climate, vegetation and soil - prashanth...

CLIMATE, VEGETATION AND SOIL

This unit deals with

• Weather and climate – spatial and temporal distribution of temperature,pressure, winds and rainfall; Indian monsoons: mechanism, onset andvariability – spatial and temporal; climatic types

• Natural vegetation – forest types and distribution; wild lifeconservation; biosphere reserves

• Soils – major types and their distribution, soil degradation andconservation

UNIT

III

We drink more water during summers.Your uniform during the summer isdifferent from the winters. Why do

you wear lighter clothes during summers andheavy woollen clothes during winters in northIndia? In southern India, woollen clothes arenot required. In northeastern states, wintersare mild except in the hills. There are variationsin weather conditions during different seasons.These changes occur due to the changes in theelements of weather (temperature, pressure,wind direction and velocity, humidity andprecipitation, etc.).

Weather is the momentary state of theatmosphere while climate refers to theaverage of the weather conditions over alonger period of time. Weather changesquickly, may be within a day or week butclimate changes imperceptively and maybe noted after 50 years or even more.

You have already studied about themonsoon in your earlier classes. You are alsoaware of the meaning of the word, “monsoon”.Monsoon connotes the climate associated withseasonal reversal in the direction of winds.India has hot monsoonal climate which is theprevalent climate in south and southeast Asia.

UNITY AND DIVERSITY IN THE MONSOON CLIMATE

The monsoon regime emphasises the unity ofIndia with the rest of southeast Asian region.This view of broad unity of the monsoon typeof climate should not, however, lead one toignore its regional variations which differentiate

the weather and climate of different regions ofIndia. For example, the climate of Kerala andTamil Nadu in the south are so different fromthat of Uttar Pradesh and Bihar in the north,and yet all of these have a monsoon type ofclimate. The climate of India has many regionalvariations expressed in the pattern of winds,temperature and rainfall, rhythm of seasonsand the degree of wetness or dryness. Theseregional diversities may be described assub-types of monsoon climate. Let us take acloser look at these regional variations intemperature, winds and rainfall.

While in the summer the mercuryoccasionally touches 55°C in the westernRajasthan, it drops down to as low as minus45°C in winter around Leh. Churu in Rajasthanmay record a temperature of 50°C or more on aJune day while the mercury hardly touches19°C in Tawang (Arunachal Pradesh) on thesame day. On a December night, temperaturein Drass (Jammu and Kashmir) may drop downto minus 45°C while Tiruvanantapuram orChennai on the same night records 20°C or22°C. These examples confirm that there areseasonal variations in temperature from placeto place and from region to region in India. Notonly this, if we take only a single place andrecord the temperature for just one day,variations are no less striking. In Kerala and inthe Andaman Islands, the difference betweenday and night temperatures may be hardlyseven or eight degree Celsius. But in the Thardesert, if the day temperature is around 50°C,at night, it may drop down considerably upto15°-20°C.

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34 INDIA : PHYSICAL ENVIRONMENT

Now, let us see the regional variations inprecipitation. While snowfall occurs in theHimalayas, it only rains over the rest of thecountry. Similarly, variations are noticeable notonly in the type of precipitation but also in itsamount. While Cherrapunji and Mawsynramin the Khasi Hills of Meghalaya receive rainfallover 1,080 cm in a year, Jaisalmer in Rajasthanrarely gets more than 9 cm of rainfall duringthe same period.

Tura situated in the Garo Hills ofMeghalaya may receive an amount of rainfallin a single day which is equal to 10 years ofrainfall at Jaisalmer. While the annualprecipitation is less than 10 cm in the north-west Himalayas and the western deserts, itexceeds 400 cm in Meghalaya.

The Ganga delta and the coastal plains ofOrissa are hit by strong rain-bearing stormsalmost every third or fifth day in July andAugust while the Coromandal coast, athousand km to the south, goes generally dryduring these months. Most parts of the countryget rainfall during June-September, but on thecoastal areas of Tamil Nadu, it rains in thebeginning of the winter season.

In spite of these differences and variations,the climate of India is monsoonal in rhythmand character.

FACTORS DETERMINING THE CLIMATE OF INDIA

India’s climate is controlled by a number offactors which can be broadly divided into twogroups — factors related to location and relief,and factors related to air pressure and winds.

Factors related to Location and Relief

Latitude : You already know the latitudinal andlongitudinal extent of the land of India. Youalso know that the Tropic of Cancer passesthrough the central part of India in east-westdirection. Thus, northern part of the India liesin sub-tropical and temperate zone and thepart lying south of the Tropic of Cancer falls inthe tropical zone. The tropical zone beingnearer to the equator, experiences hightemperatures throughout the year with smalldaily and annual range. Area north of theTropic of Cancer being away from the equator,

experiences extreme climate with high dailyand annual range of temperature.

The Himalayan Mountains : The lofty Himalayasin the north along with its extensions act as aneffective climatic divide. The towering mountainchain provides an invincible shield to protectthe subcontinent from the cold northern winds.These cold and chilly winds originate near theArctic circle and blow across central and easternAsia. The Himalayas also trap the monsoonwinds, forcing them to shed their moisturewithin the subcontinent.

Distribution of Land and Water : India isflanked by the Indian Ocean on three sides inthe south and girdled by a high andcontinuous mountain-wall in the north. Ascompared to the landmass, water heats up orcools down slowly. This differential heating ofland and sea creates different air pressurezones in different seasons in and around theIndian subcontinent. Difference in air pressurecauses reversal in the direction of monsoonwinds.

