be less than 5 e + 10 mm.

3. At least four of these seven days should have rainfall, with not less than 2.5 mm

of rain on each day.

Wet spell

A wet spell is defined as a rainy day with “X” mm of rainfall or a 7-day spell

where the total amount of rainfall equals “x” mm or more and the condition that three out

of these seven days mist be rainy with rainfall more than 2.5 mm on each day. In this “x”

is the amount of rainfall, which brings the top 50 cm soil layer to field capacity. The

water holding capacity varies with the type of soil as also the value of “x”.

For example, the value of “x” is equal to

83 mm for light soils

125 mm for medium soils and

166 mm for heavy soils of Punjab

Drought

Drought has varied meanings for different people. In general drought may be

defined as a complex phenomenon, which results from the prolonged absence of

precipitation in conjunction with high rate of evaporation. This causes abnormal loss of

water form water bodies, lowering of the water supply to plants.

Classification of Drought

Drought can be broadly divided into three categories.

1. Meteorological drought: is a situation when the actual rainfall is significantly

lower than the climatologically expected rainfall over a wide area.

2. Hydrological drought: is associated with marked depletion of surface water and

consequent drying up of lakes, rivers, reservoirs etc. Hydrological drought occurs

when meteorological drought is prolonged.

3. Agricultural drought: is a condition in which there is no rainfall and insufficient

soil moisture availability in soil to the crop.

111

4. Atmospheric drought: it occurs when the rate of transpiration exceeds rate of

absorption of water due to low RH, high temperature and moderate to high wind

velocity even through available soil moisture is high in the soil. The drought is

temporary and reversible.

5. Soil drought – Condition when the soil moisture supply exceeds – 15 hours

(Permanent wilting point). It is gradual and progressive. It is highly detrimental

than others.

6. Physiological drought – even through the available soil moisture is high in the

soil, the plants are not able to absorb due to

i. High salt concentration and

ii. Low soil temp.

Drought: Under normal condition excessive moisture is far less a problem than drought.

Thornthwaite defines drought as “a condition in which the amount of water

Aberrations in rainfall: Aberration means the deviation from the normal behavior of the

rainfall. As we all know the principal source of water for dry land crops is rain, a major

portion of which is received during the monsoon period. Bursts of rain alternated with

“Breaks” are not uncommon. There are at least four important aberrations in the rainfall

behavior.

1. The commencement of rains may be quite early or considerably delayed.

2. There may be prolonged breaks during the cropping season (Intermittent drought).

3. The rains may terminate considerably early (early cessation of rain) or continue

for longer periods.

4. There may be spatial and or temporal aberrations.

112

1. Early or delayed onset of monsoon: To quantity in the onset of monsoon, 50

years of data to be analyzed for the dates onset of monsoon has to be studied for

different regions of the country. (For example, it was seen that the normal date of

onset of monsoon in the Madhya Pradesh and Maharashtra region is 10th June. In

8% of the years onset of monsoon can occur during last week of May (May 28 th

in 1925) and in10% of the years it is as much delayed as beyond 21st June). The

aberrations require changes in crops and varieties with the normal onset of NEM

in September, October - Crops like Sorghum, Bajra, Pulses and Oil seeds can be

grown in Kovilpatti tract of Tamil Nadu. If monsoon is delayed up to late

October, Bajra, Pulses, sunflower etc., can be raised. If it is very much delayed

up to first week of November only sunflower can be sown.

2. Breaks in the monsoon rains (Intermittent drought): The breaks can be of

different duration. Breaks of shorter duration (5-7 days) may not be a serous

concern, but breaks of longer duration of 2-3 weeks or even more, lead to plant –

water stress causing reduction in production. These breaks intermittent droughts

can be different magnitude and severity and effect different crops in varying

degrees. The yields of many drought resistant crops are not seriously affected,

but in several sensitive crops the yield reduction was heavy.

Another aspect of the breaks or intermittent drought is the stage of the

crops at which the drought occurs. The effect on crop will be different stages.

Another important factor is the effect or intermittent drought depends on

the physical properties of the soil particularly its water holding capacity. Deep

black soils have capacity to store as much as 300 mm of available soil moisture in

one meter depth, whereas light soils like desert soils can store only as little as 100

mm or so. Hence drought is more pronounced in the soils having less storage

capacity.

3. Early withdrawal of monsoon: For example, the normal of SWM in

Rayalaseema region will be between 25th Sept. and Oct.15th. But is 4% of the

years out of 55 years monsoon can withdraw during first fortnight of September

and in 10% of the years it withdraws during the month of December.

