Definition All types of moisture reaching the surface of

earth from atmosphere.

Precipitation is the basic input to the hydrology.

Factors determining precipitation or the amount of atmospheric moisture over a region

Climate Geography

Ocean surfaces is the chief source of moisture for precipitation

Forms of precipitation

RainRain is the most common type of

precipitation in our atmosphere. Rain is when liquid droplets fall to the surface of the Earth.

There are two different forms of rain, either in the form of

showers drizzles

Showers are heavy, large drops of rain and usually only last a period of time.

Drizzles however usually last longer and are made up of smaller droplets of water.

Rain can either be formed as ice crystals melt or it can be smaller water droplets.

LightI = 2.5mm/hr

ModerateI =

2.8-7.6mm/hr

HeavyI > 7.6 mm/hr

SnowSnow is the second most common

precipitation in the North East. Snow forms when water vapor turns directly

into ice without ever passing through a liquid state. This happens as water condenses around an ice crystal.

Density of freshly fallen snow varies between 125-500mm of snow required to equal 25mm of liquid water

Average density (specific gravity) = 0.1

HailHail is created when moisture and wind are together.

Inside the cumulonimbus clouds ice crystals form, and begin to fall towards the surface of Earth. When this starts to happen wind gusts start to pick up the ice crystals pushing them up high into the clouds. As they start to fall down again they continue to grow in size. A wind gust might catch the hail stone again which will push it back up into the cloud. This whole process gets repeated several times before the hail stone becomes so big that it is too heavy for the wind to carry so it must fall towards Earth.

Shapes of hail particles1.Spherical2.Conical3.Irregular

Diameter range 5 to 125 mmSpecific gravity = 0.8

Average density (specific gravity) = 0.1

Fog

There are four main types of fog,  radiation fog advection fog upslope fog evaporation fog

There is really no different between fog and the clouds that are high in the sky. In simple terms fog is; a cloud that has formed near the surface of the Earth.

DewThe small drops of water which can be found on

cool surfaces like grass in the morning. This is the result of atmospheric vapor

condensing on the surface in the colder night air. Dew Point is the temperature in which

condensation starts to take place or when dew is created.

Mist / DrizzleMist is a bunch of small droplets of water which

are in the air. This occurs with cold air when it is above a warm surface, for example water.

Fog and mist are very similar, the only difference is their visibility. If you cannot see 1 kilometer or less you know you're

dealing with fog. You can see visuals through mist and it is

more haze looking than a thicker substance.

Diameter range between 0.1 and 0.5 mm/hr

Glaze Glaze is the ice coating, generally clear

and smooth, formed on exposed surfaces by the freezing of super cooled water deposited by rain or drizzle.

Specific gravity may be as high as 0.8-0.9

Rime Rime is the white opaque deposit of ice

granules more or less separated by trapped air and formed by rapid freezing of super cooled water drops impinging on exposed objects.

Specific gravity may be as low as 0.2-0.3

Sleet Sleet consists of transparent, globular,

solid grains of ice formed by the freezing of raindrops or freezing of largely melted ice crystals falling through a layer of sub freezing air near the earth’s surface.

Lapse rate

In general, a lapse rate is the negative of the rate of temperature change with altitude change, thus:

where γ is the lapse rate given in units of temperature divided by units of altitude, T = temperature, and z = altitude. Average lapse rate is about 2°C/1000ft

The lapse rate is defined as the rate of decrease with height for an atmospheric variable. The variable involved is temperature unless specified otherwise.

The terminology arises from the word lapse in the sense of a decrease or decline; thus, the lapse rate is the rate of decrease with height and not simply the rate of change. While most often applied to Earth's atmosphere.

Alt

itu

de (

m)

Temperature (C)

Formation of precipitation

Moisture is always present in the atmosphere, even on the cloudless day.

Saturation however does not necessarily lead to precipitation.

Necessary mechanism to form Precipitation

1. Lifting mechanism to cool the air

2. Formation of cloud elements (Droplets/Ice crystals)

3. Growth of cloud elements

4. Sufficient accumulation of cloud elements

1. Lifting mechanism to cool the air

Lifting mechanism gives the three main types of Precipitation.

Cyclonic Precipitation (Frontal /non Frontal)

Convective Precipitation

Orographic Precipitation

Cyclonic Precipitation(Frontal/Non frontal)

Frontal precipitation results when the leading edge( front) 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.

Convective Precipitation

Convectional precipitation results from the heating of the earth's surface that causes air to rise rapidly. As the air rises, it cools and moisture condenses into clouds and precipitation

Orographic Precipitation

It results when warm moist air of the ocean is forced to rise by large mountains. As the air rises it cools, moisture in the air condenses and clouds and precipitation result on the windward side of the mountain while the leeward side receives very little. This is common in British Columbia.

Formation of cloud elements(Droplets/Ice crystals) For droplets, hygroscopic nuclei ,small

particles (0.1-10µm) having affinity for water must be available in upper troposphere.

