Earth Rainfall Climatology.doc

Earth rainfall climatology encompasses the rainfall distribution across different regions of the planet Earth. 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, bringsrainy 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.[1] 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.[2] 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. 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. The northern half of Africa is primarily desert or arid, containing the Sahara, which central Africa (known as SubSaharan Africa) sees an annual rainy season regulated by the movement of the intertropical convergence zone or monsoon trough. Across Asia, a large annual rainfall minimum, composed primarily of deserts, stretches from the Gobi desertin 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. 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 Willamette Valley. 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.[3]