Rainfall is measured in terms of the level or height to which water is collected or accumulated on a flat surface through rain. It is usually expressed in millimeters to the nearest whole number. Rather than measuring all the rainfall falling over a large areas, which is impractical, rainfall is measured at a number of points over the area. There are many instruments for measuring rainfall; the most commonly used is the rain gauge.
Rain gauge consists of a funnel (5”-6”in diameter), a measuring tube (usually one tenth of the funnel in diameter to measure accurately even the trace amounts of rainfall) and the outer cylindrical cover with a base. The rainfall falling into the funnel is directed into the measuring tube which is calibrated accordingly.
The excess water, if any, overflows the tube and is collected within the outer cover. This is measured subsequently. Based on the material by which the parts of a rain gauge are made, it may be fibre glass type or metal type. For continuous recording of rainfall, recording rain gauge is used where the rise of water level is automatically monitored continuously. Recording rain gauge may be with float type recorder or weighing type recorder.
In float type recorder the vertical movement of the float (with the rise of water level) is recorded by a pen on a chart fixed on a rotating drum; whereas in the weighing type, the weight of the receiver is recorded by an weight balance. The rain gauge must be placed at horizontally (at about 12” height from the ground) at a distance of twice the height of the nearest objects like trees, buildings etc.
Rainfall occurring in any place is simply measured as the height of the rainwater on the land in that place provided it is not lost due to run-off, evaporation etc. and the land is flat. Measuring rain this way is however impossible. A rain gauge must be used.
A simple rain gauge which any one can use to measure rain at his place consists of a funnel (3”to 4” in diameter) fitted into a bottle (about 1 litre capacity) to collect the rain water and a measuring cylinder. (An air-vent is to be provided to prevent accumulation of water in the funnel in case of heavy down pours.)
The rain gauge is kept on the ground in the open without obstructions from buildings, trees, etc. if the rainfall, over a period of time is 1cm at a place where the rain gauge is kept, then the height of the rainwater collected would also be 1 cm only if the bottle is flat at the bottom and its area cross section is the same as the area of the opening of the funnel.
Since this specification cannot be followed the volume of water so collected has to be measured (this will be constant for a given size of funnel irrespective of the size or shape of the bottle) to know the amount of rainfall. Suppose the area of the opening of the funnel is 80 cm2then for 1 cm of rainfall the volume of water would be 80 cm2 x 1 cm that is 80 cm3. This amounts to 8 cm3 of water for every mm of rain.
Thus if the total volume of rain water (in cm3) collected, over a specified period, is divided by 8, we get rainfall in mm in that place over the given period. To get accurate rainfall data quickly by directly observing the water level, a modified form of the above described rain gauge is used in all meteorological observatories. In this, rain water is collected in a narrow graduated tube so that the height of rain water is increased several times for the same amount of rain.
This facilitates accurate measurement of even low rainfall like 1 mm or less. If the area of fifth of the area of the opening of the funnel then for 1 mm rainfall the height of rain water in the tube would be 5mm. if the graduations and made accordingly, the water level in the narrow tube directly gives the rainfall.
For measuring continuous rain (which lasts several days on many occasions) automatic rain gauges are in use. In one type, called weighing type, the rainwater as it falls is weighed and translated into a continuous record on a clock-chart. Thus gives not only the total rainfall but also the time of its occurrence and its intensity.