For ice crystals, Freezing Nuclei are required

Source of condensation nuclei are particles of sea salts, products of sulphurous and nitric acid

Source of freezing nuclei are clay minerals, usually kaolin, silver iodide etc

Growth of cloud elements For occurrence of precipitation over an area it is necessary

that cloud elements must be grown in size to over come

Coalescence of cloud droplets Cloud droplets are usually smaller than 50µm in diameter, due to

different diameters of droplets they fall with varying fall velocities. As the bigger cloud elements are heavier , having more fall velocity, hence they collide with smaller droplets. Smaller droplets join the bigger droplets and in this way the size of cloud droplets increases.

Co-existence of cloud droplets & ice crystals If in a layer of clouds there is mixture of water droplets and ice

crystals. As the saturation vapour pressure over ice is lesser than over water. As a result of this difference , there results evaporation of water drops and condensation of much of this water on ice crystals. Causing their growth and ultimate fall through clouds. The ice crystals will further grow as they fall and collide with water drops.

Growth of droplets and ice crystals

For the occurrence of precipitation over an area necessary conditions are :

Cloud elements must increase in size until their falling speeds exceed the ascending rate of air

Cloud elements should be large enough in size not to get evaporated completely before reaching the ground

Measurement of Precipitation

1. Amount of precipitation

2. Intensity of precipitation

3. Duration of precipitation

4. Arial extent of precipitation

Measurement Methods

Measurement of precipitation (Rain and Snow) can be done by various devices. These measuring devices and techniques are;

Rain GaugesSnow GaugesRadarsSatellitesScratching of snow packs Water equivalent in snow packs

RAIN GAGESRain gages are most commonly used for the

measurement of precipitation, both in terms of rain fall and snow.

Types of rain gagesThere are two main types of rain gages which

are used to measure the precipitation. These are;

1. Non recording rain gages

2. Recording rain gages

Non recording rain gaugesIt is a rain gage which does not provide the

distribution of amount of precipitation in a day. It simply gives the amount of precipitation after 24 hours (daily precipitation).

Recording rain gaugesThese rain gauges are also called integrating

rain gauges since they record cumulative rainfall. In addition to the total amount of rainfall at a station, it gives the times of onset and cessation of rains (thereby gives the duration of rainfall events)

Types of recording Rain gauges

There are three main types of recording rain gauges

1. Float type rain gages

2. Tipping bucket type rain gages

3. Weighing type rain gages

1. Tipping bucket gaugesA tipping bucket rain gauge is used for

measurement of rainfall. It measures the rainfall with a least count of 1 mm and gives out one electrical pulse for every millimeter of rainfall

2. Weighing type gaugesIt 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.

3. Float recording gaugesThe rise of float with increasing catch of rainfall is

recorded. Some gauges must be emptied manually while others are emptied automatically using self starting siphons. In most gauges oil or mercury is the float and is placed in the receiver, but in some cases the receiver rests on a bath of oil or mercury and the float measures the rise of oil or mercury displaced by the increasing weight of the receiver as the rainfall catch freezes. Float may get damaged by rainfall catch freezer

Errors in precipitation measurement by Rain Gauges

Instrumental errorsErrors in scale readingDent in receiversDent in measuring cylindersAbout 0.25mm of water is initially required to wet the

surface of gaugeRain gauges splash from collectorFrictional effectsNon verticality of measuring cylinders (10° inclination

gives 1.5% less precipitation)Loss of water by evaporationLeakage in measuring cylinderWind speed reduces measured amount of rain in the

rain gauges.

Measurement of snowIn case of snow fall following two properties

of more interest are measured.

1. Depth of snow at a particular place in mm/inches

2. Equivalent amount of water in mm

1. Depth of snow

Depth of snow fall at a particular place can be measured by the following methods.

a. Standard rain gauges without collectors

b. Snow gauges

c. By scratching snow packs

Depth of snow methods

Standard rain gauges can also be used for measuring the snow depth, with some alterations in the arrangement of rain gauges, particularly, the collectors are not used

On a paved surface with snow over it, scratching that snow layer with some scrapper helps to measure the depth of snow fall with a tape. Visual observation and with help of measuring tape helps to measure the depth of snow

Snow gauges

A snow gauge is a type of instrument used to measure the solid form of precipitation.

Equivalent water in snowSnow Water Equivalent (SWE) is a common

snow pack measurement. It is the amount of water contained within the snow pack. It can be thought of as the depth of water that would theoretically result if you melted the entire snow pack instantaneously.

Equipment used is; Standard rain gages without receiversWeighing type rain gagesSnow gages

Measurement of equivalent amount of water in a snow pack The equivalent amount of water in a snow

pack can be measured by

1. Heating

2. Weighing

3. Adding measured amount of hot water

1. By Heating

The equivalent amount in mm of water can be obtained by heating the cylinder. it will melt the snow and the depth of the liquid water can be measured with a measuring stick but this approach is adjustable because some water may get evaporated during the heating.

2. By WeighingWeight is measured either by weighing type rain gauges or by using a

snow gauge.

