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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HYDROLOGIC PRECIPITATION
by
Prof. A. Balasubramanian
Centre for Advanced Studies in Earth Science
University of Mysore, India
Introduction:
Precipitation is any product of the condensation of atmospheric water vapor that
falls under gravity. The main forms of precipitation include drizzle, rain, sleet,
snow, and hail. Precipitation occurs when a portion of the atmosphere becomes
saturated with water vapor, so that the water condenses and "precipitates".
However, fog and mist are not precipitation but they are suspensions.
Precipitation is a major component of the hydrologic cycle, and is responsible
for depositing the fresh water on the planet. Approximately 505,000 cubic
kilometres of water falls as precipitation each year. Climate classification
systems such as the Köppen climate classification system use average annual
rainfall to help differentiate between differing climate regimes. The amount of
precipitation that falls around the world may range from 1mm to several cm
per year.
World Distribution of Rainfall
Different places on the earth’s surface receive different amounts of rainfall in a
year. It also varies in different seasons. In general, as we move away from the
equator towards the poles, the amount of rainfall goes on decreasing steadily.
The coastal areas of the world receive greater amounts of rainfall than the
interior of the continents. The rainfall is more over the oceans than on the
landmasses of the world because of being great sources of water.
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.
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.). 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.
Concept of Hydrologic Cycle
Water gets transformed from liquid to solid, solid to liquid, liquid to vapour,
vapour to liquid and vapour to solid states. The sun’s radiation, acceleration due
to gravity, ability of the water to flow and several other properties of water,
make this transformation more effective and regular. The basic input to the
world’s water masses comes from precipitation. Precipitated rain (or) snow
falls overland. Processes like infiltration and peroration moves the water down
to the groundwater systems. Some amount of water flows towards the sea as
runoff. The surface water collected in lakes, ponds, swamps, seas and oceans
get evaporated into the atmosphere. The vegetation transpires the water
collected from the soil moisture. Evaporated and transported water enters into
the atmosphere as vapour. Collected water vapour gets condensed to form the
clouds. Clouds more towards the land and starts precipitating again. These
processes continue. This endless circulation of water is known as the hydrologic
cycle.
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.
(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. 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.
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.
How rain is formed?
Rain is liquid water in the form of droplets that have condensed from
atmospheric water vapor and then becomes heavy enough to fall under gravity.
The oceans are the chief source of rain, but lakes and rivers also contribute to it.
The Sun's heat evaporates the water. It remains in the atmosphere as an invisible
vapor until it condenses, first into clouds and then into raindrops. Condensation
happens when the air is cooled.
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.
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. 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. 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.
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.
Variation of Rainfall:
Factors responsible for inequitable distribution of rainfall over large area are the
following:

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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.
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.
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.
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.
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. 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. 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.
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. 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. 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:

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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.
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. 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.
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)
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
(1.3)
and S is the standard deviation of rainfall computed as

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(5.4)
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
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%
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
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.

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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.
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.
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.
Precipitation and Water Balance:
The direct input sources of water for the lands and seas are from precipitation. The
major output sources are from evaporation, transpiration, sublimation, interception
and evapotranspiration. There is a balance of water existing as storage in the form
of groundwater, surface water bodies as lakes and streams, ice caps and glaciers
and as seas and oceans. These components can be analysed using a simple mass
balance equation called as water balance equation. This equation considers the
inflow, outflow and changes in storage reservoirs of fresh and saltwater.
The basis of the equation is
Inflow = outflow  changes in storage.
This equation can be expanded as
P – E – T – RO = S
Where
P = Precipitation
E = Evaporation
T = Transpiration
RO = Runoff
S = Changes in storage

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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. 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.