Ch. 3 - Precipitation

8/12/2019 Ch. 3 - Precipitation

1/48

PRECIPITATION

INTRODUCTION

The term precipitationas used in hydrology is meant for all forms of moisture emanatingfrom the clouds and all forms of water like rain, snow, hail and sleet derived fromatmospheric vapors, falling to the ground.

Precipitation is one of the most important events of hydrology. Floods and droughts aredirectly related to the occurrence of precipitation. Water resources management, water

supply schemes, irrigation, hydrologic data for design of hydraulic structures andenvironmental effects of water resources development projects are related to precipitationin one way or the other. So it is important to study various aspects of precipitation.

FORMS OF PRECIPITATION

In the middle latitudes precipitation occurs in many forms, depending on the eistingmeteorological conditions. These are following.

i. Drizzle

These are the minute particles of water at start of rain. These consist of water drops under!." mm diameter and its intensity is usually less than #.! mm$hr. Their speed is very slowand we cannot even feel them. Therefore they cannot flow over the surface %ut usuallyevaporate.

Ii. Rain

It is form of precipitation in which the si&e of drops in this case is more than !." mm andless than '.(" mm in diameter. It can produce flow over the ground and can infiltrate andpercolate. )oth the duration as well as rate of rainfall are important. If the rainfall per unittime is greater than the rate of infiltration, the rain water can flow over the surface ofearth.

iii. Glaze

It is the ice coating formed on dri&&le or rain drops as it comes in contact with the coldsurfaces on the ground.


2/48

*!iv. Sleet

Sleet is fro&en rain drops cooled to the ice stage while falling through air at su%free&ingtemperatures.

v. Snow

Snow isprecipitation in the form of ice crystals resulting from su%limation i.e. change ofwater vapor directly to ice.

vi. Snowflake

+ snowflake is made up of a num%er of ice crystals fused together.

vii. Hail

-ail is the type of precipitation in the form of %alls or lumps of ice over " mm diameterformed %y alternate free&ing and melting as they are carried up and down %y highlytur%ulent air currents. The impact of these is also more. + single hailstone weighing over apound has %een o%served.

FACTORS INFLUENCING PRECIPITATION FORMATION

Following four conditions are necessary for the production of precipitation.

a. + lifting mechanism to produce cooling of the air.

%. + mechanism to produce condensation of water vapors and formation of clouddroplets.

c. + mechanism to produce growth of cloud droplets to si&e capa%le of falling to theground against the lifting force of air.

d. + mechanism to produce sufficient accumulation of moisture to account foro%served heavy rainfall rates.

a. Mechanism of Cooling

The pressure reduction when air ascends from near the surface to upper levels in theatmosphere is the only mechanism capa%le of producing the degree and rate of coolingneeded to account for heavy rainfall. ooling lowers the capacity of a given volume of airto hold a certain amount of water vapor. +s large degrees of su/per saturation are notknown to occur in the atmosphere, ecess moisture over saturation condenses through thecooling process.

*!


3/48

*#b. Condensation of Water Vapor

ondensation of water into cloud droplets takes place on hygroscopic nuclei which aresmall particles having an affinity for water. The source of these condensation nuclei is theparticles of sea salt or such products of com%ustion of certain sulfurous and nitrous acids.

There appears to %e always sufficient nuclei present in the atmosphere. ondensation willalways occur in air, the lower atmosphere is cooled to saturation, often %efore thesaturation point is reached.

c. Growth of Clod Droplets

0rowth of droplets is re1uired if the li1uid water present in the cloud is to reach theground. The two processes regarded as most effective for droplet growth are2

i. oalescence of droplets through collision due to difference in speed of motion %etween

larger and smaller droplets.

ii. o/eistence of ice crystals and water droplets.

o/eistence effect generally happens in the temperature range from #!!to (!oF. If in alayer of clouds there is a miture of water droplets and ice crystals, the saturation vaporpressure over ice is lower than that over water. This leads to the evaporation of waterdrops and condensation of much of this water on ice crystals causing their growth andultimate fall through the clouds. This effect is known as )ergeron3s effect. The ice crystalswill further grow as they fall and collide with water droplets.

d. !ccmlation of Moistre

-eavy rainfall amount over a river %asin eceeds %y far the amount of water vapor at theatmospheric volume vertically a%ove the %asin at the %eginning of the rainfall. learlythere must %e a large net hori&ontal inflow of water vapor into the atmosphere a%ove the%asin area. This process is called convergence, which is defined as the net hori&ontalinflu of air per unit area. The moisture added to the atmosphere over a %asin may %etransported very large distance in the lowest layer of the atmosphere. When this moistcurrent reaches a region of active vertical motion it rises thousands of feets and loosesmuch of its contained water vapor in just a few hours.

