When a mass of moist air ascends to high altitudes it cools down to lower temperatures. In doing so it attains dew point which leads to condensation and precipitation. Thus the cooling of air occurs mainly when it rises. There are three important ways in which a mass of air can be forced to rise and each of these ways produces its own characteristic precipitation or rainfall.
- (a) Convectional Rainfall
- (b) Orographic or Relief Rainfall
- (c) Convergence or Cyclonic Rainfall
Excessive heating of the earth’s surface in tropical region results in the vertical air currents. These currents, lift the warm moist air to higher strata of atmosphere. When-the temperature of such a humid air starts falling below dew point continuously, clouds are formed. These clouds cause heavy rainfall which is associated with lightning and thunder. This type of rainfall is called conventional rainfall.
It is very common in equatorial region where it is a daily phenomenon in the afternoon.
Orographic or Relief Rainfall
Orographic rainfall on formed where air rises and cools because of a topographic barrier. When their temperature fall below dew point, clouds are formed. These clouds cause widespread rain on the windward slopes of the mountain range. This type of rain is called orographic rainfall.
However when these winds cross over the mountain range and descend along the leeward slopes, they get warm and cause little rain.
Region lying on the leeward side of the mountain receiving little rain is called rainshadow area.
A famous example of orographic rainfall is Cherrapunji on the southern margin of the Khasi Hills in Meghalaya India.
(c) Convergence or Cyclonic Rainfall
Convergence rainfall, produced where air currents converge and rise. In tropical regions where opposing air currents have comparable temperatures, the lifting is more or less vertical and is usually accompanied by convention.
Convectioned activity frequently occurs along fronts where the temperature of the air masses concerned are quite different. Mixing of air along the front also probably contributes to condensation and therefore to the frontal rainfall.
When two large air masses of different densities and temperature meet, the warmer moist air mass is lifted above the colder one. When this happens, the rising warm air mass condenses to form clouds which cause extensive down pour. This rainfall is associated with thunder and lightning. ‘This type of rainfall is also called frontal rainfall. This type of rainfall is associated with both warm and cold fronts. It is generally steady and may persist for a whole day or even longer.
(a) Rainfall Associated with a Warm Front
(b) Rainfall Associated with a Cold Front
|When cold air mass and warm air mass blow against each other, the boundary line of convergence separating the two air masses is termed as front.
In all these types, the cooling of large masses of humid air is essential to produce rainfall. In conventional rainfall, after rising of air, the subsequent processes are similar, to those of relief rainfall.
In nature, these three methods work together and infact most of the earth’s precipitation or rainfall is the result of two or more causes of lifting of air rather than of anyone.
Bibliography : NIOS Geography Book
Reference : http://www.nios.ac.in