Since, crops and varieties in any given region are selected based on the

113

normal length of growing season. Persistence of rains much beyond normal dates

creates an extraordinary situation.

Under Kovilpatti (TN) condition short duration bajra and sunflower will

be suitable under early withdrawal of monsoon.

Cultural practices to mitigate the effect of moisture stress due to

intermittent drought and early withdrawal of monsoon are

1. Shallow interculture to eradicate weeds

2. Maintain soil mulch to conserve soil moisture

3. Application of surface mulch

4. Thinning of crops by removing alternated rows as in sorghum and bajra.

5. Recycling of stored run off water.

6. Ratooning in crops like sorghum and bajra.

7. For indeterminate crops like castor and red gram give 2-3% Urea sparay after a

rain.

4. Uneven distribution of monsoon rains, in space and time over different parts

of the country

Such as situations are encountered almost every year in one or another part of the

country during monsoon period leading to periodical drought and flood situations.

High variability of rainfall (or more precisely the soil-water) is the single factor

which influences the high fluctuations in the crop yields in the different parts of the

country.

114

Chapter -15

AGROCLIMATIC ZONES

Climate is general is the totality of weather during a longer period and over wider

area. An agro climate can be defined as the conditions and effects of varying weather

parameters like solar radiation, rainfall, etc.,. On crop growth and production.

Agro climatic zone classification is a method of arranging various data of climatic

parameters to demarcate a country or region into homogenous zones, i.e., places having

similar conditions.

Advantages of agro climatic classifications

1. This would enable in exploring agricultural potentiality of the area.

2. Locating similar type of climate zone will enable in identifying the specific

problems of soil and climate related to agriculture.

3. This will help in introduction of new crops from other similar areas. E.g.,

introduction of oil palm in Kerala from Malaysia.

4. Development of crop production technologies, specific to the region.

5. To take up research work to solve the regional problems and

6. To transfer the technology easily among the farmers

Agro climates of India

Krishnan and Muktar Singh (1969) have classified India into eight major agro

climatic zones using Thornthwaite moisture index and thermal index.

The moisture index is given by the following formula.

P-PE

MI= --------- x 100

PE

MI = Moisture Index

P = Precipitation /rainfall

PE = Potential evapotranspiration

Based on the year 1989, the Planning Commission made an attempt to delineate

India in to different agro-climatic zones. Based on the similarity in rainfall, temperature,

soil topography, cropping, farming system and water resource, India has been divided

115

into fifteen agro-climatic regions. This was done mainly to identify the production

constraints and to plan future strategies. (Agro climatic zones of India and Tamil Nadu-

study from record)

Efficient crop zones

The new different crop zone approach should aim in utilizing the natural

resources to the fullest extent. The uneconomical crops should be replaced by more

employment opportunities and economic stability of the farmers.

Based on the productivity, efficiency of the crops, each state has been divided into

five categories.

1. Efficient zone : The productivity of the crops is high and also stable due

to the prevalence of the optimum conditions.

2. Potentiality efficient zone: The productivity is high but unstable.

3.Moderately efficient zone : Stable, medium productivity.

4. Less efficient zone : Unstable, medium productivity

5. Inefficient zone : Low productivity

MAP scans

116

117

Chapter –16

AGROCLIMATIC NORMALS FOR FIELD CROPS

Definition: Climatic normals means the degree of temperature amount of rainfall,

Humidity, etc.,. Which distinguish optimal conditions from those defined as abnormal,

both because of excess and insufficiency.

Uses of study Agro-climatic normals for field crops can be as follows: -

1. Useful for Agricultural Planning

2. Useful in introduction of any crop. If the climate in which a crop is introduced

matches to the requirements of the crops, then the benefit will be the maximum.

Examples: a. Introduction of groundnut in Peninsular India from Africa

b. Long grained patnai rice into California

3. Will be useful to forecast the abnormal weather.

Climatic normals for crop plants

Rice

Besides rainfall, temperature and solar radiation influence rice yield, directly

affecting the physiological processes involved in grain production and indirectly through

the incidence of pest and diseases.

Temperature: The difference in yield is mainly due to temperature and solar radiation

received during its growing season. It requires high temperature, ample water supply and

high atmospheric humidity during growth period. This crop tolerates up to 400C

provided water is not limiting. A mean temperature of 220C is required for entire

growing period. If high temperature drops lower than 150C during the growth phase, the

rice yield is greatly reduced by formation of sterile spike lets. The period during which

low temperature is most critical is about 10-14 days before heading.

