Measurement of rainfall

I. Non-recording type of rain gauge:

• These are most widely used for the measurement of rainfall. They are known as non-recording because amount of the rainfall is not measured by recording but by collecting the rainwater over a period of time.
• The collected rain is then measured by means of graduated cylinder so as to directly represent the rainfall volume in cm of water depth.

Depth of rainfall(cm) =  Volume of water collected (cm3)/Area of the aperture of collector (cm2)

a) Symon’s gauge:

• The Symon rain gauge consists of a cylindrical vessel, called metal casing, of 12.7 cm diameter, with a base of 21.0cm.
• The metal casing is fixed vertically to a masonry foundation block of the size 60 cm x 60 cm x 60 cm.
• A funnel with a circular rim of 12.7 cm and a glass bottle are placed in the metal casing.
• The height of the metal casing is fixed so that the rim is 30.5 cm above the ground surface. The glass bottle, called the receiver, is of 7.5 to 10 cm diameter.
• The rain gauge is kept in open. The rain falling into the funnel is collected in the receiver.
• The receiver, with the rain water in it, is taken out of the metal casing. The rainfall depth is measured with a special measuring glass jar graduated in mm of rainfall.
• The total rainfall of the day is obtained by adding all the individual measurements of day.

b) Standard rain gauge:

• It is a modified version of the Symon’s rain gauge. The standard rain gauge consists of a collector with a gun metal rim.
• The exposed surface area of the collector is either 100 cm2 or 200 cm2.
• The collector has a deep-set funnel which discharges into a polyethylene bottle kept inside the base. The capacity of the bottle can be 2, 4 or 10 liters.
• The rain collected in the bottle is measured with a graduated cylinder which directly gives the rainfall in mm or cm.
• The advantage of the standard rain gauge over the Symon gauge is that it can be installed such that the rim is always at a standard height of 30 cm above the ground surface irrespective of the nominal capacity.
• But it does not give the duration and intensity of the rainfall.

II. Recording type of rain gauge:

• The recording type rain gauges have recording arrangement, which gives the rainfall intensity and the duration of the rainfall, besides giving the total depth of the rainfall, but records automatically on a graph, which is known as the mass curve of rain fallen.
• such a curve will also help in indicating the times of onset and cessation of rain, and thereby indicating its duration.
• Moreover, the slope of the curve gives us the intensity of rainfall for any given period.