In this article we will discuss about:- 1. Mechanism of Precipitation 2. Types of Precipitation 3. Measurement 4. Average Annual Rainfall, Maximum and Minimum Annual Rainfall.
Mechanism of Precipitation:
Water vapour from which precipitation is derived is present in the atmosphere as a gas in varying amount at all times. The absolute amount of vapour in the air depends upon the temperature of the air and upon whether it has passed over features of the earth from which moistures may be derived. A general relation between temperature and moisture is that the moisture carrying capacity of the atmosphere doubles with an increase in temperature of about 11°C.
For the precipitation to take place, it is first necessary for a portion of the atmosphere to be cooled until the air is saturated with water vapour. Thereupon condensation occurs resulting in the formation of minute droplets, which are visible collectively in the form of clouds or fog.
These droplets will not in themselves form into rain drops and may remain indefinitely as droplets in the state of colloidal suspension. Rain drops or other forms of precipitation result when the minute droplets of moisture collect on particles in the air termed as hygroscopic nuclei. The most common hygroscopic nuclei are salt particles from oceans and products of combustion in the atmosphere over cities.
Types of Precipitation:
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Precipitation is usually classified in the following three categories on the basis of the basic processes involved in causing the same:
(i) Convective Precipitation:
Convective precipitation is most common in the tropical countries. On a hot day (during summer) the ground surface becomes heated as does also the air in contact with it. This causes the air to expand and rise by convection. As it rises it cools dynamically at the dry adiabatic rate of about 1°C per 100 m which in turn results in condensation and precipitation.
(ii) Orographic Precipitation:
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The precipitation caused by lifting of air over mountain barrier is called orographic precipitation. When moisture bearing winds usually blowing from oceans to land surfaces are forced to rise far above the ground surface by the presence of the coastal mountain ranges, the cooling and condensation processes take place and the precipitation occurs on the windward side of the mountains.
(iii) Cyclonic Precipitation:
Cyclonic precipitation results from lifting of air masses converging into a low pressure area or cyclone.
The cyclonic precipitation may be classified as:
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(a) Frontal precipitation; and
(b) Non-frontal precipitation.
(a) Frontal Precipitation:
A surface separating the warm air mass and the cold air mass is called a frontal surface or a front, which may be subdivided as warm front and cold front. A warm front is the one in which warm air replaces cold air and in a cold front cold air replaces warm air.
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Frontal precipitation results from lifting of warm air over cold air and it may be subdivided as warm front precipitation and cold front precipitation. In warm front precipitation the warm air moves upwards over a relatively stationary wedge of cold air. In this case the precipitation is spread over a large area which may extend 300 to 500 kilometres ahead of the front and the precipitation is generally light to moderate and nearly continuous until after the passage of the warm front.
On the other hand in cold front precipitation the warm air is forced upwards by an advancing wedge of cold air. In this case the precipitation occurs on a small area which may extend only 100 to 150 kilometres ahead of the front and the precipitation is relatively more intense.
(b) Non-Frontal Precipitation:
Non-frontal precipitation occurs when there is low pressure (or barometric depression) caused in any region. In this case air from an adjacent high pressure area flows into the area of low pressure which causes the lifting of the air of the low pressure area to high altitudes where it cools down and results in condensation and precipitation.
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The above noted general causes of precipitation may, however, combine in various ways and hence the resulting precipitation cannot be identified as being of any one type.
Measurement of Precipitation:
For almost all the hydrological designs it is necessary to have the records of precipitation for a long period. Such records may be developed by the measurement of the precipitation occurring at any place from time to time. All forms of precipitation are measured on the basis of the vertical depth of water or water equivalent (in the case of snow) which would accumulate on a level surface if all the precipitation without any loss remained where it fell.
