" Precipitation" by Engr Rehan Zeb Khan PMAS-AAUR Rawalpindi

by Engr REhan Zeb Khan PMAS-AAur
“ Precipitation ”
By
Engr Rehan Zeb Khan
Deportment of Land and Water Conservation
Engineering
PMAS –Arid Agriculture University Rawalpindi

Contents
1.Introduction
2.Forms of Precipitation
3.Factor Influencing precipitation formation
4.Classification based on lifting mechanism
Convectional Precipitation
Orographic Precipitation
Cyclonic Precipitation
5.Measurement of Precipitation
Non Recording Gauges
Recording Gauges
Measurement by Radar
Rain Gauge Network

Contents
6.Analysis of Precipitation Data
Point Data Analysis
Estimation of Missing Record
Consistency of Precipitation
Areal Precipitation Data Analysis
Arithmetic Mean Method
Thiessen Polygon Method
Isohyetal Method
7.Intensity of Precipitation
Depth Area Relationship
Mass Curve
Depth Area Duration Curves

Inroduction
Precipitation is any product of the condensation of atmospheric water
vapour that falls under gravity. The main forms of precipitation
include drizzle, rain, sleet, snow, glaze, snowflake and hail.
Precipitation occurs when a portion of the atmosphere becomes
saturated with water vapour, so that the water condenses and
"precipitates".

Forms of Precipitation
Drizzle:
Drizzle is a light liquid precipitation consisting of
liquid water drops smaller than those of rain – generally
smaller than 0.5 mm (0.02 in) in diameter.
Rain:
Rain is the form of precipitation in which the size of drops
is more then 0.5mm and less than 6.25mm in diameter

Glaze:
Glaze is the ice coating formed on drizzle or rain drops when it
comes in contact with cold surface
Sleet:
Sleet is the frozen rain drops cooled to the ice stage while
falling through air at subfreezing temperature.
Snow:
Snow is precipitation in the form of ice crystals resulting from
sublimation.
Snowflake:
A snow flake is made up of a number of ice crystals fused
togather.

Hail:
Hail is the precipitation in the form of small balls or
lumps usually
consisting of concentric layers of clear ice and compact
snow.
Hail varies from 0.5 to 5 cm in diameter and can be
damaging crops and small buildings

Factor Influencing precipitation formation
 Mechanism of Cooling
The pressure reduction when air ascends from near
the surface to upper levels in the atmosphere is
the only mechanism capable of producing the
degree and rate of cooling needed to account for
heavy rainfall. Cooling Lowers the capacity of a
given volume of air to hold a certain amount of
water vapour.
Condensation of Water Vapor
Condensation of water into cloud droplets takes
place on hygroscopic nuclei which are small
particles having an affinity for water. The source
of these condensation nuclei are the particles of
sea salt or products of combustion of certain
sulfurous and nitrous acid and carbon dioxide.
There are always sufficient nuclei present in the
atmosphere. by Engr REhan Zeb Khan PMAS-AAur

Factor Influencing precipitation
formation
 Growth of Cloud Droplet
Growth of droplets is required if the liquid water
present in the cloud is to reach the ground. The
two processes regarded as most effective for
droplet growth are:
 Coalescence of droplets through collision due to
difference in speed of motion between larger and
smaller droplets.
 Co-existence of ice crystals and water droplets.
 Co-existence effect generally happens in the
temperature range from 100 to 20 °F.

Factor Influencing precipitation
formation
 Accumulation of Moisture
Heavy Rainfall amount over a river basin exceeds
by far the amount of the water vapor at
atmospheric volume vertically. Thus there must be
a large net horizontal inflow of water vapor into
the atmosphere above the basin area . This
Process is called Convergence.

