Introduction and meaning: Rain is the drops of water that fall on the earth from the cloud. Rainfall is defined as fall of innumerable drops of water (rain) at a time on the earth surface.
Causes of rain: Water from seas, ocean, river, lakes, canals, ponds etc. get vaporized due to the heat of the sun. This vapor rises up and comes in contact with cold air and condenses in the form of cloud.
When this cloud, is cooled further, it cannot float in space and falls down on the earth surface as rain by the gravitational attraction of the earth.
How rainfall is measured? The amount of rainfall at any place is measured by an instrument called rain-gauge. Rainfall is generally measured in inches or centimeters.
Rainfall on the surface of the earth may be classified into three kinds according to the causes of origin, characteristics and nature. There are:
Convectional rainfall: Intense heating of the surface of the earth during daytime, makes the air hot that is in close contact to the earth’s surface. This hot air expands in mass, becomes lighter and rises upward. The rising mass of air progressively losses the heat in the appear layer of atmosphere where it cools up and is condensed thereby generating heavy shower often in the afternoon and evening when temperature reduces remarkably. Such a kind of rainfall found during evening and afternoon of summer season, restricted to a certain particular locality is defined as convectional rainfall. This type of rain is very common during afternoon in equatorial region.
Orographic rainfall: When flow of humid air coming from the sea is obstructed obliquely by a mountain, the air gets up sliding along the slant of the mountain and being cooled and condensed it causes rainfall on the windward slant. Such rainfall is known as Orographic Rainfall. In the state of Meghalaya such rainfall occurs at Cherapunji and Mousinram.
Cyclonic rainfall: Rainfall caused by cyclone is known as Cyclonic rainfall. Generally, when hot humid air and cold air, moving parallel to the earth surface, approach each other, the hot air rises above the cold air and causes cyclone. Mainly in temperate region such cyclone occurs.