The precipitation is therefore expressed in millimetres (or centimetres). Many types of gages have been developed for the measurement of rain and snow which constitute the major part of precipitation. Some of these gages which are commonly used are described below. Since the amount of precipitation varies from place to place, it would be necessary to install the gages for the measurement of precipitation at various key points in the area. Especially in the drainage basin of a stream or river a network of such gage stations evenly distributed over the entire area of the drainage basin should invariably be provided.
In the measurement of precipitation it is, however, assumed that the observations of precipitation made at any gage station is representative precipitation of certain area around the gage station where the measurement is made. The Indian Standard IS: 4987-1991 gives detailed specifications for establishing a network of gage stations for the measurement of rainfall.
For the measurement of rainfall rain gages are used which may be classified as follows:
i. Non-recording type rain gages.
ii. Self-recording type or automatic recording type rain gages.
i. Non-Recording Type Rain Gages:
As the name indicates these rain gages do not record the rainfall directly but only collect the rain water which when measured gives the total amount of rainfall at the rain gage station during the measuring interval. In our country until about 1969, for the measurement of rainfall at the various rain gage stations, the non- recording type rain gage extensively used by the Indian Meteorological Department (IMD) is the Symon’s rain gage.
However, since 1969 the Indian Meteorological Department (IMD) started using another non-recording type rain gage which has been standardized by the Indian Standard Institution (ISl) in collaboration with the Indian Meteorological Department (IMD).
The Indian Standard, IS: 5225- 1992 gives details of this newly standardized non-recording type rain gage, known as Standard non-recording type rain gage, which is an improvement over the Symon’s rain gage. Both the Symon’s rain gage and the Standard non-recording type rain gage are described below.
Symon’s Rain Gage:
The Symon’s rain gage consists of a cylindrical metal case of internal diameter 127 mm (5 inches) with its Base enlarged to 203.2 mm (8 inches) diameter. At the top of the case a funnel is fixed which is provided with a brass rim measuring exactly 127 mm (5 inches) inside diameter.
The funnel shank is inserted in a glass bottle placed inside the case. The case of the rain gage is fixed in masonry or concrete foundation block 600 mm×600 mm ×600 mm which is sunk into the ground such that the funnel rim is exactly 304.8 mm (12 inches) above the ground level. The rain water enters the bottle through the funnel and gets collected in the bottle.
Standard Non-Recording type Rain Gage:
The Standard non-recording type rain gage consists of a collector with a gun metal or aluminium alloy rim, and a receiver consisting of a base and a bottle. The collector is exposed above ground level while the receiver is fixed partially below ground level. Both the collector and the base are made of fibre glass reinforced polyester. The collector has a deep set funnel and the complete rain gage has a slight taper with the narrower portion at the top.
The collector and the base are locked to each other by means of two complementary locking rings, one fixed inside the collector at its lower end and the other fixed at the top end of the base. The collectors having apertures of either 100 cm2 or 200 cm2 area at the top are used. The collector intercepts the rainfall and the rain water entering the collector is led through the funnel into the bottle where it is stored.
The bottles used in this rain gage are made of polythene and have capacities of 2, 4 and 10 litres. With different combinations of aperture size of the collector and the bottle capacity the rain gage may have measuring capacities of 100, 200, 400 and 1000 mm of rainfall. For example a rain gage having collector of aperture area 200 cm2 and bottle of capacity 2 litres will have a measuring capacity of (2 × 103/200)= 10 cm or 100 mm of rainfall.
A rain gage of measuring capacity of 200 mm or rainfall with collector of aperture area 200 cm2 and bottle of capacity 4 litres is most widely used. However, if a bottle of 2 litres capacity is used then an additional cylinder is used to collect the overflow, if any, from the bottle.
The rain water collected in the bottle of a non-recording type rain gage is measured with the help of a standard measuring glass supplied with each rain gage which indicates the millimetres of rain that has fallen at the rain gage station. The Indian Standard, IS: 4849-1992 gives detailed specifications for the measuring glasses to be used with the rain gages having collectors of different aperture areas.
