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katabatic wind
A katabatic wind, from the Greek word κατάβᾰσις ''katabasis'' meaning "descending", is the technical name for a drainage wind, a wind that carries high density air from a higher elevation down a slope under the force of gravity. Such winds are sometimes also called fall winds. Katabatic winds can rush down elevated slopes at hurricane speeds, but most are not as intense as that, and many are of the order of 10 knots (18 km/h) or less.
Not all downslope winds are katabatic. For instance, winds such as the föhn, chinook, and bergwind are rain shadow winds where air driven upslope on the windward side of a mountain range drops its moisture and descends leeward drier and warmer. Examples of true katabatic winds include the bora (or bura) in the Adriatic, the Bohemian Wind or ''Böhmwind'' in the Ore Mountains, the Santa Ana in southern California, and the oroshi in Japan. Another example is "the Barber", an enhanced katabatic wind that blows over the town of Greymouth in New Zealand when there is a southeast flow over the South Island. It is a wind that is known in the area for its coldness.
== Theory ==
A katabatic wind originates from radiational cooling of air atop a plateau, a mountain, glacier, or even a hill. Since the density of air is inversely proportional to temperature, the air will flow downwards, warming approximately adiabatically as it descends. The temperature of the air depends on the temperature in the source region and the amount of descent. In the case of the Santa Ana, for example, the wind can (but does not always) become hot by the time it reaches sea level. In Antarctica, by contrast, the wind is still intensely cold.
The entire near-surface wind field over Antarctica is largely determined by the katabatic winds, particularly outside the summer season, except in coastal regions when storms may impose their own wind field.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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