<P> Earth rainfall climatology Is the study of rainfall, a sub-field of Meteorology . Formally, a wider study includes water falling as ice crystals, i.e. hail, sleet, snow (parts of the hydrological cycle known as precipitation). The aim of rainfall climatology is to measure, understand and predict rain distribution across different regions of planet Earth, a factor of air pressure, humidity, topography, cloud type and raindrop size, via direct measurement and remote sensing data acquisition . Current technologies accurately predict rainfall 3--4 days in advance using numerical weather prediction . Geostationary orbiting satellites gather IR and visual wavelength data to measure realtime localised rainfall by estimating cloud albedo, water content, and the corresponding probability of rain . Geographic distribution of rain is largely governed by climate type, topography and habitat humidity . In mountainous areas, heavy precipitation is possible where upslope flow is maximized within windward sides of the terrain at elevation . On the leeward side of mountains, desert climates can exist due to the dry air caused by compressional heating . The movement of the monsoon trough, or intertropical convergence zone, brings rainy seasons to savannah climes . The urban heat island effect leads to increased rainfall, both in amounts and intensity, downwind of cities . Global warming may also cause changes in the precipitation pattern globally, including wetter conditions at high latitudes and in some wet tropical areas, and drier conditions in parts of the subtropics and middle latitudes . Precipitation is a major component of the water cycle, and is responsible for depositing most of the fresh water on the planet . Approximately 505,000 cubic kilometres (121,000 cu mi) of water falls as precipitation each year; 398,000 cubic kilometres (95,000 cu mi) of it over the oceans . Given the Earth's surface area, that means the globally averaged annual precipitation is 990 millimetres (39 in). Climate classification systems such as the Köppen climate classification system use average annual rainfall to help differentiate between differing climate regimes . </P> <P> Most of Australia is semi-arid or desert, making it the world's driest continent . Australia's rainfall is mainly regulated by the movement of the monsoon trough during the summer rainy season, with lesser amounts falling during the winter and spring in its southernmost sections . Almost whole North Africa is semi-arid, arid or hyper - arid, containing the Sahara Desert which is the largest hot desert in the world, while central Africa (known as Sub-Saharan Africa) sees an annual rainy season regulated by the movement of the intertropical convergence zone or monsoon trough, though the Sahel Belt located at the south of the Sahara Desert knows an extremely intense and a nearly permanent dry season and only receives minimum summer rainfall . Across Asia, a large annual rainfall minimum, composed primarily of deserts, stretches from the Gobi Desert in Mongolia west - southwest through Pakistan and Iran into the Arabian Desert in Saudi Arabia . In Asia, rainfall is favored across its southern portion from India east and northeast across the Philippines and southern China into Japan due to the monsoon advecting moisture primarily from the Indian Ocean into the region . Similar, but weaker, monsoon circulations are present over North America and Australia . In Europe, the wettest regions are in the Alps and downwind of bodies of water, particularly the Atlantic west coasts . Within North America, the drier areas of the United States are the Desert Southwest, Great Basin, valleys of northeast Arizona, eastern Utah, central Wyoming, and the Columbia Basin . Other dry regions within the continent are far northern Canada and the Sonoran Desert of northwest Mexico . The Pacific Northwest United States, the Rockies of British Columbia, and the coastal ranges of Alaska are the wettest locations in North America . The equatorial region near the Intertropical Convergence Zone (ITCZ), or monsoon trough, is the wettest portion of the world's continents . Annually, the rain belt within the tropics marches northward by August, then moves back southward into the Southern Hemisphere by February and March . </P> <Table> <Tr> <Td> Af Am Aw </Td> <Td> </Td> <Td> BWh BWk BSh BSk </Td> <Td> </Td> <Td> Csa Csb </Td> <Td> </Td> <Td> Cwa Cwb </Td> <Td> </Td> <Td> Cfa Cfb Cfc </Td> <Td> </Td> <Td> Dsa Dsb Dsc Dsd </Td> <Td> </Td> <Td> Dwa Dwb Dwc Dwd </Td> <Td> </Td> <Td> Dfa Dfb Dfc Dfd </Td> <Td> </Td> <Td> ET EF </Td> </Tr> </Table>

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