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PRECIPITATION

(Atmospheric)

by

Prof. A. Balasubramanian

Centre for Advanced Studies in Earth Science

University of Mysore, India

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Precipitation or Rainfall:

Precipitation is the natural process of conversion

of atmospheric water vapour into water. The water

falls(comes down) in the form of a rainfall or

snow fall.

The term precipitation is also used to refer

rainfall.

It is term and includes all forms of falling

moisture viz., rainfall, snowfall, sleet, hail etc.

Rainfall occurs in the form of a pattern.

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CORIOLIS EFFECT:

Precipitation patterns are determined by the

movement of large air masses. These are affected

greatly by the Coriolis effect.

Air cools with increasing height throughout the

troposphere.

The rate at which it cools is called the lapse rate

and equals about 3.6'F per 1,000 ft (6.5'C per km).

Reasons for change of state:

i. Hot air mass has large capacity to hold the

vapour particles in suspension.

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When this moist and warm air mass cools down,

its capacity to hold vapour particles is reduced.

Finally vapour gets precipitated in the form of

rainfall.

ii. Sometimes variation in pressure brings about

the change of state, from vapour to rainfall.

By a process known as nucleation, ice or water

crystals are formed upon the floating particles, in

the air mass (e.g., dust particles, salt particles,

etc.).

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The small crystals then grow in the size by

combining with other crystals.

A stage comes when they fall down on the earth as

snow or as rain water.

Forms of Precipitation:

Precipitation takes place in many different forms.

Dew is condensation on the ground of atmospheric

vapor caused by radiational cooling of the lower

layers of atmosphere, usually at night. Frost is dew

formed under freezing conditions.

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(i) Rain:

It consists of water drops mostly larger than 0.5

mm in diameter.

Drops bigger than 6 mm tend to break up as they

fell.

(ii) Drizzle:

They are tiny water droplets of size between 0.1 to

0.5 mm which fall with such slow settling rates

that they occasionally appear to float.

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(iii) Snow:

It is that type of precipitation which results from

sublimation, i.e., water vapour directly changes

into ice.

It falls as white or translucent ice crystals often

agglomerated into snowflakes.

The specific gravity of snow is often taken to be

0.1.

(iv) Hail:

It is the precipitation in form of lumps of ice.

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The hail stones are produced in convective clouds

mostly cumulonimbus.

Their shape may be conical, spheroidal or

irregular.

The size of hail stones may be anything more than

5 mm.

The specific gravity of hail stone is about 0.8.

(v) Snow Pallets:

Sometimes they are called soft hail also.

Snow pallets are more crisp and are of size 2 to 5

mm.

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Due to their, crispness upon hitting the hard

ground they often break up.

(vi) Sleet:

When the rain drops fall through the layer of sub-

freezing air near the earth’s surface the rain drops

get frozen to ice stage.

It is called sleet or grains of ice.

Mechanisms for Production of Rainfall

Mechanism to produce cooling of the air –

Mechanism to produce condensation.

Mechanism for droplet growth.

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Mechanism to produce accumulation of moisture

of sufficient intensity to account for the observed

rates of rainfall.

Types of Precipitation:

There are three major types of precipitation:

cyclonic, convective, and orographic.

Each type represents a different method of lifting

of the air mass, resulting in cooling and

condensation of atmospheric water vapor.

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Cyclonic Precipitation:

It is caused by lifting associated with the

horizontal convergence of inflowing atmosphere

into an area of low pressure.

There are two kinds of cyclonic precipitation.

Non-frontal precipitation involves only this

convergence and lifting.

Frontal precipitation results when one air mass is

lifted over another.

A front is defined as the boundary between two air

masses of different temperatures and densities.

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A warm front is the result of a warm air mass

overriding a cold air mass, causing extensive areas

of cloudiness and precipitation.

As the warm front approaches a given area, the

precipitation becomes more continuous and

intense. Warm fronts move at a speed of 15-50

km/h (10-30 mph).

A cold front results from a strong push of a cold

air mass against and beneath a warm air mass.