Distance from the Sea : With a long coastline,large coastal areas have an equable climate.Areas in the interior of India are far away fromthe moderating influence of the sea. Suchareas have extremes of climate. That is why,the people of Mumbai and the Konkan coasthave hardly any idea of extremes oftemperature and the seasonal rhythm ofweather. On the other hand, the seasonalcontrasts in weather at places in the interior ofthe country such as Delhi, Kanpur andAmritsar affect the entire sphere of life.

Altitude : Temperature decreases with height.Due to thin air, places in the mountains arecooler than places on the plains. For example,Agra and Darjiling are located on the samelatitude, but temperature of January in Agrais 16°C whereas it is only 4°C in Darjiling.

Relief : The physiography or relief of India alsoaffects the temperature, air pressure, directionand speed of wind and the amount anddistribution of rainfall. The windward sides ofWestern Ghats and Assam receive high rainfall

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during June-September whereas the southernplateau remains dry due to its leewardsituation along the Western Ghats.

Factors Related to Air Pressure and Wind

To understand the differences in local climatesof India, we need to understand themechanism of the following three factors:

(i) Distribution of air pressure and windson the surface of the earth.

(ii) Upper air circulation caused by factorscontrolling global weather and the inflowof different air masses and jet streams.

(iii) Inflow of western cyclones generallyknown as disturbances during the winterseason and tropical depressions duringthe south-west monsoon period intoIndia, creating weather conditionsfavourable to rainfall.

The mechanism of these three factors canbe understood with reference to winter andsummer seasons of the year separately.

Mechanism of Weather in the Winter Season

Surface Pressure and Winds : In wintermonths, the weather conditions over India aregenerally influenced by the distribution ofpressure in Central and Western Asia. A highpressure centre in the region lying to the northof the Himalayas develops during winter. Thiscentre of high pressure gives rise to the flow ofair at the low level from the north towards theIndian subcontinent, south of the mountainrange. The surface winds blowing out of the highpressure centre over Central Asia reach Indiain the form of a dry continental air mass. Thesecontinental winds come in contact with tradewinds over northwestern India. The position ofthis contact zone is not, however, stable.Occasionally, it may shift its position as far eastas the middle Ganga valley with the result thatthe whole of the northwestern and northernIndia up to the middle Ganga valley comesunder the influence of dry northwestern winds.

Jet Stream and Upper Air Circulation : Thepattern of air circulation discussed above iswitnessed only at the lower level of theatmosphere near the surface of the earth. Higher

up in the lower troposphere, about three kmabove the surface of the earth, a different patternof air circulation is observed. The variations inthe atmospheric pressure closer to the surfaceof the earth have no role to play in the making ofupper air circulation. All of Western and CentralAsia remains under the influence of westerlywinds along the altitude of 9-13 km from westto east. These winds blow across the Asiancontinent at latitudes north of the Himalayasroughly parallel to the Tibetan highlands(Figure 4.1). These are known as jet streams.Tibetan highlands act as a barrier in the path ofthese jet streams. As a result, jet streams getbifurcated. One of its branches blows to the northof the Tibetan highlands, while the southernbranch blows in an eastward direction, south ofthe Himalayas. It has its mean position at 25°Nin February at 200-300 mb level. It is believedthat this southern branch of the jet streamexercises an important influence on the winterweather in India.

Western Cyclonic Disturbance and TropicalCyclones : The western cyclonic disturbanceswhich enter the Indian subcontinent from the westand the northwest during the winter months,originate over the Mediterranean Sea and are

Figure 4.1 : Direction of Winds in India inWinter at the Height of 9-13 km


brought into India by the westerly jet stream. Anincrease in the prevailing night temperaturegenerally indicates an advance in the arrival of thesecyclones disturbances.

Tropical cyclones originate over the Bay ofBengal and the Indian ocean. These tropicalcyclones have very high wind velocity and heavyrainfall and hit the Tamil Nadu, Andhra Pradeshand Orissa coast. Most of these cyclones are verydestructive due to high wind velocity and torrentialrain that accompanies it. Have you seen theirmovement in the weather report in the television?

Mechanism of Weather in the Summer Season

Surface Pressure and Winds : As the summersets in and the sun shifts northwards, the windcirculation over the subcontinent undergoesa complete reversal at both, the lower as wellas the upper levels. By the middle of July, thelow pressure belt nearer the surface [termedas Inter Tropical Convergence Zone (ITCZ)]

shifts northwards, roughly parallel to theHimalayas between 20° N and 25° N. By thistime, the westerly jet stream withdraws fromthe Indian region. In fact, meteorologists havefound an interrelationship between thenorthward shift of the equatorial trough (ITCZ)and the withdrawal of the westerly jet streamfrom over the North Indian Plain. It is generallybelieved that there is a cause and effectrelationship between the two. The ITCZ beinga zone of low pressure, attracts inflow of windsfrom different directions. The maritime tropicalairmass (mT) from the southern hemisphere,after crossing the equator, rushes to the lowpressure area in the general southwesterlydirection. It is this moist air current which ispopularly known as the southwest monsoon.

Jet Streams and Upper Air Circulation : Thepattern of pressure and winds as mentionedabove is formed only at the level of thetroposphere. An easterly jet stream flows over

Figure 4.2 : Summer Monsoon Winds : Surface Circulation

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the southern part of the Peninsula in June, andhas a maximum speed of 90 km per hour(Figure 4.3). In August, it is confined to 15oNlatitude, and in September up to 22o N latitudes.The easterlies normally do not extend to the northof 30o N latitude in the upper atmosphere.