Classification based on lifting mechanism
Convectional precipitation:
Convectional precipitation results from the heating of the earth's
surface. The warm ground heats the air over it. As the air warms, the
air molecules begin to move further apart. With increased distance
between molecules, the molecules are less densely packed. Thus, the
air becomes “lighter” and rises rapidly into the atmosphere. As the air
rises, it cools. Water vapour in the air condenses into clouds and
precipitation.

Orographic Precipitation
Orographic precipitation results when warm moist
air moving across the ocean is forced to rise by
large mountains. As the air rises, it cools. Cold air
cannot hold as much moisture as warm air. As air cools, the water
vapour in the air condenses and water droplets form. Clouds forms and
precipitation (rain or snow) occurs
Classification based on lifting
mechanism

Cyclonic Precipitation
Cyclonic precipitation results when the leading edge
of a warm, moist air mass (warm front) meets a cool and
dry air mass (cold front). The molecules in the cold air are
more tightly packed together (i.e., more dense), and thus, the
cold air is heavier than the warm air. The warmer air mass
is forced up over the cool air. As it rises, the warm air cools,
the water vapour in the air condenses, and clouds and precipitation
result.
Classification based on lifting
mechanism

Measurement of Precipitation
Amount of Precipitation
The amount of precipitation means the vertical height of water that wuld
accumulate on a level surface
Intensity or Rate of precipitation
Amount of precipitation per unit time is called the intensity or Rate of
Precipitation.
Precipitation is measured by rain gauges. There are two
types of rain gauges.
1, Recording Gauges
2,Non Recording Gauges

 Non Recording Gauges
Non-recording rain gauges are commonly used. They do not record the data and
collect only rain and this collected rain is then measured in a graduated
cylinder.
The standard gauge of U.S. Weather Bureau has a collector of 200 mm diameter
and 600 mm height. Rain passes from a collector into a cylindrical measuring
tube inside the overflow can.
The snow collected in the outer container or overflow can is melted, poured
into the measuring tube and then measured. This type of rain gauge is one of
the most commonly used rain gauges.

Recording Gauges
 Float recording gauges
This type of rain gauge also has a receiver and a float chamber along with
some recording mechanism or arrangement. In this type the rain is led
into a float chamber containing a light, hollow float. The vertical
movement of the float as the level of water rises is recorded on a chart
with the help of a pen connected to float. The chart is wrapped around a
rotating clock driven drum. To provide a continuous record for 24 hours
the float chamber
has either to be very large, or some
automatic means are provided for
emptying the float chamber quickly
when it becomes full, the pen then
returning to the bottom of the chart.

Recording Gauges
 Weighing type gauges:
It consists of a storage bin, which is weighed to
record the mass. It weighs rain or snow which falls
into a bucket, set on a platform with a spring or
lever balance. The increasing weight of the bucket
and its contents are recorded on a chart. The
record shows accumulation of precipitation.

Recording Gauges
 Tipping Bucket Type:
In TBG the collector is funneled into two
compartment buckets. When one compartment of
bucket is filled with rain water it becomes over
balanced and tips such that the other
compartment takes its place beneath the funnel.
As the bucket is tipped it automatically activates
an electronic circuit. As the bucket is tipped by
each 0.25 mm of rain it actuates an electrical
circuit, causing a pen to mark on a revolving
drum.

 Measurement of Precipitation by
Radar
This is a modern technique for measurement of
rainfall rate. It can also detect local movement of
areas of precipitation. The electromagnetic
energy released and received back by radar is a
measure of rainfall intensity. The measurement is
appreciably affected by trees and buildings.
However extent of rainfall can be estimated with
reasonable accuracy. Use of radar is useful where
number of rain gauges installed in an area is not
sufficient.

 Rain Gauge Network
The number of rain gauges and their distribution affect the nature
of collected precipitation data. The larger the number of rain
guages the more representative will be the data collected. World
Meteorological Organization (WMO) has made following
recommendations for minimum number of rain gauges in a
catchment:
In comparatively flat regions of temperate, Mediterranean and
Tropical Zones, the ideal is at least one station for 230 – 345 sq.
miles.
In mountainous regions of Temperate, Mediterranean and Tropical
Zones, the ideal is at least one station for 35 – 95 sq. miles
In arid and polar zones, one station for 575 – 3860 sq. miles is
acceptable.

