Measurement of Rainfall – Raingauges

Rainfall is expressed in terms of depth to which water would stand on an area if all rainwater was too collected on it and it is measured by Raingauges.

• If 1 cm rainfall occurs over an area of 1 km², then it represents that the volume of water over that area is 10⁴ m³

Volume of water = Rainfall depth x Area

• In case of snowfall, an equivalent depth of water is used as the depth of precipitation.

The rainfall is measured by raingauges, also called as pluviometer, ombrometer, Hyetometer and udometer. A raingauge essentially consist of a cylindrical vessel assemble which is kept in the open area to collect the rain water and calibrated for measurement.

• The ground must be levelled, open and the instrument must present a horizontal catch surface.
• The rain gauge must be surrounded by an open fence area whose size should be atleast 5.5 m x 5.5 m.
• It should be kept away from any obstruction by at least 30 m or twice the height of the building or obstruction.
• It must be set as near the ground as possible to reduce wind effect but it must be sufficiently high to prevent from flooding.

Raingauges may be broadly classified under two heads:

They are known as Non-Recording type because they do not record the rain but only collect the rain.

• In India, the most commonly used non-recording raingauge is Symon’s Rain gauge which has a collecting diameter if 127 mm (5 inch)
• The Symon’s rain gauge was most widly used non recording gauge till 1969, after that, Indian Meteorological Department (IMD) has shifted over to the use of standard gauge which has a collecting area of either 100 cm² or 200 cm².
• Now the symon’s type raingauge is obsolete in India.
• These raingauges can also be used to measure the snowfall, when snow is expected. In the case of snow, the funnel and bottle is removed and snow is allowed to collect in the outer metal container. When the snow is melted, the depth of resulting water is measured.

This type of raingauges can give us a permanent automatic rainfall record and produces a continuous rainfall variation against time graph.

• Using this data, the rainfall intensity with time can also be plotted. This also shows the total accumulated rainfall vs time curve.
• These raingauges are sometimes called as integrating raingauges or continuous raingauges.
• They are of great use in hills and for opposite areas, where it is not practically feasible to daily visit the gauge station.

The commonly used recording raingauges are:

a. Tipping Bucket type

b. Weighing Bucket type

c. Natural Syphon Type

In Tipping Bucket Type raingauges, the rain water is first caught in a collector and passed through a funnel. The funnel discharge the water into a two bucket compartment when some amount of rain (0.10 – 0.25 mm) gets filled up in one compartment the bucket tips, emptying into a reservoir and moving the second compartment into place beneath the funnel.

• The movement of the tipping of the bucket can be transmitted electronically over distances. So such gauges can be installed in hilly or inaccessible area, from where they can supply measurements directly in the control room.
• The record from the tipping bucket type raingauges gives the data of the intensity if rainfall.

In this type of raingauges, the weight of rain water which falls into a bucket placed on the platform of a spacing on other weighing mechanism. The increasing weight of bucket helps in recording the increasing quantity of collected rain by time, by moving a pen on recording drum.

• This instrument gives a plot of accumulated rainfall against the elapsed time, means it provide mass curve of rainfall.

In this raingauge, the rainfall collected by a funnel shaped collector and lead into a float chamber due to this float will rise. As the float rises, a pen attached to the float through a lever system records the elevation of the float on a rotating drum.

• A syphon arrangement empties the float chamber as the float has reached a preset maximum level and again the reading starts from initial point.
• This gauge gives mass curve of rainfall.

This is basically a recording type rain gauge which contains electronic units to transfer data to the base station, both at regular interval or continuous. Telemetric raingauges are mostly used in collecting rainfall data from hill or inaccessible place.

Radar is a powerful instrument used to measure the aerial extent, location and movement of rain storms.

• Rainfall over a large area can be measured with good accuracy.
• Metereological radar operate at a wavelength of 3 to 10 cm.
• For observing heavy rain, we use 10 cm wavelength while for the light rain, 3 cm wavelength is used.
• A radar can cover the area for observation as much as 1 lakh km².
• The hydrological range of the radar is of the order of 200 km.

This is very modern or advanced technique to find the rainfall amount, area cover and location in which the image is taken by satellite and sent to the control station. By this technique, we can have the knowledge about the precipitation.

These type of gauges are used in a remote area where daily visit is not possible. By this type of gauges, we find the rainfall reasonably early.