Easterly Jet Stream and Tropical Cyclones : Theeasterly jet stream steers the tropicaldepressions into India. These depressions playa significant role in the distribution of monsoonrainfall over the Indian subcontinent. Thetracks of these depressions are the areas ofhighest rainfall in India. The frequency at whichthese depressions visit India, their directionand intensity, all go a long way in determiningthe rainfall pattern during the southwestmonsoon period.

Inter Tropical Convergence Zone (ITCZ)

The Inter Tropical Convergence Zone (ITCZ) is a low pressure zone located at the equatorwhere trade winds converge, and so, it is a zone where air tends to ascend. In July, theITCZ is located around 20°N-25°N latitudes (over the Gangetic plain), sometimes calledthe monsoon trough. This monsoon trough encourages the development of thermal lowover north and northwest India. Due to the shift of ITCZ, the trade winds of the southernhemisphere cross the equator between 40° and 60°E longitudes and start blowing fromsouthwest to northeast due to the Coriolis force. It becomes southwest monsoon. In winter,the ITCZ moves southward, and so the reversal of winds from northeast to south andsouthwest, takes place. They are called northeast monsoons.

THE NATURE OF INDIAN MONSOON

Monsoon is a familiar though a little knownclimatic phenomenon. Despite the observationsspread over centuries, the monsoon continuesto puzzle the scientists. Many attempts havebeen made to discover the exact nature andcausation of monsoon, but so far, no singletheory has been able to explain the monsoonfully. A real breakthrough has come recentlywhen it was studied at the global rather thanat regional level.

Systematic studies of the causes of rainfallin the South Asian region help to understandthe causes and salient features of the monsoon,particularly some of its important aspects,such as:

(i) The onset of the monsoon.(ii) Rain-bearing systems (e.g. tropical

cyclones) and the relationship betweentheir frequency and distribution ofmonsoon rainfall.

(iii) Break in the monsoon.

Onset of the Monsoon

Towards the end of the nineteenth century, itwas believed that the differential heating ofland and sea during the summer months isthe mechanism which sets the stage for themonsoon winds to drift towards thesubcontinent. During April and May when thesun shines vertically over the Tropic of Cancer,the large landmass in the north of Indian oceangets intensely heated. This causes theformation of an intense low pressure in thenorthwestern part of the subcontinent. Sincethe pressure in the Indian Ocean in the southof the landmass is high as water gets heated

Figure 4.3 : The Direction of Winds at 13 kmAltitude in Summer Season


The shift in the position of the ITCZ is alsorelated to the phenomenon of the withdrawalof the westerly jet stream from its position overthe north Indian plain, south of the Himalayas.The easterly jet stream sets in along 15°Nlatitude only after the western jet stream haswithdrawn itself from the region. This easterlyjet stream is held responsible for the burst ofthe monsoon in India.

Entry of Monsoon into India : The southwestmonsoon sets in over the Kerala coast by 1stJune and moves swiftly to reach Mumbai andKolkata between 10th and 13th June. By mid-July, southwest monsoon engulfs the entiresubcontinent (Figure 4.5)

Rain-bearing Systems and RainfallDistribution

There seem to be two rain-bearing systems inIndia. First originate in the Bay of Bengalcausing rainfall over the plains of north India.Second is the Arabian Sea current of the south-west monsoon which brings rain to the westcoast of India. Much of the rainfall along theWestern Ghats is orographic as the moist air isobstructed and forced to rise along the Ghats.The intensity of rainfall over the west coast ofIndia is, however, related to two factors:(i) The offshore meteorological conditions.(ii) The position of the equatorial jet stream

along the eastern coast of Africa.

EI-Nino and the Indian Monsoon

EI-Nino is a complex weather system that appears once every three to seven years, bringingdrought, floods and other weather extremes to different parts of the world.

The system involves oceanic and atmospheric phenomena with the appearance of warmcurrents off the coast of Peru in the Eastern Pacific and affects weather in many places includingIndia. EI-Nino is merely an extension of the warm equatorial current which gets replaced temporarilyby cold Peruvian current or Humbolt current (locate these currents in your atlas). This currentincreases the temperature of water on the Peruvian coast by 10°C. This results in:

(i) the distortion of equatorial atmospheric circulation;(ii) irregularities in the evaporation of sea water;(iii) reduction in the amount of planktons which further reduces the number of fish in the sea.

The word EI-Nino means ‘Child Christ’ because this current appears around Christmasin December. December is a summer month in Peru (Southern Hemisphere).

EI-Nino is used in India for forecasting long range monsoon rainfall. In 1990-91, therewas a wild EI-Nino event and the onset of southwest monsoon was delayed over most parts ofthe country ranging from five to twelve days.

Figure 4.4 : Onset of Monsoon

slowly, the low pressure cell attracts thesoutheast trades across the Equator. Theseconditions help in the northward shift in theposition of the ITCZ. The southwest monsoonmay thus, be seen as a continuation of thesoutheast trades deflected towards the Indiansubcontinent after crossing the Equator. Thesewinds cross the Equator between 40°E and60°E longitudes.