At each rain gage station the observations for the rainfall are taken daily at 8.30 a.m. (1ST). However, if the rainfall is likely to exceed the capacity of the bottle then a few intermediate observations are also taken. The sum of the observations taken will represent the total rainfall of the past 24 hours of the day on which the observation at 8.30 a.m. is taken. The detailed specifications for the installation of non-recording type rain gage and measurement of rain may be obtained from the Indian Standard, ARE: 4986- 1990.
ii. Self-Recording Type or Automatic Recording Type Rain Gages:
These rain gages automatically record the intensity of rainfall and the time of its occurrence in the form of a pen trace on a clock driven chart, from which the total amount of rainfall for the desired duration may also be determined. These gages may be operated over extended period without attention and the recorded observations may be collected from the gages each time after a certain fixed duration.
The most widely used self-recording type rain gages are as given below:
(a) Tipping bucket rain gage
(b) Weighing type rain gage
(c) Float type rain gage
Each of these rain gages are described below:
(a) Tipping Bucket Rain Gage:
It consists of a 300 mm diameter sharp edged receiver at the end of which a funnel is provided. The rain water enters the receiver and the funnel conducts it to a pair of small buckets pivoted just below the funnel. The buckets are so designed that when 0.25 mm of rainfall collects in one bucket, it tips and empties its water into the storage tank below and at the same time the other bucket is brought under the funnel.
The tipping of the bucket actuates an electric circuit which causes a pen to make a mark on a record sheet mounted on a clock driven revolving drum. Since each mark on the record sheet corresponds to 0.25 mm of rainfall, by counting the same the intensity of rainfall may be determined.
The total rainfall as determined from the records at the end of the day may also be checked by measuring the rain water collected in the storage tank in the same manner as in the case of a non-recording type rain gage.
The movement of the tipping bucket may also be transmitted electronically to the control room so that the rainfall is directly recorded in the control room without any manual assistance. As such these rain gages are quite suitable for the measurement of rainfall in hilly and other inaccessible areas where manual recording of rainfall may not be possible.
(b) Weighing Type Ruin Gage:
In this rain gage the rain water passes through a funnel into a bucket, which is supported on the weighing platform of a spring or lever balance. The increase in the weight of the bucket due to the addition of the rain water causes the platform to move.
The movement of the platform is transmitted through a system of links and levers to a pen which makes trace of accumulated amounts of rainfall on a suitably graduated chart wrapped round a clock driven revolving drum.
The mechanism is arranged to reverse the travel of the pen after a certain amount of rainfall (say 150 mm) has accumulated and reverse again after another equal amount, so that the gage may operate unattended for a week at a time, except in regions of very intense rainfall, where the total rainfall may exceed the capacity of gage (usually 300 mm). The rainfall record produced by this gage is in the form of a mass curve of rainfall in which as shown in Fig. 3.6. The total rainfall is plotted with respect to time. The slope of the curve gives the intensity of the rainfall.
(c) Float Type Rain Gage:
In this rain gage the rain water after passing through the funnel enters a chamber which contains a float. As the level of the rain water collected in the float chamber rises, the float moves up, which actuates a pen connected to it through a connecting rod. The pen makes a trace of accumulated amounts of rainfall on a suitably graduated chart wrapped round a clock driven revolving drum.
Thus in the case of this gage also the rainfall record is in the form of a mass curve of rainfall, which is same as in the case of weighing type rain gage. When the float chamber gets completely filled it is automatically emptied by means of a syphon.
As such this type of rain gage is also known as natural-syphon type rain gage. In our country this type of rain gage is adopted by the Indian Meteorological Department (IMD) as the Standard self recording-type rain gage. The Indian Standard, IS: 5235-1992 gives the details of the Standard self- recording type rain gage.
The self-recording type rain gages are installed on a concrete or masonry platform 450 mm square with the rim of the receiver being kept at a height of exactly 750 mm above the ground level. The self-recording type rain gage is generally used in conjunction with a non-recording type rain gage so that the readings of the self-recording type rain gage can be checked and if necessary adjusted.