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At the front towering clouds develop together with

intense short duration precipitation. Cold fronts

move at a speed of 30-80 km/h (20-50 mph).

Orographic Precipitation:

It is caused when air masses are lifted as they

move over mountain barriers.

Such orographic barriers tend to increase both

cyclonic and orographic precipitation due to the

increased lifting involved.

Precipitation is generally heavier on the windward

slope than on the leeward slope.

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Convective Precipitation:

Due to some local effects air gets heated up and

stores more vapour particles.

Then it rises up in the atmosphere as it is lighter

than the cold air surrounding that area.

At high altitudes it gets cooled and precipitation

occurs.

The intensity of this type of precipitation may

range from light showers to cloud bursts.

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Variation of Rainfall:

Factors responsible for inequitable distribution of

rainfall over large area are the following:

1. Nearness to Sea:

From the sea very large quantity of water goes to

the atmosphere in the form of vapour.

Naturally when excessively moisture laden clouds

pass over the sea coast, clouds drop off some of

their load.

As a result coastal area receives more rainfall.

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2. Presence of Mountains:

Windward side slope of the side towards which

clouds travel gets excessive rains whereas on the

other or leeward side slope there is area of rain-

shadow.

Mountainous region receives more rainfall than

plain areas.

3. Direction of Wind:

Clouds are driven by wind.

It is clear that the area over which wind brings

clouds will get rainfall.

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4. Development of Forest:

The forests also behave to some extent as a barrier

and intercept the clouds to derive rainfall.

The area with thick forest gets more rainfall.

5. Height of a Place Above Sea Level or

Altitude:

The places of high altitude receive more

precipitation.

At high altitudes temperature of atmosphere is low

and when clouds reach that area they get cooled

and precipitation occurs.

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Rainfall Measurement:

Rain gauge:

The purpose of the rain gauge is to measure the

depth and intensity of rain falling on a flat surface

without considering infiltration, runoff or

evaporation.

The problems of measurements include effects of

topography, nearby vegetation and the design of

gauge itself.

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Types of Rain gauges:

There are mainly two types of rain-gauges (non-

recording and recording).

Non-recording Gauge:

The standard rain gauge, known as Symon’s gauge

is recommended and installed by the Indian

Meteorological Department. This is a vertical,

cylindrical container with top opening 127 cm in

diameter. A funnel shaped hood is inserted to

minimize evaporation losses. The water is

funneled into an inner cylinder.

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Recording or Automatic Rain gauge:

Weighing Bucket Type Rain gauge -

This gauge weighs the rain, which falls into a

bucket set on a platform of a spring or level

balance.

The increasing weight of bucket and its counts are

recorded on the chart held by a clock driven drum.

The record shows the accumulation of

precipitation with time in the shape of a mass

curve of precipitation.

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The gauge must be serviced about once a week

when the clock is re-wound and the chart is

replaced.

For high rainfall, the recording mechanism

reverses the direction of record immediately on

reaching the upper edge of the recording chart.

Tipping Bucket Type Rain gauge -

The tipping bucket rain gauge consists of a 30 cm

diameter sharp edge receiver.

At the end of the receiver a funnel is provided.

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A pair of buckets are pivoted under the funnel in

such a way that when one bucket receives 0.25

mm of rainfall it tips, discharging its contents in to

a tank bringing the other bucket under the funnel.

Tipping of the bucket completes an electric circuit

causing the movement of a pen to mark on a clock

driven revolving drum which carries a record

sheet.

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Siphon Type Automatic Rainfall Recorder -

In the siphon gauge, also known as the float type

of recording rain gauge, the rain is fed into a float

chamber containing a light, hallow float.

The vertical movement of the float, as the level of

water rises, is transmitted by a suitable mechanism

in to the movement of the pen on a revolving

chart.

By suitably adjusting the dimensions of the

receiving funnel, float and float chamber, any

desired scale value on the chart can be obtained.

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Siphoning arrangement is provided for emptying

the float chamber quickly whenever it becomes

full, the pen returns to the bottom of the chart.