*#


4/48

*(

CLASSIFICATION OF PRECIPITATION BASED ON THELIFTING MECHANISM

The precipitation is often classified according to the factor responsi%le for lifting of air tohigher altitudes. Following are the various types of precipitation %ased on this

classification.

i. Con"ectional #recipitation

In the case of convectional precipitation the main causative element is thermal convectionof the moisture laden air. For this to occur, solar radiation is the only source of heat. +major portion of the solar radiation is utili&ed in heating the earth. +s the earth conductsheat slowly, the heat accumulates at the surface of the earth and air which comes in itscontact is heated up and the lapse rate near the surface of the earth increases rapidly. Withthe passage of time as the sun gets higher and higher the lapse rate increases a%ove that ofdry adia%atic and air %ecomes unsta%le. 4ertical currents are then set up which carry heatand the moisture laden air is picked up from the surface to higher levels. 5ue toconvection, the moist air in the lower levels of the atmosphere rises up to thecondensation level where clouds develop and with further convection these clouds finallygrow into cumunim%us resulting in a thunderstorm. 6ightning and thunder, gusty surfacewinds, showers and sometimes hails accompany a thunderstorm. 7ach thunderstorm isformed of a cell which updraft or down draft 8downward current9 tur%ulence etc. Thesecells are called under/storm cells. + cell hardly covers an area more than #/( s1uaremiles.

+s the warm moist air from the ground is lifted up, more and more water will condenseand the water drops and ice crystals will increase in si&e till such time that these drops areno longer supported %y the eisting updrafts. The drops will then %egin to fall. Suchthunderstorms are called :air mass thunderstorms;. They usually develop %y mid/day andreach maimum intensity %y afternoon. )y late evening such storms start dissipating.Thunderstorms may etend to a height of (!,!!! to (",!!! feet in temperate latitude and*!,!!! to

ii. $rographic #recipitation

In the orographic precipitation, epansion and condensation occurs %ecause moistureladen air masses are lifted %y contact with orographic %arriers. This type of precipitationis most pronounced on the windward side of mountain range, generally heaviestprecipitation occurs where favora%le orographic effects are present. For instance, heaviestprecipitation due to south/easterlies in the su%continent occurs along the Southern slopesof -imalayas and its other ranges. It has %een found that the monsoon rainfall 8=une to>cto%er9 decreases gradually as the distance from the line of heaviest rainfall increases.

*(


5/48

**>rographic precipitation also occurs in the inland areas where mountain ranges rise a%ovethe surrounding areas in the path of the moisture laden air masses. >n these areas,however, orographic precipitation is intermingled with the other types prevalent in thearea and, therefore, it %ecomes difficult to identify 1uantitatively the amount ofprecipitation that has occurred due to orographic effects only. >n inland areas the

orographic precipitation is irregular in occurrence and 1uantity due to interference %y theatmospheric distur%ance resulting from cyclonic storm.

0enerally, it has %een o%served that heavy orographic precipitation occurs on thewindward side of the orographic %arrier, leaving a relatively dry area leeward side. Thisoccurs %ecause the moist air has %een forced up the windward side and precipitated itsmoisture, and upon passing the peak of the %arrier, no further orographic lifting occurs sothat the rain fall is the residual of previous condensation. If the orographic %arrier issufficiently massive and the low regions on the leeward side are very etensive, the windmay descend on the leeward side, there%y undergoing compression and heating and%ecoming still more unfavora%le for precipitation.

iii C%clonic #recipitation

Precipitation in plain regions is generally cyclonic in character. yclones are of twogeneral classes, tropical and etra/tropical, so called depending upon whether they occurwithin or %eyond the tropics. In as much as all cyclones occurring in the Indo/PakSu%continent are of tropical variety. This kind alone will %e discussed here. ?oreover,cause of these storms is not of primary concern ecept if it affects the precipitation.Tropical cyclones are violent storms which are generally formed in the ward maritime air/

mass of low latitude where the temperature is high. These are known as typhoons orcyclones when formed in the Indian >cean.

In the center of the cyclonic storm there is small low pressure air. The iso%ars around sucha low pressure are very nearly circular in shape. Their intensity is generally greater thanthe etra/tropical cyclones. >n an average the tropical cyclones have a diameter of over*!! to


6/48

*cto%er and Aovem%er these storms are very destructive in )angladesh. Such stormscause considera%le loss of life and property over the coastal districts. yclonic storms also

form in +ra%ian sea %ut their num%er is far less.