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Figure 4.5 : India : Normal Dates of Onset of the Southwest Monsoon


The frequency of the tropical depressionsoriginating from the Bay of Bengal varies fromyear to year. Their paths over India are mainlydetermined by the position of ITCZ which isgenerally termed as the monsoon trough. Asthe axis of the monsoon trough oscillates, thereare fluctuations in the track and direction ofthese depressions, and the intensity and theamount of rainfall vary from year to year. Therain which comes in spells, displays a decliningtrend from west to east over the west coast, andfrom the southeast towards the northwest overthe North Indian Plain and the northern partof the Peninsula.

Break in the Monsoon

During the south-west monsoon period afterhaving rains for a few days, if rain fails to occurfor one or more weeks, it is known as break inthe monsoon. These dry spells are quitecommon during the rainy season. Thesebreaks in the different regions are due todifferent reasons:(i) In northern India rains are likely to fail if

the rain-bearing storms are not veryfrequent along the monsoon trough orthe ITCZ over this region.

(ii) Over the west coast the dry spells areassociated with days when winds blowparallel to the coast.

THE RHYTHM OF SEASONS

The climatic conditions of India can best bedescribed in terms of an annual cycle ofseasons. The meteorologists recognise thefollowing four seasons :

(i) the cold weather season(ii) the hot weather season(iii) the southwest monsoon season(iv) the retreating monsoon season.

The Cold Weather Season

Temperature : Usually, the cold weatherseason sets in by mid-November in northernIndia. December and January are the coldestmonths in the northern plain. The mean dailytemperature remains below 21°C over mostparts of northern India. The night temperature

may be quite low, sometimes going belowfreezing point in Punjab and Rajasthan.

There are three main reasons for the excessivecold in north India during this season :

(i) States like Punjab, Haryana andRajasthan being far away from themoderating influence of sea experiencecontinental climate.

(ii) The snowfall in the nearby Himalayanranges creates cold wave situation; and

(iii) Around February, the cold winds comingfrom the Caspian Sea and Turkmenistanbring cold wave along with frost and fogover the northwestern parts of India.

Understanding the Monsoon

Attempts have been made tounderstand the nature andmechanism of the monsoon on thebasis of data collected on land, oceansand in the upper atmosphere. Theintensity of southwest monsoon windsof southern oscillation can bemeasured, among others, by measuringthe difference in pressure betweenTahiti (roughly 20°S and 140°W) inFrench Polynesia in East Pacific andport Darwin (12°30'S and 131°E) innorthern Australia. Indian MeteorologicalDepartment (IMD) can forecast thepossible behaviour of monsoons on thebasis of 16 indicators.

The Peninsular region of India, however,does not have any well-defined cold weatherseason. There is hardly any seasonal changein the distribution pattern of the temperaturein coastal areas because of moderatinginfluence of the sea and the proximity toequator. For example, the mean maximumtemperature for January at Thiruvanantapuramis as high as 31°C, and for June, it is 29.5°C.Temperatures at the hills of Western Ghatsremain comparatively low (Figure 4.6).

Pressure and Winds : By the end of December(22nd December), the sun shines verticallyover the Tropic of Capricorn in the southernhemisphere. The weather in this season ischaracterised by feeble high pressureconditions over the northern plain. In south

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Figure 4.6 : India : Mean Monthly Temperatures of the Day in January


India, the air pressure is slightly lower. Theisobars of 1019 mb and 1013 mb passthrough northwest India and far south,respectively (Figure 4.7).

As a result, winds start blowing fromnorthwestern high pressure zone to the low airpressure zone over the Indian Ocean in thesouth.

Due to low pressure gradient, the lightwinds with a low velocity of about 3-5 km perhour begin to blow outwards. By and large,the topography of the region influences thewind direction. They are westerly ornorthwesterly down the Ganga Valley. Theybecome northerly in the Ganga-Brahmaputradelta. Free from the influence of topography,they are clearly northeasterly over the Bay ofBengal.

During the winters, the weather in Indiais pleasant. The pleasant weather conditions,however, at intervals, get disturbed by shallowcyclonic depressions originating over the eastMediterranean Sea and travelling eastwardsacross West Asia, Iran, Afghanistan andPakistan before they reach the northwesternparts of India. On their way, the moisturecontent gets augmented from the Caspian Seain the north and the Persian Gulf in the south.What is the role of Westerly Jet Streams insteering these depressions in India?

Rainfall : Winter monsoons do not causerainfall as they move from land to the sea. It isbecause firstly, they have little humidity; andsecondly, due to anti cyclonic circulation onland, the possibility of rainfall from themreduces. So, most parts of India do not haverainfall in the winter season. However, there aresome exceptions to it:(i) In northwestern India, some weak

temperate cyclones from theMediterranean sea cause rainfall inPunjab, Haryana, Delhi and westernUttar Pradesh. Although the amountis meagre, it is highly beneficial forrabi crops. The precipitation is in thef o rm o f snow fa l l i n the l owerHimalayas. It is this snow thatsustains the flow of water in the

Himalayan rivers during the summermonths. The precipitation goes ondecreasing from west to east in theplains and from north to south in themountains. The average winter rainfallin Delhi is around 53 mm. In Punjaband Bihar, rainfall remains between 25mm and 18 mm respectively.

(ii) Central parts of India and northernparts of southern Peninsula also getwinter rainfall occasionally.

(iii) Arunachal Pradesh and Assam in thenortheastern parts of India also haverains between 25 mm and 50 mmduring these winter months.