Solar Radiation: Low sunshine hours during the vegetative stage have alight ill effect on

grain production, whereas the same situation during reproductive stage reduce the

number and development of spike lets and thereby the yield. For getting higher grain

118

yield of 5t/ha, a solar radiation of 300 cl./cm2 /day is required. A combination of low

daily mean temperature and high solar radiation during reproductive phase has given

higher yield.

Rainfall: Rice requires high moisture and hence classified as hydrophyte. Rice requires

a submerged condition from sprouting to milky stage. The moisture requirement is 125

cm. An average monthly rainfall of 200 mm is required to grow low land rice and 100

mm to grow upland rice successfully.

Wheat

Temperature: Optimum temperature for sowing is 15-200C. At maturity it requires

250C. At harvest time wheat requires high temperature of 30-350C and bright sunny

period of 9-10 hours.

Moisture: One hectare of wheat consumes about 2500-3000 tonnes of water. Water

deficiency at the heading stage results in shriveled grains and low yield. In Punjab, 35 to

40 cm of well-distributed rainfall in the entire crop season or irrigations, one at crown

initiation stage and subsequently three at 40 days interval, result in good yield in wheat.

Maize

This crop is best suited for intermediate climates of the earth to which the bulk of

its acreage is confined.

Temperature: Maize requires a mean temperature of 340C and a night temperature above

150C. No maize cultivation is possible in areas where the mean summer temperature is

below 190C or where the average night temperature during the summer falls blow 210C.

However, high night temperatures also result in less yield. The crop gave 40% lesser

yield at 290C night temperature as compared with 180C.

Moisture: Maize is adapted to humid climates and as high water requirements. It needs

75 cm of rainfall during its life period. The average consumptive use of water by maize

is estimated to range between 41 and 64 cm. From germination up to the earing stage,

119

maize requires less water. However, at flowering it requires more water and the

requirement reduces towards maturity.

Groundnut

It is a tropical crop distributed between 450N latitude to 300S.

Temperature: it can be raised under a wide range of temperature. However, both very

high and low temperatures adversely affect it. A temperature range of 14-160C is

necessary for the seeds to germinate. Higher temperature results in better performance in

terms of length of stem, number of flowers and the number of pods. Maximum of pods

have been harvested at a mean soil temperature 230C. The numbers of pods decrease as

the temperature increases.

Moisture: An ideal rainfall consists of 75-125 mm during months preceding sowing,

125-175 mm during a fortnight after sowing and 370-600 mm of well distributed rainfall

during the crop growth.

Cotton

It is not season crop. It requires 4-5 months of uniformly high temperature (28-

450C) during its crop growth period.

Mean air temperature for 21 to 290C is required at vegetative period. The

optimum air temperature for reproductive phase is 27-320 C; mean sunshine hour is 8-9

hrs/day; and mean RH is 70%. But at boll development and boll opening period

(September to November) RH less than 70% and 8-hrs.of sunshine are ideal for good

cotton production.

The growth rate of cotton crop is increased at 25-300C. Temperature below 150C

retards growth and reduces the square (bud) formation.

Moisture: The minimum rainfall required for cotton is 500-650 mm. Heavy rainfall

during early stage is undesirable. Dry autumn months are desirable for good quality

produce. Excess rainfall at later stage may cause shedding of leaves, squares and bolls. It

also stimulates top growth and delays maturity and discolors lint. High humidity favors

many pests and diseases.

120

Sugarcane

i. Mean air temperature for optimum germination is 300C.

ii. Mean air temperature for optimum growth is 350C.

iii. At temperature less than 200C growth is reduced.

iv. Ideal climate is 4-5 months of hot period with temperature of 30-350C

followed by 6-8 weeks of cooler period for better maturity.

121

Chapter –17

WEATHER FORECASTING

Weather forecasts for India are made in advance by the Indian Metrological

Department and broadcasted through mass media like Radio, Television, and Newspapers

etc. Forecasting requires knowledge of the average and seasonal weather conditions of

the locality, accurate information about the actual weather conditions of the locality with

regard to all the weather elements at the time of forecasting, prevailing weather data

pertaining other places whose of the locality in question can possibly influence the

weather of the locality in question and finally considerable practical forecasting

experience.

There are two types of weather forecast.

1. Short range forecast and

2. Long range forecast.

SHOT RANGE FORECAST:

The daily forecasts of weather are short-range forecasts and are based mainly on

current weather data. The influences are based on pressure, and temperature changes and

cyclonic tendencies. The cyclone often takes the same course in each region and past

experiences indicates the probable course of movement of depressions.

