With the exception of the province of Asir with its towns of Jizan on the western coast and Najran, Saudi Arabia has a desert climate characterized by extreme heat during the day, an abrupt drop in temperature at night, and slight, erratic rainfall. Because of the influence of a subtropical high-pressure system and the many fluctuations in elevation, there is considerable variation in temperature and humidity. The two main extremes in climate are felt between the coastal lands and the interior.
Along the coastal regions of the Red Sea and the Persian Gulf, the desert temperature is moderated by the proximity of these large bodies of water. Temperatures seldom rise above 38° C, but the relative humidity is usually more than 85 percent and frequently 100 percent for extended periods. This combination produces a hot mist during the day and a warm fog at night. Prevailing winds are from the north, and, when they blow, coastal areas become bearable in the summer and even pleasant in winter. A southerly wind is accompanied invariably by an increase in temperature and humidity and by a particular kind of storm known in the gulf area as a kauf. In late spring and early summer, a strong northwesterly wind, the shamal, blows; it is particularly severe in eastern Arabia and continues for almost three months. The shamal produces sandstorms and dust storms that can decrease visibility to a few meters.
A uniform climate prevails in Najd, Al Qasim Province, and the great deserts. The average summer temperature is 45° C, but readings of up to 54° C are common. The heat becomes intense shortly after sunrise and lasts until sunset, followed by comparatively cool nights. In the winter, the temperature seldom drops below 0° C, but the almost total absence of humidity and the high wind-chill factor make a bitterly cold atmosphere. In the spring and autumn, temperatures average 29° C.
The region of Asir is subject to Indian Ocean monsoons, usually occurring between October and March. An average of 300 millimeters of rainfall occurs during this period--60 percent of the annual total. Additionally, in Asir and the southern Hijaz condensation caused by the higher mountain slopes contributes to the total rainfall.
For the rest of the country, rainfall is low and erratic. The entire year's rainfall may consist of one or two torrential outbursts that flood the wadis and then rapidly disappear into the soil to be trapped above the layers of impervious rock. This is sufficient, however, to sustain forage growth. Although the average rainfall is 100 millimeters per year, whole regions may not experience rainfall for several years. When such droughts occur, as they did in the north in 1957 and 1958, affected areas may become incapable of sustaining either livestock or agriculture.
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