|
Desert climate (in the Köppen climate classification ''BWh'' and ''BWk'', sometimes also ''BWn''), also known as an arid climate, is a climate that does not meet the criteria to be classified as a polar climate, and in which precipitation is too low to sustain any vegetation at all, or at most a very scanty shrub. An area that features this climate usually experiences less than 250 mm (10 inches) per year of precipitation and in some years may experience no precipitation at all. In some instances, an area may experience more than 250 mm of precipitation annually, but is considered a desert climate because the region loses more water via evapotranspiration than falls as precipitation (Tucson, Arizona and Alice Springs, Northern Territory are examples of this). There are usually 2 or three variations of a desert climate: a hot desert climate (''BWh''), a cold desert climate (''BWk'') and, sometimes, a mild desert climate (''BWh/BWn''). Furthermore, to delineate "hot desert climates" from "cold desert climates", there are three widely used isotherms: either a mean annual temperature of 18 °C, or a mean temperature of 0 °C or -3 °C in the coldest month, so that a location with a "BW" type climate with the appropriate temperature above whichever isotherm is being used is classified as "hot arid" (BWh), and a location with the appropriate temperature below the given isotherm is classified as "cold arid" (BWk). To determine whether a location has an arid climate, the precipitation threshold is determined. The precipitation threshold (in millimetres) involves first multiplying the average annual temperature in °C by 20, then adding 280 if 70% or more of the total precipitation is in the high-sun half of the year (April through September in the Northern Hemisphere, or October through March in the Southern), or 140 if 30%–70% of the total precipitation is received during the applicable period, or 0 if less than 30% of the total precipitation is so received. If the area's annual precipitation is less than half the threshold, it is classified as a ''BW'' (desert climate). ==Hot desert climates== Hot desert climates are typically found under the subtropical ridge where there is largely unbroken sunshine for the whole year due to the stable descending air and high pressure aloft. These areas are located between 30 degrees south and 30 degrees north latitude, under the subtropical latitudes called the horse latitudes. Hot desert climates are generally hot, sunny and dry year-round. Hot desert climates feature hot, typically exceptionally hot, periods of the year. In many locations featuring a hot desert climate, maximum temperatures of over 40 °C (104 °F) aren't uncommon in summer and can even soar to over 45 °C (113 °F) in the hottest regions. In fact, the world absolute heat records are generally in the hot deserts where the heat potential is the highest on the planet. Some desert locations consistently experience very high temperatures all year long, even during wintertime. These locations feature some of the highest annual average temperatures recorded on Earth, averages which can exceed 30 °C (86 °F). This last feature is seen in some parts of deserts of Africa and Arabia. During colder periods of the year, night-time temperatures can drop to freezing or below due to the exceptional radiation loss under the clear skies. However, very rarely do temperatures drop far below freezing. Hot desert climates can be found in the deserts of North Africa such as the wide Sahara Desert, the Libyan Desert or the Nubian Desert; deserts of the Horn of Africa such as the Danakil Desert or the Grand Bara Desert; deserts of Southern Africa such as the Namib Desert or the Kalahari Desert; deserts of the Middle East such as the Arabian Desert, the Syrian Desert or the Lut Desert; deserts of South Asia such as the Thar Desert; deserts of the United States and Mexico such as the Mojave Desert, the Sonoran Desert or the Chihuahuan Desert; deserts of Australia such as the Simpson Desert or the Great Victoria Desert and many other regions. Only one region in Europe has a hot desert climate, coastal areas of Almeria in South Eastern Spain. In fact, hot deserts are lands of extremes: most of them are the hottest, the driest and the sunniest places on Earth because of nearly constant high pressure; the nearly permanent removal of low pressure systems, dynamic fronts and atmospheric disturbances; sinking air motion; dry atmosphere near the surface and aloft; the exacerbated exposure to the sun where solar angles are always high. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「desert climate」の詳細全文を読む スポンサード リンク
|