The short-range forecast of the day-to-day weather is very helpful to Irrigation

Engineers, Mariners and aviators: it enables them to take timely precautions in times of

stroms, cyclones, heavy rains etc; precautionary measures against possible flood and

storm damages by providing suitable embankments and drains where necessary. It is

valid for 24-48 hours.

LONG RANGE FORECAST:

Knowledge of the normal climatic conditions from the normal during the 1 or 2

months preceding helps to forecast weather for the next 1-2 months other about. The

behavior of climate in different parts of the world is often a guide and in certain cases it is

possible to correlate climate of a given locality with the past climate with the other parts

122

of the world. E.g., Abundance of rainfall in India is associated with an excess of wind

pressure over the Pacific Ocean, Chile and Argentina, combined with deficit pressure in

the Indian Ocean and the Cape of Good Hope. From such correlations a forecast of the

immediate future monsoon can be made.

Long-range seasonal forecast are useful in another way; they enable cropping to

be adjusted to the anticipated climate.

Extended forecast: It gives emphasis on type of weather, sequence of rainy days, normal

weather, sequence of rainy days, normal weather hazards in farming such as strong

winds. Extended dry or wet spells and holds good for 5-7 days. It is useful for many

agricultural operations such as sowing, irrigation, spraying, etc.

Now casting: Weather forecasting is given 2 to 3 hours in advance. It will be useful for

Aviation and Navigation. Weather scientists are also turning to the other end of the

forecasting spectrum; extremely short-term forecasts – ‘Now casting’. A pilot

approaching London Air Port, for instance, wants to know what the weather will be like

in the next few minutes. Will there be a wind shear?.etc.

Laser Techniques: Now casting, which predicts local weather up to six hours ahead on

the basis of radar and satellite soundings, gives such answers. Unlike numerical models,

which represent weather conditions in terms of temperature, humidity and wind, reader

can “See” rain and pin point its location down to a few kilometers. With radar probes

and infrared photos available from satellites, now casters can predict small and short-term

phenomena like lightning or flash flooding. And by using computers, now casters can

extrapolate from what they already see to indicate the course a rainstorm will follow and

the likely variations in its intensity.

WEATHER FORECASTING ORGANIZATIONS:

Suitable organizations have been set up in the different countries for forecasting

weather. Accepted international methods of measuring weather elements, assigning

values to them and representing them in international code are being adopted by all the

participating countries. There are about 300 meteorological observation stations of

123

different types distributed over India. First class observatories like those at Poona, Agra,

New Delhi, Calcutta, and Bangalore etc. Take continuous readings of pressure,

temperature, wind humidity, rainfall sunshine, etc.

The second-class observatories take reading at 8 hours and 17 hours (Indian

Standard time IST) daily. Third class observatories record rainfall and temperature only.

Fourth-class observatories record rainfall and temperature only. Certain taluk offices

record rainfall alone and these are compiled periodically and forwarded to the

Meteorological Laboratory, Poona.

SYNOPTIC CHART:

The communication system provides the forecaster with a large mass of figure.

The next step is to put them into a form suitable for study. Plotting the observations on a

large outline map, which in popular term is called a “weather map” technically a

“Synoptic chart”, does this, simplified synoptic charts appear in some newspapers

On the forecaster’s synoptic chart the position of each station is marked by a

small circle. The report for each station is plotted in and around the circle. Some

elements like temperature, and pressure are entered in plain figures. Others like rain,

snow fog and cloud not easily expressed in figures are plotted in internally agreed

symbols. Some of the symbols used are shown under. The meanings to be attached to

the figures and symbols depend upon where they are placed in relation to the station

circle.

Thus the amount of shading in the circle is an indicator of the proportion of sky

covered by cloud the temperature (in whole degrees) is written to upper left of the circle,

the sea level pressure in millibars and tenths to the upper right (The hundreds figure for

the pressure that is the critical 9 or 10 is omitted as being understood since the pressure is

almost always between 950 and 1050 millibars. Thus 987=998.7 mb 125 = 1052.5 mb).

The wind is represented by an arrow flying with the wind and drawn towards the station

circle. The speed by feathers on the wind arrow, a short feather indicating 5 knots, a

large one 10 knots, a long and short 15 knots and so on.

