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polar vortex
There are two polar vortices in the atmosphere of planet Earth, one around the North Pole, and one around the South Pole. Each polar vortex is a persistent, large-scale cyclone, circling the planet's geographical poles. The bases of the two polar vortices are located in the middle and upper troposphere and extend into the stratosphere. They surround the polar highs and lie in the wake of the polar front. These cold-core low-pressure areas strengthen in the winter and weaken in the summer due to their dependence upon the temperature differential between the equator and the poles.〔Halldór Björnsson. . Veðurstofa Íslands. Retrieved on 2008-06-15.〕 They usually span less than 1,000 kilometers (620 miles) in diameter within which the air circulates in a counter-clockwise fashion in the Northern Hemisphere, and in a clockwise fashion in the Southern Hemisphere. As with other cyclones, their rotation is caused by the Coriolis effect.
The Northern Hemisphere (Arctic) vortex often contains two low pressure centers, one near Baffin Island, Canada and the other over northeast Siberia. Within the Antarctic vortex in the Southern Hemisphere a single low pressure zone tends to be located near the edge of the Ross ice shelf near 160 west longitude. When the polar vortex is strong, the Westerlies increase in strength. When the polar cyclone is weak, the general flow pattern across mid-latitudes buckles and significant cold outbreaks occur. Ozone depletion occurs within the polar vortices – particularly over the Southern Hemisphere – reaching a maximum depletion in the spring.
==History==
The polar vortex was first described as early as 1853.〔("Air Maps" ), ''Littell's Living Age'' No. 495, 12 November 1853, p. 430.〕 The phenomenon's sudden stratospheric warming (SSW) appears during the winter in the Northern Hemisphere and was discovered in 1952 with radiosonde observations at altitudes higher than 20 km.
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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