MEASUREMENT OF PRECIPITATION

!mont of #recipitation

The amount of precipitation means the vertical depth of water that would accumulate on alevel surface, if the precipitation remains where it falls. The amount of precipitation ismeasured in length units 8inches, ft., cm, etc.9.

&ntensit% or Rate of #recipitation

+mount of precipitation per unit time is called the intensity of precipitation or rate ofprecipitation.

)oth the amount and rate of precipitation are important in hydrologic studies.

The precipitation is measured %y rain gauges. There are two types of rain gauges2

a. Aon/recording rain gauge. 8Standard rain gauge9

%. Becording rain gauge

The main difference %etween these rain gauges is that with the help of recording raingauges we get the rain recorded automatically with respect to time, so intensity of rain fallis also known whereas an o%server has to take readings from non recording rain gauge forrain and he has to record the time also, for calculation of intensity of rain fall.

a. 'on(Recording Rain Gages

The standard gauge of C.S. Weather )ureau has a collector of (!! mm diameter and '!!mm height. Bain passes from a collector into a cylindrical measuring tu%e inside theoverflow can. The measuring tu%e has a cross sectional area #$#!thof the collector, so that(." mm rain fall will fill the tu%e to (" mm depth. + measuring stick is marked in such away that #$#!thof a cm depth can %e measured. In this way net rainfall can %e measured tothe nearest # mm. The collector and tu%e are removed when snow is epected. The snowcollected in the outer container or overflow can is melted, poured into the measuring tu%eand then measured. This type of rain gauge is one of the most commonly used rain gauge.

*


7/48

*"

Fig. *.# Aon/Becording Bain 0auge

Sorces of )rror

Bainfall measured %y the rain gauge might have some errors. For eample some water isused to wet the surface of instrumentD the rain recorded may %e less than the actual rainfalldue to the direction of the rainfall as affected %y wind. 5ents in the collector and tu%e mayalso cause error. Some water is a%sor%ed %y the measuring stick. 6osses due toevaporation can also take place. The volume of stick replaces some water which causes

some error.

b. Recording Rain Gages

Becording rain gauges can %e divided into the following types2

i. Float typeii. Weighing typeiii. Tipping %ucket type

i. *loat +%pe Rain Gage

This type of rain gauge also has a receiver and a float cham%er along with some recordingmechanism or arrangement. In this type the rain is led into a float cham%er containing alight, hollow float. The vertical movement of the float as the level of water rises isrecorded on a chart with the help of a pen connected to float. The chart is wrappedaround a rotating clock driven drum. To provide a continuous record for (< hours thefloat cham%er has either to %e very large, or some automatic means are provided foremptying the float cham%er 1uickly when it %ecomes full, the pen then returning to the

*"


8/48

*'%ottom of the chart. This is usually done with some sort of siphoning arrangement. Thisarrangement activates when the gauge records a certain fied amount of rain 8mostly #!mm of rainfall.9. Snow can not %e measured %y this type of rain gauge.

Fig. *.(

ii. Weighing +%pe Rain Gage

The weighing type rain gauge consists of a receiver, a %ucket, a spring %alance and somerecording arrangement. The weighing type gauge weighs the rain or snow which falls intoa %ucket which is set on a lever %alance. The weight of the %ucket and content is recordedon a chart %y a clock driven drum. The record is in the form of a graph, one ais of whichis in depth units and the other has time. The records show the accumulation ofprecipitation. Weighing type gauges operate from # to ( months without stopping. )utnormally one chart is enough only for (< hours. This type of rain gauge has advantage ofmeasuring snow also. The receiver is removed when snow is epected. Figure *.* shows

schematic sketch of a weighing type rain gauge.

FIG 3.3

iii. +ipping ,cket +%pe Rain Gage

*'


9/48

*EThis type of gauge used at some Weather )ureau First >rder Stations, is e1uipped with aremote recorder located inside the office which is away from the actual site. The gaugehas two compartments pivoted in such a way that one compartment receives rain at onetime. + certain amount of rain 8usually !.(" mm fills one compartment and over %alancesit so that it tips, emptying into a reservoir and %ringing the second compartment of the

%ucket into place %eneath the funnel of receiver. +s the %ucket is tipped %y each !.(" mmof rain it actuates an electrical circuit, causing a pen to mark on a revolving drum. Thistype of gauge is not suita%le for measuring snow without heating the collector. Plotting issimilar to that of other recording rain gauges. + Tipping )ucket Type Bain 0auge isshown in Figure *.


