Precipitation whether it is rain or snow is expressed as the depth to which it would cover a horizontal projection of the earth’s surface, if there were no loss by evaporation, run-off or infiltration and if any part of the precipitation falling as snow or ice were melted. It is expressed in units of millimetres. Measurement is made of the total precipitation, whether of rain, hail or snow in the form of liquid water. That is any snow or hail is melted and added to any rain that has already fallen.
The amount of rainfall in a station is measured by a rain gauge. In 1969, the India Meteorological Department accepted the recommendations of the world Meteorological Organisation for the use of Fibre Glass Reinforced Polyester (FRP) Rain gauges as the standard instruments at all rain gauge stations in the country. The rain gauge consists of the following parts:
(a) Conical Base: This is made up of metal with a provision for inserting the outer cylindrical case. This is erected in masonry platform built on the ground.
(b) Outer Cylindrical Case: This is made up of metallic sheet or reinforced fibreglass. This consists of two pieces. The top portion fits in closely with the bottom portion. The top portion is provided with perfectly circular brass rim having a diameter of 127 mm for metallic gauge and 156 mm for FRP rain gauge. A funnel is provided inside the top portion 25 mm below the brass rim.
(c) Inner container: This is made up of metal having a cylindrical shape and a narrow mouth. The container is placed inside the outer case with the top portion of the mouth such a way that the water falling with in the brass rim is collected inside the inner container.
Exposure of Rain Gauge
In order that the observations at different stations are comparable the exposure must be uniform as far as possible at all stations. The following points must be strictly adhered to in the erection of rain gauge:
(a) The rim of the rain gauge should be exactly horizontal and 30 cm above the ground.
(b) The distance between the rain gauge and any nearest object shall not be less than twice the height of the object above the rim of the rain gauge.
(c) The site selected for erection of rain gauge should be on a level ground away from trees, buildings and other obstructions and not upon a slope or terrain.
Procedure for Measuring the Rainfall
Watch the surface wind direction and kneel down by keeping back towards the wind. Remove the funnel and place it on the platform. Take out the receiver; pour gently the contents into the glass with care to avoid spilling. Replace the receiver and funnel. Lift the graduated rain measuring glass, hold it upright between the thumb and the first finger or place it on a horizontal surface. Bring the eye to the level of the water into the measuring glass and take the reading of the bottom meniscus or curved surface of the water. It is extremely important to note that the correct type of measuring glass, appropriate to the type of rain-gauge funnel in use, should be used for measuring the amount of rain otherwise completely wrong results will be obtained.
If the receiver contains more than 20 mm of rain, measure it, in two or more instalments and add the amounts together.
The snow gauge, which is mounted in the observatory, collects the snow.
Snow Gauge Exposure
Exposure conditions as for rain gauge must be satisfied. The snow gauge should be fixed on a masonry or concrete foundation sunk into the ground with the top surface 5 cm above ground. If within observatory enclosure, the rain gauge exposure should not be vitiated.
On the days of snow or fall of hail or when the water collected in the gauge is frozen, the snow or ice in the funnel, bottle or receiver must be melted by warming and the water thus obtained is then measured in the ordinary way like rain water. One of the following methods may be employed to melt the frozen precipitation.
(a) The funnel and receptacle are brought indoors; the contents melted and the water measured.
(b) A cloth dipped in hot water is applied to the outer side of the funnel or receiver or both as necessary to melt the snow or ice and collect the water from the gauge and measure it.
(c) A known quantity of warm water already measured in the measuring glass is poured into the gauge. The total amount of water in the gauge is then measured and the amount added to the gauge is subtracted to give the equivalent rainfall.
(d) Snowfall is also measured as the depth of the snow, which has freshly fallen in the stated period. In this case the depth of snow usually stated in terms of inches and the water equivalent in inches is obtained by dividing this amount by 10, assuming specific gravity of snow to be 0.1.
1 cm of snow = 1 mm of rainfall
(e) In heavy snowfall, the depth of snow is measured with snow poles and the water equivalent of snow determined. The depth of snow may be obtained by taking the mean of several measurements in places where there is drifting of snow, using a graduated ruler or scale. Care should be taken to measure only the depth of snow, which has fallen since the previous hour of observation. This can be done by sweeping a suitable patch clear beforehand or laying a cover of suitable material (wooden board or white painted sheets) about 3 square feet on the top of the snow surface and measuring the depth of the freshly fallen snow.
Self-Recording Raingauge (SRRG)
Self-recording rain gauge is an instrument used to obtain continuous record of the amount of precipitation.
The instrument consists of the following parts.
(a) An outer cylindrical case
(b) A float chamber
(c) A siphoning tube
(d) A clock and drum mechanism
(a) Outer cylindrical Case: This consists of two halves. The bottom half is provided with supporting base for the float chamber. The top half is provided with a brass of diameter 203 mm and a funnel for collecting the rainfall. The outer case provided with a glass window. The top portion fits neatly over the bottom half.
(b) Float Chamber: This consists of a cylindrical chamber provided with an inlet tube and a siphon tube. The tail of the funnel goes into the inlet tube when the top half of the outer case is placed in position. A metallic float rod moves up and down through the top lid of the chamber. Siphoning tube connected to the side of the float chamber consists of an outer tube and a small gap is provided between the opening of the capillary tube and the glass cap. When the float chamber is filled with water the whole water from the chamber is siphoned out through the capillary tube, due to capillary effect and the float falls to the bottom of the chamber. The chamber has a capacity of 10mm of water.
(c) Clock and Drum: This is attached to the top of the float chamber, the pen traces a continuous line showing increase in the amount of precipitation and each time the water is siphoned off, the pen traces a vertical line on the chart attached to the drum thus providing a continuous record of the amount of precipitation.
The SR rain gauge should be installed on a concrete masonry platform 45 cm square at a plinth level of 15 cm above ground in the observatory enclosure by the side of ordinary rain gauge and at distance of about 1.5 to 3.0 meter from it. The rim of the funnel should be perfectly horizontal at a height of exactly 75 cm above ground level. A channel is to be provided on the plinth for drainage of siphoned water.
Care and Maintenance
It is very important to ensure the routine care of pens, charts changing and time marking etc. besides:
(a) Necessary adjustments can be made by giving siphoning test in case there is no rainfall and whenever the chart is changed.
(b) It is important that the inside of the glass top of siphon should be kept clean as this is essential for proper siphoning. It can be cleared with soft dry cloth after unscrewing the top.
(c) While fixing the upper part it must be aligned with the inlet tube. This can be achieved by viewing through glass window provided to check the recording on the chart.