In general, sun intensity and, consequently, temperatures decrease from south to north; in summer, however, the decrease in intensity is partly offset by longer days in the north. Montana, North Dakota, and Minnesota actually have higher record temperatures than New Mexico and Alabama. In winter, on the other hand, the short days in the north exaggerate the effect of low sun angles, creating wide temperature differences from south to north. Forests use up much solar energy to evaporate water, and therefore the humid states of the eastern United States do not get as warm as the dry western deserts. Oceans and lakes moderate temperatures, and mountains are somewhat cooler by day and much colder at night than surrounding lowlands.
The pattern of precipitation is largely a consequence of the interaction of wind and topography. The wind system of the Earth balances temperatures by taking heat from the equator and carrying it towards the poles. Two features of this global atmospheric circulation are particularly significant for the United States. One is a current of sinking air, a gentle but persistent downward movement of air from the upper atmosphere.
This subsidence is part of the global convection cycle and starts with updrafts of warm and humid air near the equator; the air loses moisture as it rises to the upper atmosphere and begins to move polewards. At about latitude 30° north the air begins to sink, bringing hot and dry conditions to the south-western United States, especially in summer.
The other significant part of atmospheric circulation is the jet stream, a shifting zone of fast winds blowing high above the ground, generally from west to east. The path of the jet stream on any given day is a key to surface weather. In summer, the jet stream is usually near the Canadian border, though it may loop as far north as Alaska or as far south as Louisiana. It brings wet Pacific air onshore in Washington and Alaska, but in other western states dry air masses from Mexico and Canada dominate.
A finales de la década de 1980, entre las exportaciones importantes figuraban el petróleo, y sus productos derivados, el gas natural y la chapa de madera; de menor importancia eran el café, el caucho, el estaño, el aceite de palma, el tabaco, el té y la pimienta. Entre las importaciones predominaban la maquinaria, el equipamiento de transportes, equipos eléctricos, productos químicos, arroz, hierro y acero, y medicinas. Los principales socios comerciales de Indonesia son Japón, Estados Unidos, Singapur, China y Corea del Sur.
In the east, by contrast, the jet can pull moist air masses northward from the Gulf of Mexico all the way to Canada. In winter, the entire wind system follows the sun southward. Pacific air masses now bring clouds and rain to the coastal mountains from California to southern Alaska. The jet usually crosses the country at the latitude of Oklahoma, and cold, dry Canadian air covers the northern half of the country; however, day-to-day shifts of the jet may pull warm, moist Gulf air as far north as Illinois or bring Canadian air to Florida. Regional weather hazards are intimately associated with the seasonal position of the jet stream and associated fronts. Torrential rains are most common near the Gulf of Mexico, which is the major source of moisture for the country. Tornadoes occur in the centre of the United States, where Canadian and Gulf air masses often collide violently; hurricanes arise out of the late-summer warmth of the Atlantic Ocean and drift towards the south-eastern states in the autumn. Southern California experiences smog and forest fires in late summer.
Heavy winter snows in the eastern United States are caused by the rapid cooling of Gulf air, amplified in the Great Lakes region by local lake breezes. December and March are the major months for snow in Minnesota and the Dakotas; in January there is a time of intense cold and little snowfall, because Gulf air cannot penetrate that far north. Finally, the occasional kona (west coast) storms of Hawaii are wintertime incursions of North Pacific air that occur when the jet stream curves far to the south. Normal weather consists of trade winds that cause rain only on the north-eastern slopes of each island "United States" © Emmanuel Buchot and Encarta
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