Air temp.0C --------- ------------- Type of high cloud

Wind speed --------- ------------- Type of medium cloud

Visibility

124

Present Weather ----------- ------------ Weather since last report

Dew point Temp.0C ---------- -----------Type of low cloud

When the plotting of synoptic chart is completed the forecaster then proceeds to

the analysis. The object of which is to systematize the collection of individual station

plots into a coherent picture. The first stage is to draw the isobars – lines along which the

pressure is the same. The completed isobars usually revealed a few standard patterns,

like low pressure (cyclone) and high pressure (anticyclone), etc., and how they will

change in due course. Generally isobar formations show the general characters of the

weather in their areas.

In India such synoptic charts are drawn two tines daily (0830 I.T and 1730 hrs

IST)

125

Chapter –18

AGRICULTURAL SEASONS OF INDIA

Season is a period in a year comprising few months during which the

prevailing climate does not very much. Growing season for a crop is more important for

its yield and other management practices to be followed.

Indian meteorological Department has divided the year into four seasons.

i. Summer / Zaid : March-May

ii. Monsoon : June – September

iii. Post Monsoon : October – November

iv. Winter : December - February

The monsoon season is designated as Kharif, whereas the post monsoon and

winter seasons are together designated as “Rabi” throughout India.

Based on temperature ranges three distinct crop seasons have been identified in

India.

i. Hot weather (Mid February – Mid June)

ii. Kharif or rainy season (Mid June – Mid October)

iii. Rabi (Mid October to Mid February)

In southern states (Tamil Nadu, Andhara Pradesh and Karnataka) there is slight

variation in the season based on rainfall duration as

1. Winter - January and February

2. Summer - March to May

3. Rainy season - a. South West monsoon – June to September

b. North East monsoon – October - December

Based on the criteria, monthly precipitation and temperature, the growing season is

broadly divided as follows: -

i. Hot month - if the average temperature is above 200C

ii. Cold month - if the mean temperature is between 0 – 100C

iii. Warm month - if the mean temperature is between 15 – 200C

126

Agronomic concepts of the growing seasons

Agronomic ally the growing season can be defined as the period when the soil

water, resulting mainly from rainfall, is freely available to the crop. This condition

occurs when the water consumed by the crop is in equilibrium with rainfall and water

storage in the soil.

The growing season for a rain fed crop involves three different periods during which

the soil moisture conditions depend on the rainfall received.

a. Per-humid period: During this period precipitation will always remain lower than

the potential evapotranspiration for the corresponding period. This period

corresponds to the sowing period of the crop. Sowing can be done when the

precipitation during the week is > 0.5 PET.

b. Humid period: During this second period the precipitation remains higher than the

PET. The crops in this period will be in active vegetative and flowering phase

and the water requirement will be at its peak. At the end of this period water

balance is on the positive side and the water storage in the soil is on the increase,

since the rainfall is higher than the water needs.

c. Post-humid period: This period follows the humid period. During this period

there is a gradual reduction in the water stored in the soil due to the utilization by

the crop plants. The crops will also make use of the rainfall received. This period

usually coincides with maturity stage of the crop.

Types of growing period

There are four types of growing period.

1. Normal: In this type, rainfall is in excess during the humid period the humid

period. At the end of the pre-humid period when precipitation is higher than the

PET sowing of the crops are taken up. This type of growing season is prevalent

in semi arid tropics.

2. Intermediate type: The precipitation is lower than the PET all round the year.

The growing season is limited to the period when rainfall is in excess of PET.

127

Only drought hardy crops like pearl millet, castor, etc., can be grown. Dry

farming is highly risky.

3. All year round dry: In this type, the precipitation is more than PET all round the

year, indicating the moisture sufficiency for cropping. This type occurs in high

rainfall areas and mostly perennial crops are raised.

4. All year round dry: The precipitation is lower than PET throughout the year.

Cropping is not possible in these areas. This type of growing season is found in

extremely arid areas, mostly the deserts.

The fluctuations in the crop yields depend on the following conditions.

1. The length of the rainy seasons i.e., from sowing to the end of the rains

2. The quantity and distribution of rains during the per-humid and humid periods

3. The excess rainfall during humid period should go to soil storage. It may cause

water logging and crop lodging

4. The amount of rainfall received during post humid season, may supplement the

soil moisture during maturity. This may favorably influence the yield.

In India, four cropping seasons have been identified by IMD in dry farming areas.

S.No Name of the

season

Duration

(LGP)

Water need

from RF

Crops

1 Short duration Upto 10 weeks 75% Very short duration crops

2 Medium duration 10-15 weeks 75% Medium duration crops with intercrops

3 Extended medium

duration

15-20 weeks 75% A medium duration crop followed by

short duration crops if soil type is suitable

4 Long duration 20-30 weeks 75% Medium duration crops followed by short

duration crops.

128

129