Uses of Rainfall Records:

At each rain gauging station, the rainfall is

measured after 24 hours.

Usually the measurement is taken at 0830 hours

IST obviously total rainfall occurred in the past 24

hours is entered against the date on which

measurement is done.

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The rainfall records are maintained on daily,

monthly, seasonally or yearly basis for any basin.

The rainfall varies from year-to-year.

The average of the series of yearly records gives

mean rainfall value.

The long term mean is called the normal rainfall.

Main uses of rainfall records:

1. The trend of rainfall can be studied from rainfall

records. Knowing the trend of rainfall future

predictions can be done.

2. Runoff over the basin can be calculated.

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3. Maximum flood due to any storm can be

calculated, and predicted.

4. Rainfall records help in estimating irrigation

requirements.

Considerations for Installation:

The site should be an open place.

The distance between the rain gauge and the

nearest object should be at least twice the height of

the object.

As for as possible it should be a level ground.

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In the hills, the site should be so chosen where it is

best shielded from high winds and wind does not

cause eddies, and if a fence is erected, it should be

at least at a distance of twice the height.

Errors in Rainfall Measurements :

There are three main sources of errors in rainfall

measurements –

a) instrumental defects,

b) improper sitting (location) of the gauge, and

c) human errors.

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Rain gauge Network:

The adequacy of an existing rain-gauge network of

a watershed is assessed statistically.

The optimum number of rain gauges

corresponding to an assigned percentage of error in

estimation of mean areal rainfall can be obtained

as:

(1.1)

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Where, N is the optimum number of rain gauges,

CV is the coefficient of variation of the rainfall

values of the gauges, and is the assigned

percentage of error in estimation of mean areal

rainfall.

(1.2)

In which P is the mean rainfall defined as

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(1.3)

and S is the standard deviation of rainfall

computed as

(5.4)

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Where, m is the number of raingauges in the

watershed recording P1, P2… Pm values of rainfall

for fixed time interval.

Generally, value of is taken as 10%.

Example:

A catchment has six rain gauge stations. In a year,

the annual rainfalls recorded by the gauges are as

follows:

Stations A B C D E F

Rainfall (cm) 82.6 102.9 180.3 110.3 98.8 136.7

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For a 10% error in the estimation of mean rainfall,

calculate optimum number of stations in the

catchment.

Solution:

Number of stations (m) = 6,

Mean precipitation = 118.6 cm

Standard deviation of precipitation (S) = 35.04

Error (ε) = 10%

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Indian Standard Recommendation:

One station per 520 km2 –in plains.

One station per 260-390 km2– in regions of

average elevation of 1000 m.

One station per 130 km2 – in predominantly

hilly areas with heavy rainfall

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Presentation of Rainfall Data:

Mass Curve of Rainfall:

The mass curve of rainfall is a plot of the

accumulated precipitation against time, plotted in

chronological order.

Hyetograph:

A hyetograph is a plot of the intensity of rainfall

against the time in the hyetograph is derived from

the mass curve and is usually represented as a bar

chart.

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Depth-Area-Duration Relationships:

The areal distribution characteristics of a storm of

given duration is reflected in its depth-area-

relationship.

Estimation of Mean Areal Rainfall:

A single point precipitation measurement is quite

often not representative of the volume of

precipitation falling over a given catchment area.

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A network of precipitation measurement points

can be converted to areal estimates using any of

the following techniques:

1. Arithmetic or Station Average Method

2. Thiessen Polygon Method

3. Isohyetal Method.

Rain-making:

Rainmaking, also called cloud seeding, is a process

that makes rain fall from a cloud.

The seeding agent is sprayed from an aeroplane

into the bottom of a cloud.

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Ice crystals can be produced by using such seeding

agents as dry ice or crystals of silver iodide.

When the ice crystals form, they fall toward the

earth as snowflakes.

As the flakes enter a region that has a temperature

higher than 0 °C, they melt into rain.

Silver iodide crystals resemble crystals of ice and

cause super-cooled water to form ice crystals

around them.