This document defines precipitation and discusses its various forms and types. It describes factors that precipitation depends on and how it is measured. Average annual rainfall in India and Assam is provided. The document also discusses the impact of climate change on precipitation patterns, including a case study on the Brahmaputra Basin. Climate change is causing temperatures to rise and altering precipitation variability and seasonal patterns in some regions.

1. ANIRUDDHA SINGHA
Water Resource and Management Engineering
Indian Institute of Technology Guwahati
13-08-2018

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Definition
 Precipitation may be defined by the hydrologist as those
hydrometeors which reach the earth.
 Precipitation is the basic unit to the Hydrological Cycle.
Average Annual Rainfall in
India is 119.4 cm
Average Annual Rainfall in
Assam is 281.8 cm

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Forms of precipitation
RAIN
Water Droplet of size
>0.5mm
Intensity > 1mm/hr
DRIZZLE
Water Droplet of size
<0.5mm
Intensity < 1mm/hr
SNOW
Ice Crystal of size <
1mm
SLEET
Ice Crystal of size 1mm to
5mm
HAIL
Lump of ice crystal of size
5mm to 125mm
MIST
Water Droplet of size
0.005mm to 0.05mm

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Types of Precipitation
CONVECTIVE PRECIPITATION
It occurs due to temperature difference between moist air parcel
(clouds) and their surroundings.
High intensity rain occurs for short period of time.
Summer rain is the example of convective precipitation

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OROGRAPHIC PRECIPITATION
Orographic Precipitation occurs due to topographic
obstructions such as Mountains or Hills.
Rains at Himalayas, Western Ghats & Eastern Ghats are the
examples of Orographic Precipitation.

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FRONTAL PRECIPITATION
Frontal Precipitation occurs when the leading edge of a warm air
mass meets a cool air mass.
The warmer air mass is forced up over the cool air. As it rises, the
warm air cools, moisture in the air condenses, clouds and
precipitation result.

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CYCLONIC PRECIPITATION
 Precipitation associated with passage of cyclone.
 It occurs on a large area of low pressure region.

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FACTORS ON WHICH
PRECIPITATION DEPENDS UPON
MOISTURE
NUCLEI
WEATHER
PRODUCT OF CONDENSATION MUST REACH THE EARTH

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FACTORS AFFECTING PRECIPITATION
EQUATOR AND LATITUDE
EVAPORATION
WIND
HUMIDITY
PRESSURE
TEMPERATURE

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VARIABILITY OF PRECIPITATION
 SPACE VARIATIONS
 SEASONAL VARIATIONS
 SOUTH-WEST MONSOON
(JUNE-SEPTEMBER)
 POST-MONSOON
(OCTOBER-NOVEMBER)
 WINTER SEASON
(DECEMBER-FEBRUARY)
 PRE-MONSOON (MARCH-
MAY)

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Measurement of Precipitation
Non Recording Gauge
Recording Gauge

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Non Recording Gauge
Standard gauge – mainly used for daily rainfall reading
Can be used to take monthly and weekly measurement
Storage gauge - recording entire season rainfall
Large storage capacity
Mainly for remote areas
 Commonly used in India
 Data is recorded manually at longer intervals
 Human error

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Recording Gauges
Automatically records rainfall depth
Used in inaccessible and remote areas
Tipping Bucket
Weighing bucket
Float gauge

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Tipping Bucket
 Consists of pair of buckets. One bucket fills up and brings other in
position
 This motion is recorded by device and rainfall intensity can be
measured.
 Good for remote areas.

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Weighing Bucket
 Funnel empties into a bucket mounted on a weighing scale.
 Rainfall is recorded as weight of bucket is proportional to rainfall
collected.
 Mass curve can be obtained.

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Float Gauge
 Rainfall collected by funnel leads to rise in flow
 Bucket empties automatically after it reaches pre set level and
reading resets to zero
 Vertical movement of float is translated into movement of pen on
chart.

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Whether rain gauge operates by rise of float, tipping of bucket or
accumulation of weight movement can be recorded.
Motion of mechanism can be converted into an electrical signal
transmitted to a distant receiver.