W=W1-W2

 

W1= weight of snow + empty cylinderW2= Weight of empty cylinderW= Weight of snow By using weight volume relationship 

Γ = Weight/ Volume 

Γ = W/ A.h 

h = W/A ΓWhere, h = Equivalent amount of water in snow.

c. By scratching snow packs

A measured amount of hot water is added into the cylinder which will melt the snow. Now measure the total depth of water in the cylinder “h1”

 

h = h1-h2

 Where, h2 = measured amount of hot waterh = equivalent amount of water

Radar MeasurementsA weather radar is a type of radar used to locate

precipitation, calculate its motion, estimate its type (rain, snow, hail, etc.), and forecast its future position and intensity. Weather radars are mostly Doppler radars, capable of detecting the motion of rain droplets in addition to intensity of the precipitation. Both types of data can be analyzed to determine the structure of storms and their potential to cause severe weather. 

Satellite MeasurementsA weather satellite is a type

of satellite that is primarily used to monitor the weather and climate of the Earth.These meteorological satellites, however, see more than clouds and cloud systems, like other types of environmental information collected using weather satellites.

Interpretation Of Precipitation DataInterpretation of missing precipitation data

includes;

 1. Estimating missing precipitation data at a

station

2. Checking inconsistency in particular data at a station

3. Averaging precipitation over an area

1. Estimating missing precipitation data at a station

Missing precipitation data is estimated by two

commonly used methods.

Arithmetic Mean Method

Normal Ratio Method (NRM)

Arithmetic Mean MethodArithmetic mean method is used when

normal annual precipitation is within 10% of the gauge for which data are being reconstructed. This method is least accurate however.

Where: Pm = precipitation at the missing location

Pi = precipitation at index station IN = number of rain gauges

Normal ratio method (NRM)Normal ratio method (NRM) is used when the normal

annual precipitation at any of the index station differs from that of the interpolation station by more than 10%. In this method, the precipitation amounts at the index stations are weighted by the ratios of their normal annual precipitation data in a relationship of the form:

Where:Pm = precipitation at the missing location

Pi = precipitation at index stationNm = average annual rain at ‘missing data’ gaugeNi = average annual rain at gaugeN = number of rain gauges

Checking inconsistency in a particular data record at a station

By a technique called Double Mass Curve Analysis.

It is used to check the consistency of many kinds of hydrologic data by comparing date for a single station with that of a pattern composed of the data from several other stations in the area

The double-mass curve can be used to adjust inconsistent precipitation data

Double Mass Curve AnalysisThe theory of the double-

mass curve is based on the fact that a plot of the two cumulative quantities during the same period exhibits a straight line so long as the proportionality between the two remains unchanged, and the slope of the line represents the proportionality. This method can smooth a time series and suppress random elements in the series, and thus show the main trends of the time series.

Averaging precipitation over area

It is the amount of precipitation which can be assumed uniform over an area. If the average precipitation over an area is known than total rain volume of water can be computed for that area.

Rain volume = Pavg × A

 

Methods for computing Average precipitation over an area

There are some widely used methods to compute average precipitation over an area, but the most common of these used are:

Arithmetic mean method

Theissen polygon method

Isohytal method

Arithmetic Mean MethodArithmetic mean method is used when

normal annual precipitation is within 10% of the gauge for which data are being reconstructed. This method is least accurate however.

Where: Pm = precipitation at the missing location

Pi = precipitation at index station IN = number of rain gauges

Theissen Polygon MethodDivide the region (area

A) into sub-regions centred about each rain gauge;

Determine the area of each sub-region (Ai) and compute sub-region weightings (Wi) using: Wi = Ai/A

Compute total aerial rainfall using Rainfall recorded at each station is given a weight age based on the area closest to the station.

Theissen Polygon MethodConsider a catchment area with

say, 3 rain gauge stations. Let there be 3 stations outside the catchment, but in its neighborhood. Catchment area is drawn to scale and position of these 6 stations is plotted on it. Stations are joined so as to get a network of triangles. Perpendicular bisectors are drawn to each of the sides of these triangles. These bisectors form a polygon around each station. If the boundary of catchment cuts the bisectors, then boundary is taken as outer limit of polygon. These bounding polygons are called Thiessen Polygons. The area of these polygons is measured with a planimeter or by grid overlay

Isohytal Method Plot gauge locations

on a map; Subjectively

interpolate between rain amounts between gauges at a selected interval;

Connect points of equal rain depth to produce lines of equal rainfall amounts (isohyets);

 

Isohytal Method Compute aerial rain using Isohyets – It is a line joining points of equal

rainfall magnitude. The catchment area is drawn to scale

and the rain gauge stations are marked on it. The recorded rainfall values for which aerial average is to determined are marked at the respective stations.

Neighboring stations outside the catchment are also considered. Taking point rainfall values as the guide, isohyets of different rainfall values are drawn (similar to drawing contours based on spot levels.

The area between adjacent isohyets is measured using a planimeter. If isohyets go out of the catchment, the catchment boundary is used as the bounding line.

It is assumed that the average value of rainfall indicated by two isohyets acts over the inter isohytal area