 Rain Gauge Network

Analysis of Precipitation Data
 Point data analysis
Point precipitation data refers to precipitation of a station. This data could be form of hourly
record, daily record, monthly precipitation or annually precipitation. Due to many reasons
data may not be continuous and consistent.
1. ESTIMATION OF MISSING PRECIPITATION RECORD.
Some precipitation stations may have short breaks in the records because of absence of the
observer or because of instrumental failure. It is often necessary to estimate the missing
record. There are two methods for estimation of missing data.
A. SIMPLE ARITHMETIC MEAN METHOD.
i=n
Px =1/n ∑ Pi (where ‘n’ is number of station and ‘Pi’ is precipitation at ith
i=1 station)
WHERE,
Px = (P1 +P2 +P3)/3

B, NORMAL RATION METHOD.
i=n
Px =1/n ∑ (Nx /Ni)Pi
i=1
Px=1/3 [Nx /N1)P1 + (Nx /N2)P2 +(Nx /N3)P3 ]
2, CONSISTANCY OF PRECIPITATION DATA OR DOUBLE MASS ANALYSIS.
Double mass curve is used when the consistency of the data is required.
Pa = (Sa/So)* Po
Pa = Adjusted precipitation.
Po = Observed Precipitation.

Double Mass Curve
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0 0.5 1 1.5 2 2.5
accumulatedannualrainfallofXstnin10^3cm
Accumulated annual rainfall of neigbouring stns in 10^3 cm
Double Mass Curve

 Areal Precipitation Data Analysis.
To find out runoff from a catchment and most of other hydrology
analysis, it is important to know the average precipitation of a
certain part of a catchment or for the whole of catchment area.
 ESTIMATION OF AVERAGE PRECIPITATION OVER A BASIN.
Conversion of point precipitation of various gauging station into
average precipitation of that area a great skill and
experience is required.
A. Arithmetic mean method
According to this method ,
i=n
P(average) = (1/n)∑ Pi
i=1
Pav = [ P1 + P2 + P3 + …………..+Pn ]/n

 Thiessen Polygon

 Thiessen Polygon
Draw the given area according to a certain scale and locate the stations
where measuring devices RE installed.
Join all the according to get a network of non- intersecting systems of
triangles.
Draw the bisectors of all the lines joining the station and get a suitable
network of polygons, each enclosing one station. It is assumed that
precipitation over the area enclosed by the polygon is uniform.
Measure area of the each polygon.
Calculate the average precipitation for the whole basin by the formula
P(average)=(P1A1+P2A2+……PNAN)/A
P1=Precipitation at station enclosed by polygon of area A1
P2=Precipitation at station enclosed by polygon of area A2
And so on.
Pn=Precipitation at station enclosed by polygon of area An and A
represents the total area of catchment

 Isohyet Method
F
B
E
A
C
D
12
9.2
4.0
7.0
7.2
9.1
10.0
10.0
12
8
8
6
6
4
4
a1a1
a2
a3
a4
a5

 Isohyet Method
Draw the map of the area according to a certain scale.
Locate the points on map where precipitation measuring gauges are installed.
Write the amount of precipitation of stations.
Draw isohyets (lines joining points of equal precipitation).
Measure area enclosed between every two isohyets or the area enclosed by an isohyet
and boundry of the catchment.
Find the average precipitation by the formula.
P (average) = (P1A2 + P2A2 + ………..+PnAn
Where P1=Mean precipitation of two isohyets 1 and 2
A1= Area between two isohyets
P2=mean precipitation of two isoyets2 and 3
A 2=Area b/w these two isohyets
Pn=Mean precipitation of isohyets n and n+1
An=The area b/w these two isohyets. by Engr REhan Zeb Khan PMAS-AAur

