TYPES OF PRECIPITATION AND RAIN GAUGES
Precipitation
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All forms of water that reaches earth from
the atmosphere.
Forms of Precipitation:
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Rain
- 0.5 mm < Water drop size < 6mm
- Light rain: 2.5 mm/h
- Moderate rain: 2.5 mm/h to 7.5 mm/h
- Heavy rain: > 7.5 mm/h
Snow
- Density varies from 0.06 to 0.15 g/cm3
- Occurs in Himalayan Regions
Frost
Hail
- Showery precipitation
- Irregular pellets or lump of ice of size > 8
mm
- Occurs in violent thunderstorm
Dew
Drizzle
- Fine sprinkle of water droplets less than
0.5 mm
- Intensity less than 1 mm/h
Glaze
- Rain or drizzle comes in contact with cold
ground at 0°C
- Water drops freezes and form ice coating
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Rain and Snow contribute to the maximum
amount of water.
How Precipitation is formed?
 The atmosphere must have moisture. The
relative humidity should be greater than 90%
 Sufficient nuclei to aid condensation.
 Suitable weather condition for condensation
of water vapour.
 Products of condensation must reach earth
Weather Systems for Precipitation
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Front
- Interface between two distinct air masses
- Ascending warm air cools adiabatically
and forms clouds
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Cyclone
- Large low pressure region with circular
wind motion.
- Commonly known as Cyclone in India,
Hurricane in USA and typhoon in South
East Asia.
- Areal extent of about 100-200 km in
diameter.
- Winds are anticlockwise in the northern
hemisphere.
Anticyclones
- Regions of high pressure
- Weather is usually calm in center
- Clockwise wind in northern hemisphere
- Winds of moderate speed
- Clouds and precipitation prevails at the
outer edges.
Convective Precipitation
- Packet of air rises due to localized
heating.
- Air from cooler surrounding flows to take
up its place setting up convective cell
- Warm air rises, cools down and form
precipitation
- Depends upon the moisture content,
thermal and other conditions
- Areal extent is small, about 10 km
Orographic Precipitation
- Moist air masses lifted up to higher
altitudes due to presence of mountain
barrier
- Undergo cooling, condensation and
precipitation
- In mountain ranges, windward slope have
heavy rainfall and leeward slope have
light rainfall.
- Orographic rainfall in India is seen only
over the Western Ghats during monsoon
period.
- Cherrapunji and Mawsynram are the
wettest place in the world due to its
location in the windward side of
Himalayas and Tibet remains dry being in
the leeward side.
Measurement of Precipitation
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Expressed in terms of depth to which rainfall
water would stand on an area if all the rain
are collected on it.
It is collected and measured in a rain gauge.
Other terns are pluviometer, ombrometer and
hyetometer.
It consist of a cylindrical-vessel assembly kept
in the open to collect rain
Classifies into non-recording gauges and
recording gauges.
Non-Recording Rain Gauges
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Symon’s Gauge is commonly used in India
It consists of a circular collecting area of
12.7 cm(5.0 inch) diameter and height
30.5 cm.
The rainfall is measured every day at 8:30
a.m. and is recorded as the rainfall of that
day.
Can also measure snowfall.
Recording Rain Gauges
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Recording gauges produces a continuous
plot of rainfall against time.
Provides data of intensity and duration of
rainfall for hydrological analysis.
Tipping-Bucket Type
- A 30.5 cm size Rain Gauge
- Buckets are balances such that when 0.25
mm of rainfall collects in one bucket, it
tips and bring the other one in position.
Weighing-Bucket Type
- Plot of accumulated rainfall against
elapsed time.
Neutral-Syphon Type
- Also called as float rain gauge
- Adopted as standard rain gauge in India
- Gives plot of mass curve of rainfall.
Telemetering Rain Gauges
- Gather rainfall data from mountainous
regions and inaccessible places.
HYDROLOGY TOPIC 2-3-5 PRECIPITATION DATA
ANALYSIS / POINT PRECIPITATION / MEAN AREAL
RAINFALL
Mean Areal Rainfall
The representative precipitation over a
defined area is required in engineering application,
whereas the gaged observation pertains to the point
precipitation
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Arithmetic Mean Method
- This procedure is satisfactory if gauges are
uniformly distributed and the topography
is flat.