(iv) During October and November,northeast monsoon while crossing overthe Bay of Bengal, picks up moistureand causes torrential rainfall over theTamil Nadu coast, southern AndhraPradesh, southeast Karnataka andsoutheast Kerala.

The Hot Weather Season

Temperature: With the apparent northwardmovement of the sun towards the Tropic ofCancer in March, temperatures start risingin north India. April, May and June are themonths of summer in north India. In mostparts of India, temperatures recorded arebetween 30°-32°C. In March, the highest daytemperature of about 38°C occurs in theDeccan Plateau while in April, temperatureranging between 38°C and 43°C are foundin Gujarat and Madhya Pradesh. In May,the heat belt moves further north, and in thenorth-western part of India, temperaturesaround 48°C are not uncommon (Figure 4.8).

The hot weather season in south India ismild and not so intense as found in northIndia. The Peninsular situation of southIndia with moderating effect of the oceanskeeps the temperatures lower than thatprevailing in north India. So, temperaturesremain between 26°C and 32°C. Due toaltitude, the temperatures in the hills ofWestern Ghats remain below 25°C. In thecoastal regions, the north-south extent of

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Figure 4.7 : India : Pressure and Surface Winds (January)


Figure 4.8 : India : Mean Monthly Temperature of the Day in July

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isotherms parallel to the coast confirms thattemperature does not decrease from northto south rather it increases from the coastto the interior. The mean daily minimumtemperature during the summer monthsalso remains quite high and rarely goesbelow 26°C.

Pressure and Winds : The summer monthsare a period of excessive heat and falling airpressure in the northern half of the country.Because of the heating of the subcontinent,the ITCZ moves northwards occupying aposition centred at 25°N in July. Roughly,this elongated low pressure monsoontrough extends over the Thar desert in thenorth-west to Patna and Chotanagpurplateau in the east-southeast (Figure 4.9).The location of the ITCZ attracts a surfacecirculation of the winds which aresouthwesterly on the west coast as well asalong the coast of West Bengal andBangladesh. They are easterly or south-easterly over north Bengal and Bihar. It hasbeen discussed earlier that these currentsof southwesterly monsoon are in reality‘displaced’ equatorial westerlies. The influxof these winds by mid-June brings about achange in the weather towards the rainyseason.

In the heart of the ITCZ in the northwest,the dry and hot winds known as ‘Loo’, blowin the afternoon, and very often, theycontinue to well into midnight. Dust stormsin the evening are very common during Mayin Punjab, Haryana, Eastern Rajasthan andUttar Pradesh. These temporary stormsbring a welcome respite from the oppressingheat since they bring with them light rainsand a pleasant cool breeze. Occasionally, themoisture-laden winds are attracted towardsthe periphery of the trough. A suddencontact between dry and moist air massesgives rise to local storms of great intensity.These local storms are associated withviolent winds, torrential rains and evenhailstorms.

Some Famous Local Storms of HotWeather Season

(i) Mango Shower : Towards the end ofsummer, there are pre-monsoonshowers which are a commonphenomena in Kerala and coastalareas of Karnataka. Locally, theyare known as mango showers sincethey help in the early ripening ofmangoes.

(ii) Blossom Shower : With this shower,coffee flowers blossom in Kerala andnearby areas.

(iii) Nor Westers : These are dreadedevening thunderstorms in Bengaland Assam. Their notorious naturecan be understood from the localnomenclature of ‘Kalbaisakhi’, acalamity of the month of Baisakh.These showers are useful for tea,jute and rice cultivation. In Assam,these storms are known as “BardoliChheerha”.

(iv) Loo : Hot, dry and oppressing windsblowing in the Northern plains fromPunjab to Bihar with higherintensity between Delhi and Patna.

THE SOUTHWEST MONSOON SEASON

As a result of rapid increase of temperature inMay over the northwestern plains, the lowpressure conditions over there get furtherintensified. By early June, they are powerfulenough to attract the trade winds of SouthernHemisphere coming from the Indian Ocean.These southeast trade winds cross the equatorand enter the Bay of Bengal and the ArabianSea, only to be caught up in the air circulationover India. Passing over the equatorial warmcurrents, they bring with them moisture inabundance. After crossing the equator, theyfollow a southwesterly direction. That is whythey are known as southwest monsoons.

The rain in the southwest monsoon seasonbegins rather abruptly. One result of the firstrain is that it brings down the temperaturesubstantially. This sudden onset of themoisture-laden winds associated withviolent thunder and lightening, is oftentermed as the “break” or “burst” of the


Figure 4.9 : India : Pressure and Surface Winds (July)

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monsoons. The monsoon may burst in thefirst week of June in the coastal areas of Kerala,Karnataka, Goa and Maharashtra while in theinterior parts of the country, it may be delayedto the first week of July. The day temperatureregisters a decline of 5°C to 8°C between mid-June and mid-July.As these winds approach the land, theirsouthwesterly direction is modified by the reliefand thermal low pressure over the northwestIndia. The monsoon approaches the landmassin two branches:(i) The Arabian Sea branch(ii) The Bay of Bengal branch.

Monsoon Winds of the Arabian Sea

The monsoon winds originating over theArabian Sea further split into three branches:(i) Its one branch is obstructed by the

Western Ghats. These winds climb theslopes of the Western Ghats from900-1200 m. Soon, they become cool,and as a result, the windward side of theSahyadris and Western Coastal Plainreceive very heavy rainfall rangingbetween 250 cm and 400 cm. Aftercrossing the Western Ghats, these windsdescend and get heated up. This reduceshumidity in the winds. As a result, thesewinds cause little rainfall east of theWestern Ghats. This region of low rainfallis known as the rain-shadow area. Findout the rainfall at Kozhikode, Mangalore,Pune and Bangalore and note thedifference (Figure 4.10).

