Types of Rainfall
Rainfall occurs due to the ascent of moisture bearing clouds or a body of moist air and its subsequent cooling, condensation and precipitation. A light rain is called a drizzle. Ascent of rain may take place in different ways.
- Obstruction by a relief barrier such as a mountain.
- Uplift due to convection currents.
- Cyclone in temperature or tropical regions.
Imaginary lines (drawn on maps) joining places having equal amount of rainfall are called isohyets.
When a moisture bearing wind is obstructed by a mountain range, it is forced to rise along the slope. On rising, it cools to form a cumulonimbus cloud. Further cooling causes Relief rain. The windward side of the mountain barrier receives heavy rainfall. On the leeward side air gets warmer as it descends and hence very little rain falls. The leeward side of the mountain is called the rain shadow. This type of rain is received in the Indian subcontinent the south-west monsoon that blows in from the Arabian Sea. The Western ghats receive over 300 cm of rain. The Leeward side on the Deccan plateau gets hardly 60 to 70 cm of rain.
This type of rain is common in the hot and wet tropical rain forests of central Africa, Amazon basin of South America and the islands of south-east Asia. Continuous heating of the earth in the day causes hot convectional currents to raise Water keeps on evaporating and rising up. On reaching the upper layers of the atmosphere they cool, condense and from cumulonimbus clouds. In the late afternoon the clouds burst into a heavy downpour, often accompanied by thunder and lighting. It usual occurs at about 4 pm, so convectional rain is called ‘four o’clock rain’ and occurs daily in the equatorial regions.
In the mid latitudes warm moist air from the sub-tropics meets the cold polar air near the Arctic Circle. The plane along which they meet is called a front. The warm air being lighter rises up and condenses on reaching a certain height. Continued condensation causes heavy rain along the front. This rain is common at sub polar low pressure belts where the warm westerlies meet the cold Polar winds. The former rise up, cool and condense and give rainfall. Such rain is called frontal rainfall. In the tropics, winds blow in form all sides towards a lower pressure center. They move in a spiral motion at great speed. At the center, the air is forced to rise up. The sudden uplift of the warm air causes cooling and condensation and formation of cumulonimbus clouds which result in heavy rainfall. This rain is called Cyclonic rainfall.