Isohyetal Method
Thiessen Polygon Method
The reliability of rainfall measured at one gauge in
representing the average depth over a surrounding
area is a function of the distance from the gauge to
the center of the representative area, the size of the
area, topography, the nature of the rainfall of
concern, and local stone pattern characteristics.
Arithmetic Mean Method
- It uses only those gaging stations within the
topographic basin and is calculated using:
𝑃=
(𝑃1 + 𝑃2 + 𝑃3 + ⋯ + 𝑃𝑛
𝑛
or
𝑃
𝑃 = ∑ 𝑛𝑖
Where,
P = average precipitation depth (mm)
Pi = precipitation depth at gage (i) within the
topographic basin (mm)
n = total number of gaging stations within the
topographic basin
Thiessen Polygon Method
- This concept is implemented by drawing
perpendicular bisectors to straight lines connecting
each two rain gages.
The procedure involves:
1. Connecting each precipitation station with
straight lines;
2. Constructing perpendicular bisectors of the
connecting lines and forming polygons with
these bisectors;
3. The area of the polygon is determined.
𝑃=
𝐴1 𝑃1 + 𝐴2 𝑃2 + 𝐴3 𝑃3 + ⋯ + 𝐴𝑛 𝑃𝑛
𝐴1 + 𝐴2 + 𝐴3 + ⋯ + 𝐴𝑛
𝑛
= ∑
𝑖=1
∑
𝑃𝑟𝑒𝑐𝑖𝑝𝑖𝑡𝑎𝑡𝑖𝑜𝑛 =
𝐴𝑖 𝑃𝑖
𝐴
𝑃𝑜𝑙𝑦𝑔𝑜𝑛 𝑎𝑟𝑒𝑎 𝑓𝑜𝑟 𝑒𝑎𝑐ℎ 𝑠𝑡𝑎𝑡𝑖𝑜𝑛 𝑥
𝑝𝑟𝑒𝑐𝑖𝑝𝑖𝑡𝑎𝑡𝑖𝑜𝑛
∑ 𝑇𝑜𝑡𝑎𝑙 𝑃𝑜𝑙𝑦𝑔𝑜𝑛 𝑎𝑟𝑒𝑎
Isohyetal Method
- Based on interpolation between gauges.
- The first step is to plot the rain gauge
locations on a suitable map to record the
rainfall amounts.
- Next, an interpolation between gauges is
performed and rainfall amounts at
selected increments are plotted.
- Identical depths from each interpolation
are then connected to for isohyets. The
areal average is the weighted average of
depths between isohyets.
𝑃=
HYDROLOGY TOPIC 2-3-4 PRECIPITATION DATA
ANALYSIS / MISSING RAINFALL DATA / DOUBLE
MASS CURVE
Rain Gauge Inconsistency
 Have been installed at different sites in the
past,
 The rainfall depth is being recorded
continuously between the previous and the
current sites.
 The entire data is not homogeneous.
 Change of exposure conditions of the gage
due to growth of the trees or the construction
of tall buildings in the proximity of the gage
site, or
 Change in the instrument, or
 The rain gauge may have been faulty for a
part of the period of record.
Double Mass Curve
𝑃0 + 𝑃1
𝑃 + 𝑃2
𝑃
+ 𝑃𝑛
𝐴1 + 1
𝐴2 + ⋯ + 𝑛−1
𝐴𝑛
2
2
2
𝐴1 + 𝐴2 + ⋯ + 𝐴1
or
𝑃=
∑𝑛𝑖=1
𝑃𝑖−1 + 𝑃𝑖
𝐴𝑖
2
∑𝑛𝑖=1 𝐴𝑖
Or
𝑃=
∑𝑛𝑖=1
𝑃𝑖−1 + 𝑃𝑖
2
𝐴
𝑃𝑎 = 𝑃𝑜
Where
𝑀0 =
∆𝑃𝑜
∆𝑃
𝑀𝑎 =
∆𝑃𝑎
∆𝑃
Where,
P = mean areal precipitation
A = Area
P1, P2, … Pn = precipitation depth for each station
A1, A2, … An -= Area of each site
𝑀𝑎
𝑀𝑜