(ii) Another branch of the Arabian seamonsoon strikes the coast north ofMumbai. Moving along the Narmada andTapi river valleys, these winds causerainfall in extensive areas of central India.The Chotanagpur plateau gets 15 cmrainfall from this part of the branch.Thereafter, they enter the Ganga plainsand mingle with the Bay of Bengal branch.

(iii) A third branch of this monsoon windstrikes the Saurashtra Peninsula and theKachchh. It then passes over westRajasthan and along the Aravallis,causing only a scanty rainfall. In Punjab

and Haryana, it too joins the Bay ofBengal branch. These two branches,reinforced by each other, cause rains inthe western Himalayas,

Monsoon Winds of the Bay of Bengal

The Bay of Bengal branch strikes the coastof Myanmar and part of southeastBangladesh. But the Arakan Hills along thecoast of Myanmar deflect a big portion of thisbranch towards the Indian subcontinent. Themonsoon, therefore, enters West Bengal andBangladesh from south and southeastinstead of from the south-westerly direction.From here, this branch splits into two underthe influence of the Himalayas and thethermal low is northwest India. Its onebranch moves westward along the Gangaplains reaching as far as the Punjab plains.The other branch moves up the Brahmaputravalley in the north and the northeast, causingwidespread rains. Its sub-branch strikes theGaro and Khasi hil ls of Meghalaya.Mawsynram, located on the crest of Khasihills, receives the highest average annualrainfall in the world.

Here it is important to know why the TamilNadu coast remains dry during this season.There are two factors responsible for it:

(i) The Tamil Nadu coast is situatedparallel to the Bay of Bengal branch ofsouthwest monsoon.

(ii) It lies in the rainshadow area of theArabian Sea branch of the south-westmonsoon.

Characteristics of Monsoonal Rainfall

(i) Rainfall received from the southwestmonsoons is seasonal in character,which occurs between June andSeptember.

(ii) Monsoonal rainfall is largely governedby relief or topography. For instancethe windward side of the Western Ghatsregister a rainfall of over 250 cm. Again,the heavy rainfall in the northeasternstates can be attributed to their hillranges and the Eastern Himalayas.


Figure 4.10 : India : Seasonal Rainfall (June-September)

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(iii) The monsoon rainfall has a decliningtrend with increasing distance from thesea. Kolkata receives 119 cm duringthe southwest monsoon period, Patna105 cm, Allahabad 76 cm and Delhi56 cm.

(iv) The monsoon rains occur in wet spellsof few days duration at a time. The wetspells are interspersed with rainlessinterval known as ‘breaks’. These breaksin rainfall are related to the cyclonicdepressions mainly formed at the headof the Bay of Bengal, and their crossinginto the mainland. Besides the frequencyand intensity of these depressions, thepassage followed by them determinesthe spatial distribution of rainfall.

(v) The summer rainfall comes in a heavydownpour leading to considerable runoff and soil erosion.

(vi) Monsoons play a pivotal role in theagrarian economy of India because overthree-fourths of the total rain in thecountry is received during the south-west monsoon season.

(vii) Its spatial distribution is also unevenwhich ranges from 12 cm to more than250 cm.

(viii) The beginning of the rains sometimesis considerably delayed over the wholeor a part of the country.

(ix) The rains sometimes end considerablyearlier than usual, causing greatdamage to standing crops and makingthe sowing of winter crops difficult.

Season of Retreating Monsoon

The months of October and November areknown for retreating monsoons. By the endof September, the southwest monsoonbecomes weak as the low pressure trough ofthe Ganga plain starts moving southward inresponse to the southward march of the sun.The monsoon retreats from the westernRajasthan by the first week of September. Itwithdraws from Rajasthan, Gujarat, WesternGanga plain and the Central Highlands by theend of the month. By the beginning of October,the low pressure covers northern parts of theBay of Bengal and by early November, it moves

over Karnataka and Tamil Nadu. By themiddle of December, the centre of low pressureis completely removed from the Peninsula.

The retreating southwest monsoon seasonis marked by clear skies and rise intemperature. The land is still moist. Owing tothe conditions of high temperature andhumidity, the weather becomes ratheroppressive. This is commonly known as the‘October heat’. In the second half of October,the mercury begins to fall rapidly, particularlyin northern India. The weather in theretreating monsoon is dry in north India butit is associated with rain in the eastern part ofthe Peninsula. Here, October and Novemberare the rainiest months of the year.

The widespread rain in this season isassociated with the passage of cyclonicdepressions which originate over theAndaman Sea and manage to cross theeastern coast of the southern Peninsula. Thesetropical cyclones are very destructive. Thethickly populated deltas of the Godavari,Krishna and Kaveri are their preferred targets.Every year cyclones bring disaster here. A fewcyclonic storms also strike the coast of WestBengal, Bangladesh and Myanmar. A bulk ofthe rainfall of the Coromondal coast is derivedfrom these depressions and cyclones. Suchcyclonic storms are less frequent in theArabian Sea.

