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Arid and semi-arid climates exist where more surface moisture is lost to evaporation than gained from precipitation. Arid and semi-arid climates cover 30 percent of earth’s land surface, a larger land area than any other climate. These climates have low annual precipitation, abundant sunshine, and high evaporation rates. The difference between the maximum daytime and nighttime minimum temperatures is wide, ranging from 15-20 degrees C (27-36 degrees F). Low precipitation in arid and semi-arid climates is influenced by the presence of semi-permanent, subtropical high-pressure areas located across the globe around 30 degrees latitude. These highpressure zones are large areas of descending air aloft, limiting cloud development and precipitation.
Subtropical arid climates, such as the Sonoran Desert, Rub al-Khali, Sahara, and the Australian Outback, are located at lower latitudes within subtropical high-pressure areas. These areas have very low annual precipitation between 25-150 mm (1-6 inches), and experience extremely hot summers, with temperatures commonly over 45 degrees C (113 degrees F). The world’s hottest, near-surface air temperature was 57.8 degrees C (136 degrees F) recorded at El Azizia, Libya in the northern Sahara.
Mid-latitude arid climates are defined as arid regions that have more than four months with mean temperatures cooler than 10 degrees C (50 degrees F). The Gobi and the Taklimakan Deserts of central Asia and the Great Basin Desert of the United States have midlatitude arid climates. Mountain rain shadow effects are a common influence on mid-latitude arid climates. For example, in Asia, the Himalayas block the northward flow of Indian Ocean moisture from reaching the Gobi and the Taklimakan Deserts. Summers are cooler than subtropical arid climates, and winters can be below freezing. Kashi, China, in the Taklimakan Desert, has a winter minimum temperature of minus 11 degrees C (12 degrees F) and summer maximum temperature of 33 degrees C (91 degrees F).
The world’s coastal arid climates are mainly located along the west coasts of South America (Atacama Desert) and southern Africa (Namib Desert). These climates typically have mild temperatures due to the adjacent cold ocean currents. The cold ocean waters limit convection and significant precipitation. Although overcast skies and coastal fog are frequent, these climates are phenomenally dry. On average there is only 10-20 mm (0.4-0.8 inches) of total annual precipitation.
The temperatures in semi-arid climates are generally similar to those of adjacent arid climates, but annual precipitation is higher. The largest areas of subtropical semi-arid climates are found in northern Mexico, the Sahel of North Africa, the Kalahari of southern Africa, and parts of the Australian Outback. Subtropical semi-arid climates average 500 mm (20 inches) of annual precipitation, which usually occurs during pronounced wet seasons. The vegetation in subtropical semi-arid climates consist of drought tolerant grasses, shrubs, and widely dispersed trees. The mid-latitude semi-arid “steppe” climates are found over central Asia and western North America. Annual precipitation ranges from 250 to 500 mm (10 to 20 inches) and cooler temperatures reduce the amount of moisture evaporated from the surface. The mid-latitude semi-arid climates are able to support significant low-lying vegetation consisting of bunch grasses and shrubs.
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