Rainfall
Evaporation causes the formation of water vapour and condensation causes the formation of droplets of water. Condensation occurs when the water vapour cools on rising. Clouds are masses of such water droplets. When these droplets become too heavy to float in the air, they come down as precipitation.
Precipitation that comes down to the Earth in the liquid form is called rainfall. Most of the ground water comes from rain. Rainfall is the most common form of precipitation.
Types of Rainfall :-
There are three types of rainfall -
1. Conventional Rainfall - Conventional rainfall is formed when air on the surface of the Earth and few metres above it is heated by the sun. As the air is heated, it becomes lighter. The lighter air rises, cools down, and then condenses. It condenses to form thick cumulous clouds. These rising clouds become heavier and unstable. This unstable cloud then drop to the ground as rainfall.
2. Orographic or Relief Rainfall - This type of rainfall is common in places with mountains and sea. Relief rainfall frequently occurs near mountains beside the sea. The moisture- laden air blows in from the sea because the wind meets a high mountain and hence it is forced to rise upwards. At the height, it is cooled and cloud is formed.
This saturated cloud with water vapour begins to precipitate on the side of the mountains facing the sea. This front side of the mountain is called the Windward side.
The cloud mostly precipitates on the windward side of the mountain. Meanwhile, the cloud meets the other side, which is called the
leeward side. Since the cloud has already lost most of its moisture so it rains very little there.
3. Frontal Rainfall - This rainfall occurs when a warm, tropical air mass comes in contact with a cold, polar air mass. It is very common in Britain and Ireland . Because the air is in warm front, then it rises over the cold front. The air is cooled and so condenses to form a Stratus cloud. Thus when the Stratus cloud becomes saturated, it begins to precipitate.
Measurement:-
The amount of rain that falls over a given area is measured by a rain gauge. The unit of measurement of rainfall is millimeters. The rain gauge consists of a metal cylinder fitted with a collecting vessel. The cylinder is fitted with a metal funnel which leads up to the collecting vessel. The hole in the funnel that leads down to the bottle is kept very small so that evaporation is kept to minimum. At the end of the period for which the rainfall is being measured, the rain water that has collected in the collecting vessel is poured to a graduated measuring jar. The graduation on the measuring cylinder is such that the reading obtained is the depth of rain that has fallen over an area equivalent to that of the top of the funnel. To measure snowfall snow is collected, melted and then measured as rain. The self recording instrument for measuring rainfall is called pluviograph.
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