There are some conditions, which are essential for the formation of clouds and subsequent occurrence of rainfall. Firstly, there should be sufficient water bodies and the heat of the Sun for the process of evaporation. Secondly, the winds should blow towards the land to oust the warm air containing the water vapour from the surface of the water bodies to the land. Finally, the process of inducing the warm air to rise upwards for cooling and condensation should be present.
Expansion on rising is the usual cause of cooling and cooling of the air, in turn, is the main cause of rain. However, air may rise up in the atmosphere due to various reasons. As we have discussed, it may rise in convection currents as a result of rapid heating. It may rise as a result of the mountains forming a barrier. Air blowing over hills or mountains is forced upward and can lead to the development of clouds. Sometimes a collision with a large mass of cold heavy air may also force the warm, humid and light air to go up.
There are three main types of rainfall, known as Relief or Orographical rainfall, Convectional rainfall and Frontal or Cyclonic Rainfall.
Relief or Orographical Rainfall occurs, when mountains stand as a barricade in the path of the moisture-laden warm winds and force them to flow upwards. As the air rises on the windward side of the mountain, it expands, cools and finally falls as rain. However, when the ascending air reaches the summit and descends on the leeward side, it becomes warm by compression, loses most of the moistures and becomes dry. This leeward side of the mountain is called Rain Shadow Area. Thus, heavier rainfall occurs in Cherrapunji, which lies on the windward side of the Khasi Hill, whereas Pune, which lies on the leeward side of Western Ghat, is a rain shadow area, as the clouds shed their moisture near the Lonavila area.
This type of rainfall may occur in any part of the world, where the wind blows onshore and mountains are situated favourably in relation to the direction of the winds. The relief rainfall is likely to be heavy, especially where a high continuous mountain range lies near the coast and the rain-bearing winds face it at right angles.
Convectional Rainfall is very common within the Tropics, especially in the Equatorial area, where it is almost a regular feature. As the convectional system and condensation reach their optimum level in the afternoon, the rainfall daily occurs at about 4 pm., and is known as the 4 O’clock Rain.
During the hot summer days, the heated land transmits radiant heat to the air near the surface. As a result, the surfacing air becomes warm. The warm air expands, becomes lighter and moves upward in the form of convection currents, carrying a lot of water vapour. When this moist air reaches the higher strata, it becomes cool, which results in the formation of Cumulonimbus clouds, causing rainfall. This is known as Convectional Rainfall. These rains are usually torrential and accompanied by thunder and lightning. They are often referred to as Thunder Showers, instead of’ thunder rain, as they do not last long.
Frontal or Cyclonic Rainfall is associated with cyclones in Tropical Latitudes and depression in Temperate Latitudes. It occurs, when a large mass of cold heavy air collides with a mass of warm, humid and light air. Warm air and cold air are of different kinds of air masses with different temperatures and densities. Hence, when warm and cold air masses move against each other, the warm air, being light, climbs over the heavier cold air. The separating line, or the boundary zon of these two different kind of air layers are called the ‘Front’. The rising warm air is then cooled and pushed upward by a frontal lift, which causes rainfall. This type of rainfall is known as Frontal or Cyclonic rainfall.
It is interesting to note that front has two parts. In the warm front, the warm air rises gently, then cools and condenses to form altostratus clouds, and the subsequent rain falls steadily for a few hours to a few days. While at the cold front, the warm air rises aggressively and condenses quickly, which results in the formation of cumulonimbus clouds. This rain is usually heavy and of short duration.
Rain Gauge is a simple instrument used to measure rainfall. It consists of a cylinder, with a funnel of 12.5 cm or 20 cm in diameter at the mouth, fitted on its top. The funnel leads into a smaller jar, kept inside the cylinder. As rain falls in the funnel, it trickles into the jar below. Usually twice a day the water collected in the jar is shifted to a specially designed graduated cylinder for measurement. The reading obtained in the graduated cylinder tells us the depth of rain that has fallen over a particular area, which is equal to that of the funnel. The rainfall is thus expressed as a depth of centimetres or millimetres. It represents the depth of rain water that would have been accumulated on the ground, if none of it had escaped.
By adding daily records of rainfall, we get the total rainfall in a particular year. The average rainfall for 35/40 years gives us the Mean Average Rainfall.