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Humidity and Rainfall

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Humidity and Rainfall - Lesson Summary

The weather occurs in the lower part of the atmosphere which contains moisture as well, mostly in the form of water vapour, an invisible gas. The surface water from water bodies like lakes, oceans and rivers evaporate to form moisture in the form of water vapour. This process is known as evaporation.

The amount of water vapour present in  the atmosphere is called as humidity or absolute humidity. Water vapour when cools down and condenses in form of liquid water or snow. This process is called condensation.

Condensation takes place in different ways like dew or frost which happens when the moisture in the air is exposed to a cold surface; mist or fog happens when the moisture in the air condenses near the surface of the ground and clouds are formed when the moisture in the air condenses at high altitudes. 

Rain, snow, sleet and hail are some other forms of condensation.

Clouds can be classified in three main groups on the basis of their shape and height.

Cirrus clouds are the highest clouds feathery or like hair curls, formed above 6000 meters or 20,000 feet. Cirrus clouds indicate the presence of moisture at high altitudes.

Cumulus clouds are puffy and look like puffs of cotton. They are quite in the sky and if they grow tall, they become thunderheads and bring down rain.

Stratus clouds are flat sheets of clouds, quite low in sky and look like a low gray blanket. These clouds are responsible to bring rain or snow.

Precipitation is the process where water vapour condenses in the air and returns to the earth in form of rain, dew or snow. Rainfall is the most common form of precipitation and occurs in three different ways i.e. convectional rainfall, orographic rainfall and cyclonic rainfall. 

Convectional rainfall occurs when the hot air rises and cools down eventually during condensation, to fall as rain. When hot air rises, it cools and starts condensing to form clouds. When these clouds hit mountains, they cause rainfall in that region. This is called as orographic or relief rainfall. 


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