Seven precipitation regions can be recognized in terms of annual total in combination with location.
1. The wet equatorial belt of heavy rainfall, over 80 in (200 cm) annually, straddles the equator and includes the Amazon River basin in South America, the Congo River basin of equatorial Africa, much of the African coast from Nigeria west to Guinea, and the East Indies. Here the prevailingly warm temperatures and high moisture content of the mE air masses favor abundant convective rainfall. Thunderstorms are frequent year around.
2. Narrow coastal belts of high rainfall, (60-80+) in (150-200+ cm) per year, extend from near the equator to latitudes of about 25 to 30 N. and S. on the eastern sides of every continent or large island. For examples, see the eastern coasts of Brazil, Central America, Madagascar, and northeastern Australia. These are the trade-wind coasts, or windward tropical coasts, where moist mT air masses from warm oceans are brought over the land by the trades. Encountering coastal hills, escarpments, or mountains, these air masses produce heavy orographic rainfall.
3. In striking contrast to the wet equatorial belt astride the equator are the two zones of huge tropical deserts lying approximately upon the tropics of Cancer and Capricorn. These hot, barren deserts, with less than 10 in (25 cm) of rainfall annually and in many places with less than 2 in (5 cm) are located under and caused by the subtropical cells of high pressure where the subsiding cT air mass is adiabatically warmed and dried. Note that these deserts extend off the west coasts of the lands and out over the oceans. Such rain as these areas experience is largely convective and extremely unreliable.
4. Farther northward, in the interiors of Asia and North America between lat. 30 and lat. 50, are great continental middle-latitude deserts and expanses of semiarid grasslands known as steppes. Annual precipitation ranges from less than 4 in (10 cm) in the driest areas to 20 in (50 cm) in the moister steppes. Dryness here results from remoteness from ocean sources of moisture. Located in a region of prevailing westerly winds, these arid lands occupy the position of rain shadows in the lee of coastal mountains and highlands. Upon descending into the intermontance basins and interior plains, the mP air masses are warmed and dried.
Similarly, mountains of Europe and the Scandinavian peninsula serve to obstruct the flow of moist mP air masses from the North Atlantic into western Asia. The great southern Asiatic ranges likewise prevent the entry of moist mT and mE air masses from the Indian Ocean.
5. On the southeastern sides of the continents of North America and Asia, in lat. 25 to 45, and to a less marked degree in these same latitudes in the southern hemisphere in Uruguay, Argentina, and southeastern Australia, are the humid subtropical regions, with 40-60 in (100-150 cm) of rainfall annually. These regions lie on the moist western sides of the subtropical high-pressure centres in such a position that humid mT air masses from the tropical ocean are carried pole-ward over the adjoining land. Commonly too, these areas receive heavy rains from tropical cyclones.
6. Still another distinctive wet location is on middle-latitude west coasts of all continents and large islands lying between about 350 and 650 in the region of prevailing westerly winds. Where the coasts are mountainous, as in Alaska and British Columbia, Patagonia, Scotland, Norway, and South, Island of New Zealand, the annual precipitation is over 80 in (200 cm). Small wonder that these coasts formerly supported great valley glaciers that carved the deep bays (fiords,) so typically a part of their scenery.
7. The seventh precipitation region is formed by the arctic and polar deserts. Northward of the 60th parallel, annual precipitation is largely under 12 in (30 cm), except for the west-coast belts. Cold cP and cA air masses cannot hold much moisture, consequently they do not yield large amounts of precipitation. At the same time, however, the relative humidity is high and evaporation rates are low. Consequently these arctic and polar regions have abundant moisture in the air and soil and are not to be considered as dry in the same sense as the tropical deserts.
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