" 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.Introduction2.Forms of Precipitation3.Factor Influencing precipitation formation4.Classification based on lifting mechanism

Convectional PrecipitationOrographic Precipitation Cyclonic Precipitation

5.Measurement of PrecipitationNon Recording Gauges

Recording Gauges Measurement by RadarRain 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 Method7.Intensity of Precipitation Depth Area Relationship Mass Curve Depth Area Duration Curves by Engr REhan Zeb Khan PMAS-AAur


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


Forms of Precipitation 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.


Forms of Precipitation 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 PrecipitationCyclonic precipitation results when the leading edgeof a warm, moist air mass (warm front) meets a cool anddry air mass (cold front). The molecules in the cold air aremore tightly packed together (i.e., more dense), and thus, thecold air is heavier than the warm air. The warmer air massis 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 PrecipitationThe amount of precipitation means the vertical height of water

that wuld accumulate on a level surfaceIntensity or Rate of precipitationAmount 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 Gauges2,Non Recording Gauges


Measurement of Precipitation 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.


Measurement of PrecipitationRecording Gauges Float recording gaugesThis 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.


Measurement of PrecipitationRecording 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.


Measurement of PrecipitationRecording 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 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 analysisPoint 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


Analysis of Precipitation Data

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 2.50

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Double Mass Curve

Accumulated annual rainfall of neigbouring stns in 10^3 cm

accu

mul

ated

ann

ual r

ainf

all o

f X st

n in

10^

3 cm


Analysis of Precipitation Data 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


Analysis of Precipitation Data Thiessen Polygon


Analysis of Precipitation Data 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


Analysis of Precipitation Data Isohyet Method

F

B

EA

CD12

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

Analysis of Precipitation Data 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.



Analysis of Precipitation Data 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.


Analysis of Precipitation Data 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 120Time, hour

accu

mul

ated

pre

cipita

tion,

mm

1st storm, 16 mm 2nd

storm, 16 mm


Analysis of Precipitation Data 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 .


Analysis of Precipitation Data DEPTH – AREA – DURATION CURVES

0 1 2 3 4 5 602468

101214

DEPTH – AREA – DURATION CURVES

CUMM. AREA (X100 Km2)

EUD

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.


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" precipitation" by engr rehan zeb khan pmas-aaur rawalpindi

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