TRADITIONAL INDIAN SEASONS

In the Indian tradition, a year is divided intosix two-monthly seasons. This cycle of seasons,which the common people in north and centralIndia follow is based on their practicalexperience and age-old perception of weatherphenomena. However, this system does notmatch with the seasons of south India wherethere is little variation in the seasons.

Seasons Months Months(According to the (According to theIndian Calendar) Indian Calendar)

Vasanta Chaitra-Vaisakha March-AprilGrishma Jyaistha-Asadha May-JuneVarsha Sravana-Bhadra July-AugustSharada Asvina-Kartika September-OctoberHemanta Margashirsa-Pausa November-DecemberShishira Magha-Phalguna January-February


Distribution of Rainfall

The average annual rainfall in India is about125 cm, but it has great spatial variations(Figure 4.11).

Areas of High Rainfall : The highest rainfalloccurs along the west coast, on the WesternGhats, as well as in the sub-Himalayan areasis the northeast and the hills of Meghalaya. Herethe rainfall exceeds 200 cm. In some parts ofKhasi and Jaintia hills, the rainfall exceeds1,000 cm. In the Brahmaputra valley and theadjoining hills, the rainfall is less then 200 cm.

Areas of Medium Rainfall : Rainfall between100-200 cm is received in the southern partsof Gujarat, east Tamil Nadu, northeasternPeninsula covering Orissa, Jharkhand, Bihar,eastern Madhya Pradesh, northern Ganga plainalong the sub-Himalayas and the Cachar Valleyand Manipur.

Areas of Low Rainfall : Western Uttar Pradesh,Delhi, Haryana, Punjab, Jammu and Kashmir,eastern Rajasthan, Gujarat and Deccan Plateaureceive rainfall between 50-100 cm.

Areas of Inadequate Rainfall: Parts of thePeninsula, especially in Andhra Pradesh,Karnataka and Maharashtra, Ladakh and mostof western Rajasthan receive rainfall below 50 cm.

Snowfall is restricted to the Himalayanregion.

Identify the pattern of rainfall afterconsulting the rainfall map.

Variability of Rainfall

A characteristic feature of rainfall in India is itsvariability. The variability of rainfall is computedwith the help of the following formula:

C.V.Standard Deviation

Mean 100= ×

where C.V. is the coefficient of variation.The values of coefficient of variation show

the change from the mean values of rainfall. Theactual rainfall in some places deviates from20-50 per cent. The values of coefficient ofvariation show variability of rainfall in India. Avariability of less than 25 per cent exists on thewestern coasts, Western Ghats, northeastern

Peninsula, eastern plains of the Ganga,northeastern India, Uttaranchal and HimachalPradesh and south-western part of Jammu andKashmir. These areas have an annual rainfallof over 100 cm. A variability of over 50 per centexists in the western part of Rajasthan, northernpart of Jammu and Kashmir and interior partsof the Deccan plateau. These areas have anannual rainfall of less than 50 cm. Rest of Indiahave a variability of 25-50 per cent and theseareas receive an annual rainfall between50 -100 cm (Figure 4.12).

Climatic Regions of India

The whole of India has a monsoon type ofclimate. But the combination of elements of theweather, however, reveal many regionalvariations. These variations represent the sub-types of the monsoon climate. It is on this basisthat the climatic regions can be identified. Aclimatic region has a homogeneous climaticcondition which is the result of a combinationof factors. Temperature and rainfall are twoimportant elements which are considered to bedecisive in all the schemes of climaticclassification. The classification of climate,however, is a complex exercise. There aredifferent schemes of classification of climate.Major climatic types of India based onKoeppen’s scheme have been described below:

Koeppen based his scheme of Climaticclassification on monthly values of temperatureand precipitation. He identified five majorclimatic types, namely:

(i) Tropical climates, where mean monthlytemperature throughout the year is over18°C.

(ii) Dry climates, where precipitation is verylow in comparison to temperature, andhence, dry. If dryness is less, it is semi-arid (S); if it is more, the climate is arid(W).

(iii) Warm temperate climates, where meantemperature of the coldest month isbetween 18°C and minus 3°C.

(iv) Cool temperate climates, where meantemperature of the warmest month is over10°C, and mean temperature of thecoldest month is under minus 3°C.

(v) Ice climates, where mean temperature ofthe warmest month is under 10°C.

51CLIMATE

Figure 4.11 : India : Annual Rainfall


Figure 4.12 : India : Variability of Annual Rainfall

53CLIMATE

Koeppen used letter symbols to denoteclimatic types as given above. Each type isfurther sub-divided into sub-types on thebasis of seasonal variations in thedistributional pattern of rainfall andtemperature. He used S for semi-arid and Wfor arid and the following small letters to definesub-types: f (sufficient precipitation), m (rainforest despite a dry monsoon season), w (dryseason in winter), h (dry and hot), c (less thanfour months with mean temperature over10°C), and g (Gangetic plain). Accordingly,India can be divided into eight climatic regions(Table 4.1; Figure 4.13).

Monsoons and the Economic Life in India

(i) Monsoon is that axis around whichrevolves the entire agricultural cycle ofIndia. It is because about 64 per centpeople of India depend on agriculture fortheir livelihood and agriculture itself isbased on southwest monsoon.

(ii) Except Himalayas all the parts of thecountry have temperature above thethreashold level to grow the crops orplants throughout the year..

(iii) Regional variations in monsoon climatehelp in growing various types of crops.

(iv) Variability of rainfall brings droughts orfloods every year in some parts of the country.

