Rainfall is synonymously used with precipitation by many peoples. But these two terms differ typically. Precipitation includes all forms of water that are received by earth from the atmosphere which includes rainfall, snowfall, frost, hail etc. Rainfall generally describes that form of precipitation where the size of water droplets is larger than 0.5mm. Rainfall is termed as light when its intensity is less than 2.5mm/hr, moderate between 2.5 and 7.5mm/hr and heavy when the intensity is more than 7.5mm/hr. Drizzle is a form of rain with numerous droplets of size less than 0.5 mm with intensity less than 1mm/hr. A hailstorm is a form of precipitation in the form of irregular pellets or lumps of frozen rain of size greater than 8 mm.
The period with which rainfall occurs is known as the duration of rainfall and is expressed as the unit of time. The intensity of rainfall is defined as the rate at which rainfall takes place. It is expressed in mm/hr or cm/hr. Rainfall intensity decides the runoff. Rainfall frequency denotes the period in years during which a rainfall of a given duration and intensity can be expected to occur. It is expressed as percent chance. Amount of rainfall refers to the depth to which rainwater would stand on a horizontal surface with conditions of no infiltration, runoff, and evaporation.
Types of Rainfall
Orographic precipitation occurs when rain bearing clouds encounter barriers such as mountains. These barriers stop the air and lifted up. This results in the expansion of air and adiabatic cooling. Thus, rain is formed due to the condensation of the water vapor.
Cyclonic precipitation refers to the rainfall associated with cyclones. Cyclone is a large whirling mass of air which is moving at a velocity of 50 to 80 km/hr. The low-pressure zone in a cyclone acts as a chimney through which the air mass from surrounding gets lifted up. This causes precipitation heavily.
In Convective precipitation, the air mass is heated up by the solar radiation. The heated air mass become lighter and gets lighted up. THis is followed by adiabatic cooling which causes condensation of water vapour and produces precipitation.
Rainfall measurement is based on sampling method of estimation. In this method, the rain gauges are located at predetermined points in the watershed and averages value is arrived at. Rain gauges used may be of non recording or recording type. The average rainfall in an area estimated by the following ways.
Arithmetic mean: The average rainfall is obtained by dividing the sum of the readings from all the rain gauges in an area by the total number of rain gauges. The distribution of rain gauge determines the accuracy. This method does not give more accurate results due to the irregular duration and intensity of rainfall in the area.
Thiessen polygon Method: The location of the rain gauges is plotted on a map and they are connected by a straight line. Perpendicular bisectors are marked on each of the lines such that each of the rain gauge station is enclosed in a certain area. If R1, R2, R3…. are the amounts of rainfall recorded in each of the rain gauges, average rainfall over the given area (A), is derived from the formula given below:
Isohyetal Method: Rain gauge stations are plotted and located on the map and the amounts of rainfall at each of the station are gathered. Isohyetals (lines of equal rainfall) are drawn by interpolation on the map. The area between the successive Isohyetals is determined, for which planimeter can be used. If P1, P2, P3… are the amounts of rainfall recorded in each of the rain gauge and A1, A2, A3…. are the areas of the irregular figures in between Isohyetals, then the average rainfall (R) could be expressed by the formula given below