Types of rainfall

There are three distinct, main types of rainfall, or forms that rain can fall in. These are convectional rainfall, frontal or cyclonic rainfall, and orographic or relief rainfall.__NOTOC__

Convectional rainfall

Convectional rainfall usually occurs in places with warmer or tropical climates, and in countries close to the equator; in places such as Europe and the United States this type of rainfall mostly occurs during the summer months.

Convection is the condition where rising currents of warm or heated air are separated by larger areas of gradually dropping air, and is commonly the cause of particularly powerful thunderstorms that occur in summer or in hot regions.

Convectional rain occurs when the surface of the atmosphere becomes heated or hotter than normal, which in turn causes the damp air to rise. As this heated and dampened air cools, it forms cloud matter. These clouds, which are of the cumulonimbus variety, eventually become large thunder clouds, and then release the great amount of water they contain in a massive downpour.

Frontal or cyclonic rainfall

Frontal or cyclonic rainfall is caused mainly by the occurrence of low pressure areas or lows, and occurs when warm and often tropical air meets cooler air. When these two opposing masses of air meet, the warm air 'prevails', as such, over the colder air, and the two contrasting forces of air cause a front. Fronts cause sudden, enigmatic changes in general temperature, and in the humidity and pressure in the air.

Fronts can be either cold fronts or warm fronts. Warm fronts occur in the situation presented above, where the warm air and cold air meet, and the warm air 'overrides' the cooler air and moves upward. Warm fronts are usually followed by several days of intense rainfall, because, after the warm air rises above the cooler air (which sinks to the ground), it gradually cools, forms clouds, and causes rain.

Cold fronts occur when a mass of cooler air meets a mass of warm air, as opposed to a when a warm mass of air confronts a cold mass of air. This, however, causes a similar effect to a warm front, but the ensuing rain duration is less, but perhaps more intense than that of which is caused by warm fronts.

Orographic or relief rainfall

Orographic or relief rainfall is caused when masses of air pushed by wind encounter sizeable objects or land formations they cannot pass, such as large mountains (hence its name of orographic rainfall). This forces the air above the object/land form in question and, if the air is pushed high enough, it is condensed and becomes cloud matter, which will then cause rain to fall. This type of rainfall is also known as relief rainfall. One of the rainiest places on Earth, Mount Waialeale, on the island of Kauai, is an example of orographic rainfall.

Effective rainfall

Effective rainfall is defined as that part of the precipitation that reaches stream and river channels as direct runoff (as opposed to that which is intercepted by vegetation or lost as evaporation).

See also

*Weather
*Flood
*Cyclone
*High pressure area

References

* [http://hrsbstaff.ednet.ns.ca/mcallip/WestCoast/types_of_rainfall.htm The Three Types of Rainfall]
* [http://www.bbc.co.uk/schools/gcsebitesize/geography/weather/elementsofweatherrev6.shtml BBC - GCSE Bitesize - Geography | Weather | Types of rainfall]
* [http://www.projectgcse.co.uk/geography/rainfall.htm GCSE Geography> Water> Rainfall]