The term precipitation is also used to refer all forms of falling moisture viz., rainfall, snowfall, sleet, hail etc. Rainfall occurs in the form of a pattern. Atmospheric Precipitation is a wonderful process for the whole globe to use. This module explains it in general.

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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.