(v) Agricultural prosperity of India dependsvery much on timely and adequatelydistributed rainfall. If it fails, agricultureis adversely affected particularly in thoseregions where means of irrigation are notdeveloped.

(vi) Sudden monsoon burst creates problemof soil erosion over large areas in India.

(vii) Winter rainfall by temperate cyclones innorth India is highly beneficial for rabi crops.

(viii) Regional climatic variation in India isreflected in the vast variety of food, clothesand house types.

GLOBAL WARMING

You know that change is the law of nature.Climate has also witnessed change in the pastat the global as well as at local levels. It ischanging even now but the change isimperceptible. A number of geologicalevidences suggest that once upon a time,

(see geological time scale in Chapter 2 ofFundamentals of Physical Geography, NCERT,2006) large part of the earth was under ice cover.Now you might have read or heard the debateon global warming. Besides the natural causes,human activities such as large scaleindustrialisation and presence of polluting gasin the atmosphere are also important factorsresponsible for global warming. You might haveheard about the “green house effect” whilediscussing global warming.

The temperature of the world issignificantly increasing. Carbon dioxideproduced by human activities is a majorsource of concern. This gas, released to theatmosphere in large quantities by burningof fossil fuel, is increasing gradually. Othergases like methane, chlorofluorocarbons,and nitrous oxide which are present in muchsmaller concentrations in the atmosphere,together with carbon dioxide are known asgreen house gases. These gases are betterabsorbers of long wave radiations than carbon

Table 4.1 : Climatic Regions of India According to Koeppen’s Scheme

Type of Climate Areas

Amw Monsoon with short dry season West coast of India south of GoaAs – Monsoon with dry summer Coromandel coast of Tamil NaduAw – Tripical savannah Most of the Peninsular plateaus, south of the Tropic of CancerBwhw – Semi-arid steppe climate North-western Gujarat, some parts of western Rajasthan and

PunjabBwhw – Hot desert Extreme western RajasthanCwg – Monsoon with dry winter Ganga plain, eastern Rajasthan, northern Madhya Pradesh,

most of North-east IndiaDfc – Cold humid winter with short summer Arunachal PradeshE – Polar type Jammu and Kashmir, Himachal Pradesh and Uttaranchal


Figure 4.13 : India : Climatic Regions According to Koppen’s Scheme

55CLIMATE

dioxide, and so, are more effective atenhancing the green house effect. Thesegases have been contributing to globalwarming. It is said that due to global warmingthe polar ice caps and mountain glacierswould melt and the amount of water in theoceans would increase.

The mean annual surface temperatureof the earth in the past 150 years hasincreased. It is projected that by the year2,100, global temperature will warm about2°C. This r ise in temperature wi l laccompany many other changes: one ofthese is a rise in sea level, as glacier and sea

ice melt in response to warming. Accordingto the current prediction, on an average, thesea level will rise 48 cm by the end of twentyfirst century. This would increase theincidence of annual flooding. Climaticchange would promote insect-bornediseases like malaria, and lead to shift inclimatic boundaries, making some regionswetter and others drier. Agricultural patternwould shift and human population as wellas the ecosystem would experience change.What would happen to the Indian seacoasts if the sea level rises 50 cm above thepresent one?

1. Choose the right answer from the four alternatives given below.

(i) What causes rainfall on the coastal areas of Tamil Nadu in the beginningof winters?(a) South-West monsoon (c) North-Eastern monsoon

(b) Temperate cyclones (d) Local air circulation

(ii) What is the proportion of area of India which receives annual rainfall lessthan 75 cm?(a) Half (c) Two-third

(b) One-third (d) Three-fourth

(iii) Which one of the following is not a fact regarding South India?(a) Diurnal range of temperature is less here.

(b) Annual range of temperature is less here.

(c) Temperatures here are high throughout the year.

(d) Extreme climatic conditions are found here.

(iv) Which one of the following phenomenon happens when the sun shinesvertically over the Tropic of Capricorn in the southern hemisphere?(a) High pressure develops over North-western India due to low

temperatures.(b) Low pressure develops over North-western India due to high

temperatures.(c) No changes in temperature and pressure occur in north-western

India.(d) ‘Loo’ blows in the North-western India.

(v) In which of the following states in India do we find ‘As’ type of climate asper Koeppen’s classification?(a) In Kerala and coastal Karnataka(b) In Andaman and Nicobar Islands(c) On Coromandal coast(d) In Assam and Arunachal Pradesh

EXERCISES


2. Answer the following questions in about 30 words.(i) What are the three important factors which influence the mechanism of

Indian weather?(ii) What is the Inter-Tropical Convergene Zone?(iii) What is meant by ‘bursting of monsoon’? Name the place of India which

gets the highest rainfall.(iv) Define ‘climatic region’? What are the bases of Koeppen’s classification?(v) Which type(s) of cyclones cause rainfall in north-western India during

winter? Where do they originate?

3. Answer the following questions in not more than 125 words.

(i) Notwithstanding the broad climatic unity, the climate of India has manyregional variations. Elaborate this statement giving suitable examples.

(ii) How many distinct seasons are found in India as per the IndianMeteorological Department? Discuss the weather conditions associatedwith any one season in detail.

Project/ActivityOn the outline map of India, show the following:(i) Areas of winter rain(ii) Wind direction during the summer season(iii) Areas having variability of rainfall over 50 per cent(iv) Areas having less than 15°C temperature in January(v) Isohyte of 100 cm.

climate, vegetation and soil - prashanth...

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