10/48

*c. Bain drops splashing from the collector.d. For very intense rain some water is still pouring into the already filled %ucket.e. Inclination of the gauge may result in catching less or more rain than the actual

amount.f. 7rror in measurement due to wind.

Remedial measures for errors in precipitation measurement

Bemoval of error due to dents o%viously needs repair of the instrument. For rain recordedwith dents a correction should %e applied. 7rrors such as moistening of the inside surfacesof the gauge, splashing of rainwater from the collector and pouring of water into thealready filled %ucket during an intense rain can only %e corrected %y some correctionfactor. Inclined instrument needs to %e reinstalled. The correction factor however can %ecalculated from the angle of inclination. For wind protection certain wind shields aredesigned and used which are called Splash 0uards. Proper setting of gauge a%ove groundlevel is necessary.

Example 3.1

+ rain gauge recorded #(" mm of precipitation. It was found later that the gauge wasinclined at an angle of (! degree with the vertical. Find the actual precipitation.

SolutionP8measured9 G #(" mm+ngle of inclination 8H9 G (!owith the vertical

P8actual9G P8measured9$cos8H9 G #("$cos(!o

G #** mm

Measrement of #recipitation b% Radar

This is a modern techni1ue for measurement of rainfall rate. It can also detect localmovement of areas of precipitation. The electromagnetic energy released and received%ack %y radar is a measure of rainfall intensity. The measurement is apprecia%ly affected%y trees and %uildings. -owever etent of rainfall can %e estimated with reasona%leaccuracy. Cse of radar is useful where num%er of rain gauges installed in an area is notsufficient.

Rain Gage 'etwork

The num%er of rain gauges and their distri%ution affect the nature of collectedprecipitation data. The larger the num%er of rain gauges the more representative will %ethe data colleted. )ut on the other hand we have to o%serve other factors also, likeeconomy of the project, accessi%ility of certain areas and topography of the area. So, onehas to look for some optimum solution. In this regard the World ?eteorological

*


11/48

*@>rgani&ation 8W?>9 has made following recommendations for minimum num%er of raingauges in a catchment2

a. In comparatively flat regions of temperate, ?editerranean and Tropical ones, theideal is at least one station for (*! J *


12/48

AG Aormal annual precipitation.8?ean of *! years of annual precipitationdata9

P G Storm Precipitation.

6et P%e the missing precipitation for station LM3 and A, the normal annual precipitationof this station, Aa, A%and Acare normal annual precipitations of near%y three stations, +,) and respectively while Pa, P%and Pcare the storm precipitation

of that period for thesestations.

Aow we have to compare A with Aa ,A%and Ac separately. If difference of A/ Aa, A/A%, A/ Acis within #$#!K of Athen we use simple arithmetic mean method otherwisethe normal ratio method is used.

Simple !rithmetic Mean Method

+ccording to the arithmetic mean method the missing precipitation is given as2

PG Pn

i

ni

i

=

=#

#

where n is num%er of near%y stations.

In case of three stations #, ( and *,

PG 8P# P( P*9$*

and naming stations as +, ) and instead of #, ( and *

PG 8Pa P% Pc9$*Where Pa, P%and Pcare defined a%ove.

'ormal Ratio Method

+ccording to the normal ratio method the missing precipitation is given as2

P G PN

N

n i

ni

i i

x

=

=#

#

where Pis the missing precipitation for any storm at the interpolation station LM3. Piis theprecipitation. for the same period for the same storm at the :ith; station of a group ofinde stations and Aand Aiare the normal annual precipitation values for the LM3 andLith3 stations for eample for the sym%ols defined a%ove for three inde stations in acatchment area.


13/48


14/48


15/48

%served precipitation.

SaG Slope prior to the %reak in the curve

SoG Slope after the %reak in the curve.

+ll values after %reak are to %e adjusted.

Example 3.4

heck consistency of the data given in ta%le *.# %elow and adjust it if it is found to %einconsistent

Ta%le *.# Precipitation 5ataRear +nnualprecipitation at

8mm9

?ean of annualprecipitation of (!surrounding stations

8mm9

Rear +nnualprecipitation at

8mm9

?ean of annualprecipitation of(! surroundingstations 8mm9

1972 # ('< 195 ((* *'!1971 #" (( 195! #E* (*nce it has %een done for all data maimum value is notedalong with the period in which it occurred. In this case, this value comes out to %e ((

Documents

Ch. 3 - Precipitation