The climate of the Arctic, classified as polar, is characterized by long, cold winters and short, cool summers. Polar climate may be further subdivided into tundra climate (the warmest month of which has an average temperature below 50°F/10°C but above 32°F/0°C) and ice cap climate (all months average below 32°F/0°C, and there is a permanent snow cover). Precipitation, almost entirely in the form of snow, is very low, with the annual average precipitation for the regions less than 20 in. (51 cm). Persistent winds whip up fallen snow to create the illusion of constant snowfall. The climate is moderated by oceanic influences, with regions abutting the Atlantic and Pacific oceans having generally warmer temperatures and heavier snowfalls than the colder and drier interior areas. Much of the Arctic Ocean remains covered by ice throughout the year, although the extent and thickness of the summer ice has shrunk considerably since the early 1980s; in 2012 the volume of ocean ice at its smallest was less than one third what it is estimated to have been in the early 1980s.
Great seasonal changes in the length of days and nights are experienced N of the Arctic Circle, with variations that range from 24 hours of constant daylight ("midnight sun") or darkness at the Arctic Circle to six months of daylight or darkness at the North Pole. However, because of the low angle of the sun above the horizon, insolation is minimal throughout the regions, even during the prolonged daylight period. A famous occurrence in the arctic night sky is the aurora borealis, or northern lights.
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