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Climates and Climatic Regions, Climatic Regions of the United States

orographic precipitation, eastern woodlands, coniferous forests, Coast Ranges, steppes

Because of its midlatitude location and vast size, the United States experiences a wide variety of climates. At one extreme are the tropical islands of Hawaii; at the other, the arctic conditions of northern Alaska. The majority of Americans live between these two extremes in a group of climatic regions with unique moisture and temperature patterns.

Geographers have traditionally divided the 48 contiguous United States into two broad patterns of continental climate: the humid East and the arid West. The dividing line most often used is 100 degrees west longitude, an imaginary north-south line extending through the Great Plains from Texas to North Dakota.

The humid east receives abundant precipitation throughout the year. Winters in the northern part are very cold with much snowfall. In the southern part, rainfall is plentiful; summers are very hot but winters are mild. Because of its bountiful moisture, the humid east has also traditionally been a very important agricultural area. Once a land of vast forests, early settlers cleared the land as they moved westward. In some areas, cleared lands were cultivated, abused, exhausted, and eroded away. In other areas, vast forests have been replanted, as in the South, the Appalachians, and parts of the Midwest.

A climatic transition zone occurs on either side of the 100 degrees west longitude line. The eastern woodlands gradually give way to tall grass prairies, which in turn give way to steppes, where short grasses flourish. Few natural tall grass prairies exist today on the Plains. Over the past few centuries, farmers cultivated and planted most of the region with corn or wheat.

In the arid West, precipitation diminishes from east to west and eventually reaches the point where it becomes impossible to raise crops without irrigation. Some desert areas of Arizona, Nevada, and southern California receive less than 125 mm (5 in) of precipitation annually. The grazing of livestock is an important agricultural activity in these areas of mesquite bushes and cacti.

Not all of the West is dry. In fact, one of the wettest areas of the United States is located in the Pacific Northwest. On the west-facing slopes of the Cascades and the Coast Ranges, moisture-laden winds blow from the Pacific Ocean and drop their rain on the mountain slopes. This type of mountain-induced rainfall is known as orographic precipitation. It occurs when wet air rises along the slope of a mountain. As the air moves upward into cooler temperature zones, it expands and cools, releasing the moisture as precipitation. Because of this effect, the climate of the Northwest is cool and moist, and the land is covered with vast, coniferous forests.

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