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Akatabatic wind (named from Ancient Greek κατάβασις (katábasis) 'descent') is a downslope wind caused by the flow of an elevated, high-densityair mass into a lower-density air mass below under the force of gravity. The spellingcatabatic[1] is also used. Since air density is strongly dependent on temperature, the high-density air mass is usually cooler, and the katabatic winds are relatively cool or cold.
Examples of katabatic winds include the downslopevalley and mountain breezes, thepiteraq winds of Greenland, theBora in theAdriatic,[2] theBohemian Wind orBöhmwind in theOre Mountains, theSanta Ana winds insouthern California, theoroshi inJapan, or "the Barber" inNew Zealand.[3]
Not all downslope winds are katabatic. For instance, winds such as theföhn andchinook arerain shadow winds where air driven upslope on thewindward side of a mountain range drops its moisture and descendsleeward drier and warmer.
A katabatic wind originates from the difference of density of two air masses located above a slope. This density difference usually comes from temperature difference, though humidity may also play a role. Schematically, katabatic winds can be divided into two types for which the mechanisms are slightly different: the katabatic winds due toradiative cooling (the most common) and thefall winds.
In the first case, the slope surface cools down radiatively after sunset, which cools down the air near the slope. This cooler air layer then flows down in the valley. This type of katabatic is very often observed during the night in the mountains. The term katabatic actually often refer to this type of wind.[4]
In contrast, fall wind do not come from radiative cooling of the air, but rather from theadvection of a relatively cold air mass to the top of a slope.[5][6] This cold air mass can come from the arrival of acold front (seeBora),[7] or from the advection of cool marine air by a sea-breeze.[8]
Katabatic winds are for example found blowing out from the large and elevated ice sheets ofAntarctica andGreenland. The buildup of high density cold air over the ice sheets and the elevation of the ice sheets brings into play enormous gravitational energy. Where these winds are concentrated into restricted areas in the coastal valleys, the winds blow well over hurricane force,[9] reaching around 160 kn (300 km/h; 180 mph).[10] In Greenland these winds are calledpiteraq and are most intense whenever a low pressure area approaches the coast.
In a few regions of continental Antarctica the snow is scoured away by the force of the katabatic winds, leading to "dry valleys" (or "Antarctic oases") such as theMcMurdo Dry Valleys. Since the katabatic winds are descending, they tend to have a low relative humidity, whichdesiccates the region. Other regions may have a similar but lesser effect, leading to"blue ice" areas where the snow is removed and the surface icesublimates, but is replenished by glacier flow from upstream.
In the Fuegian Archipelago (Tierra del Fuego) in South America as well as in Alaska in North America, a wind known as awilliwaw is a particular danger to harboring vessels. Williwaws originate in the snow and ice fields of the coastal mountains, and they can be faster than 120 kn (220 km/h; 140 mph).[11]
In California, strong katabatic wind events have been responsible for the explosive growth of many wildfires, including the 2018Camp Fire and the 2020North Complex.
InCatalonia,Spain theMarinada is a fall wind that relieves from the heat inhabitants of theUrgell region during summer.[8]
Katabatic winds (also catabatic)
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