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Rain Gauge Network
According to IS 4978 rain gauge density should be as small as possible
Recommended rain Gauge network density
Plain area 1 in 520 km sq
Moderated elevated 1 in 260-390 km sq
Hilly area 1 in 130 km sq
According to WMO at least 10% of rain gauge station should be
equipped with recording type rain gauge

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Average precipitation over a catchment
Arithmetic mean method
least accurate, suitable for plain areas
Thiessen polygon method
good for non uniform rainfall
Isohyetal method
best method, topographic influences are taken into account

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IMPACT OF CLIMATE CHANGE ON
RAINFALL
 What is Climate Change?
 Change as defined by United Nations Framework Convention on
Climate Change 2007 as, “a change of climate which is attributed
directly or indirectly to human activity that alters the composition of
the global atmosphere and which is in addition to natural climate
variability observed over comparable time periods”.
 Climate change is caused by greenhouse gases which trap the
outgoing radiations and leads to increasing of the earth’s
temperature

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Global temperature
rising
Warming Oceans
Shrinking Ice Sheets Glacial Retreat
Courtesy :NASA

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IMPACT OF CLIMATE CHANGE
ON PRECIPITATION
TEMPERATURE
GLOBAL
WARMING
URBANIZATION
In arid and semi-arid landscape high
temperature leads to increase in
potential evaporation and low
precipitation, whereas in other cases
increase in high evaporation leads to
high rainfall.
Global warming refers to the emission
of greenhouse gases like CO2 which
are likely to increase the temperature.
Due to rapid increase in urbanization
urban heat islands are generated
results in increase in precipitation

23. EVAPORATION
HUMIDITY
As global warming increases the
temperature will increase as a result
there will be increase in evaporation
which determines the rainfall pattern.
Relative humidity (Rh) is inversely
related to the air temperature .i.e. if
temperature is increases, the Rh
decreases and vice versa. That's why
the Rh is lower at higher
temperature.

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CASE STUDY
 Deka and Sarma (2011) the impact of the climate change on
the precipitation characteristics of Brahmaputra Basin.
 Two sites were selected one at the upper reach of the basin,
Gerukamukh border of Assam and Arunachal Pradesh and the
other site is at Guwahati.
 The Large-scale predictor variables obtained from the National
Centre for Environmental Prediction (NCEP) reanalysis data
and observed meteorological station data are used to develop
and validate a Statistical downscaling model for establishing a
statistical relationship between the data.

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 Using this downscaling model the possible future scenarios of
temperature and precipitation variations are generated using
large-scale predictor variables obtained from the Global
Climate Model (GCM) outputs.
 The statistical model uses the HadCM3 daily weather data
under A2 scenario to determine the precipitation and
temperature variations at a specific site.
 The result shows decrease in precipitation in the early
monsoon period and marginal increase in precipitation in the
late monsoon period in the future.
 The future series generated by temperature model has
produced up to 1.80C rise in the maximum temperature
averaged over month

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PRECIPITATION MODELS (CASE-1 LAT-LONG: 250 N – 900E )
Figure: Calibration (1980-94)
Validation(1995-2001) NCEP data
Figure: Calibration (1980-94)
Validation(1995-2001) GCM data

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Comparison of observed and future (generated) precipitation
(case 1)
It can be seen from the comparison that all the cases suggest near
similar indication of climate change in this station. A particular trend
of increase or decrease is absent in the average annual precipitation
of the future

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CONCLUSION
 In this section the various forms of precipitation as well as its types are
discussed.
 Measurements of precipitation as well as the various inconsistencies in its
measurements are discussed.
 Climate change plays a vital role on the change of the rainfall pattern
altering the hydrologic cycle. Certain evidences on climate change have
also been shown.
 A case study on the impact of climate change on Brahmaputra Basin has
also been discussed.
 Various researches are going on regarding the effect of climate change in
the rainfall pattern as global warming is leading to erratic weather
resulting drought and flood in various regions of the world.
 Therefore it is of utmost importance for the scientific community to take
certain steps regarding the growing erroneous trend in rainfall pattern over
the changing environment.

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REFERENCES
 Deka and Sarma(2011), ‘ Impact of climate change on precipitation characteristics of
Brahmaputra Basin’ MTech Report
 Devi, Goswami.,(2015), ‘Analysis of effect of climate change of rainfall, temperature and
discharge on River Brahmaputra, International Journal of Innovative Research in Advanced
Engineering (IJIRAE) ISSN: 2349-2163 Issue 4, Volume 2
 Gabriele C. Hegerl, “Detecting Greenhouse-Gas-Induced Climate Change with an Optimal
Fingerprint Method,” Journal of Climate, v. 9, October 1996, 2281-2306
 Handbook ‘Applied Hydrology’ V.T Chow,1968
 https://climate.nasa.gov/evidence/
 IPCC. 2001. Climate Change 2001: The Scientific Basis. Cambridge University Press: Cambridge
 United Nation Framework Convention on Climate Change, 2007

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