 INTENSITY OF Precipitation.
The rate of occurrence of precipitation is called INTENSITY OF
Precipitation. To find out intensity of a certain interval the points
on graph of accumulative precipitation Vs Time are chosen in such
a way that we get maximum difference for the given interval.
 DEPTH – AREA RELATIONSHIPS.
The distribution of rainfall is usually not uniform over the area. The
precipitation is maximum at the centre of storm and decreases as
we move away from the centre of the storm. For rainfall of a
given duration, the average depth decreases with the area in an
exponential manner.

 MASS CURVE
Precipitation recorded by a wing type and float type
recording rain gauge is in form of graph. It is a plot of
cummulative precipitation against time in
chronological order this is called mass curve of rainfall
data. Intensity of rainfall for a certain duration is
determined by the graph.

Mass curve of rainfall
0
10
20
30
40
50
60
0 20 40 60 80 100 120
Time, hour
accumulatedprecipitation,mm
1st storm,
16 mm
2nd storm,
16 mm

 DEPTH – AREA – DURATION CURVES
Analysis of both the time and areal distribution of a
storm is required in many hydrological studies. Depth
area duration curves provide requisite information for
such studies. It is necessary to have information on the
maximum amount of precipitation of various durations
occurring over various sizes of areas. The development of
relationship between maximum depth area duration for a
region is called (DAD) analysis.
WHILE,
EUD is found by dividing cumulative volume by
cumulative area .

 DEPTH – AREA – DURATION CURVES
0
2
4
6
8
10
12
14
0 1 2 3 4 5 6
EUD
CUMM. AREA (X100 Km2)
DEPTH – AREA – DURATION CURVES

IMPORTANT POINTS
 Precipitation is any product of the condensation
of atmospheric water vapour that falls under gravity.
 Drizzle is a light liquid precipitation consisting of
liquid water drops smaller than those of rain
 Rain is the form of precipitation in which the size of drops is
more then 0.5mm
 Glaze is the ice coating formed on drizzle or rain drops when it
comes in contact with cold surface
 Sleet is the frozen rain drops cooled to the ice stage while falling
through air at subfreezing temperature
 Snow is precipitation in the form of ice crystals resulting from
sublimation
 A snow flake is made up of a number of ice crystals fused
togather.
 Hail is the precipitation in the form of small balls or lumps

IMPORTANT POINTS
 Cooling Lowers the capacity of a given volume of air to hold a
certain amount of water vapour.
 Condensation of water into cloud droplets takes place on
hygroscopic nuclei which are small particles having an affinity
for water.
 Growth of droplets is required if the liquid water present in
the cloud is to reach the ground.
 Heavy Rainfall amount over a river basin exceeds by far the
amount of the water vapor at atmospheric volume vertically.
 Convectional precipitation results from the heating of the
earth's surface. The warm ground heats the air over it.
 Orographic precipitation results when warm moistair moving
across the ocean is forced to rise by large mountains
 Cyclonic precipitation results when the leading edge of a
warm, moist air mass (warm front) meets a cool and dry air
mass (cold front)

IMPORTANT POINTS
 The amount of precipitation means the vertical height of water that wuld
accumulate on a level surface
 Amount of precipitation per unit time is called the intensity or Rate of
Precipitation.
 Non-recording rain gauges are commonly used. They do not record the
data and collect only rain
 Float recording gauges
 Weighing type gauges
 Tipping Bucket Type
 Point precipitation data refers to precipitation of a station. This data
could be form of hourly record, daily record, monthly precipitation or
annually precipitation.
 Analysis of both the time and areal distribution of a storm is required in
many hydrological studies. Depth area duration curves provide requisite
information for such studies.

THANK YOU

" Precipitation" by Engr Rehan Zeb Khan PMAS-AAUR Rawalpindi

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