In addition to the above noted gages, in the modern times the use of radar as an aid in the measurement of rainfall is being made. However, the main use of radar is in the determination of the areal extent, orientation and movement of rain storms.
For the measurement of snowfall, storage gages are used. In principle these gages are same as the non-recording type and the self-recording type rain gages, but they are designed to hold much greater amounts of precipitation. Self-recording type gages designed to operate for 30 to 60 days without servicing and non-recording type gages with sufficient capacity for an entire season are now in use. A non-recording type snowfall measuring gage used by U.S. Weather Bureau is made from 300 mm diameter thin-walled pipe.
The gage is made in 1.5 m long sections so that any height in multiples of 1.5 m may be obtained. To permit free fall of snow inside the gage, the upper portion of the gage is in the shape of a frustum of a cone. The equivalent depth of water contained in any snowfall may be found by melting the snow collected in the gage and measuring the quantity of water formed.
The snow may be melted by adding an adequate quantity of warm water, accurately measured previously with the measuring glass. The total water content in the gage is then measured using the measuring glass. The measurement thus obtained would give the total of the precipitation and the added warm water. By subtracting from the total the amount of the added warm water, the amount of precipitation is determined.
The snow can also be melted in the gage as it gets collected there by a heating system fitted to the gage or by placing in the gage certain chemicals such as calcium chloride, ethylene glycol, etc. This arrangement for the melting of snow would, however, provide gages with capacity for large amounts of snowfall without making the gage excessively large.
For the measurement of snowfall another method is also used which is known as pole platform method.
The Indian Standard, IS: 4986-1990 gives the details about the pole platform method which is described below:
Pole Platform Method:
A concrete platform 2 m × 2 m and 75 mm thick is made at the ground level at a place where there is least drifting due to wind. At the centre of the platform a scale is permanently fixed. The scale consists of a 50 mm square wooden pole projecting 3 m above the level of the platform and 1 m grouted below it.
The scale is graduated in millimetres and the graduations are pained on all the four sides of the pole. Another scale in the form of a wooden stick of 25 mm square with similar graduations is provided to the gage station. While taking observation, the observer first reads the scale at the centre for the depth upto which it is submerged in snow. With the help of the other scale the depth of snow at each of the four corners of the platform is measured.
The mean of the five readings is taken as the depth of snowfall. The mean depth of snowfall in millimetres divided by 10 approximately gives the water equivalent of snow in millimetres. At the end of each observation, the observer clears the platform of the snow so that the next snowfall measurement is not vitiated by the snow accumulation which has already been measured.
Average Annual Rainfall, Maximum and Minimum Annual Rainfall:
The rainfall recorded on each day at a rain gage station being the amount of rainfall that has accumulated in the rain gage during a period of 24 hours, it is known as daily rainfall. By adding the values of the daily rainfall recorded for various periods such as week, month and year, weekly, monthly and annual (or yearly) rainfall may be obtained.
From the rainfall data so obtained for a rain gage station the variation of rainfall during the same months in different years as well as the year to year variation of the rainfall may be known. Further from the rainfall data obtained for a number of consecutive years the average (or mean) annual rainfall may be determined.
The average annual rainfall is determined by taking simple average of the annual rainfall of several consecutive years recorded at a rain gage station. Since the annual rainfall varies at the same rain gage station, rainfall record for a large number of years is required to obtain a reliable estimate of the average annual rainfall at a place.
It has been indicated by Alexander Binnie that the average annual rainfall worked out from 35 years of rainfall record is within 1.8% of the true average value and hence in India a rainfall record of 35 consecutive years is usually considered to determine the average annual rainfall. Further from the rainfall data of consecutive 35 years the minimum and maximum annual rainfall may also be determined.
The information about the average annual rainfall and minimum and maximum annual rainfall at a place is very useful for planning water supply projects because it gives an idea of the probable amount of water that may be available at that place.