The climate of the Arctic is characterized by long, cold winters and short, cool summers. There is a large amount of variability in climate across the Arctic, but all regions experience extremes of solar radiation in both summer and winter. Some parts of the Arctic are covered by ice (sea ice, glacial ice, or snow) year-round, and nearly all parts of the Arctic experience long periods with some form of ice on the surface. The Arctic consists of ocean that is largely surrounded by land. As such, the climate of much of the Arctic is moderated by the ocean water, which can never have a temperature below . In winter, this relatively warm water, even though covered by the polar ice pack, keeps the North Pole from being the coldest place in the Northern Hemisphere, and it is also part of the reason that Antarctica is so much colder than the Arctic. In summer, the presence of the nearby water keeps coastal areas from warming as much as they might otherwise. Overview of the Arctic There are different definitions of the Arctic. The most widely used definition, the area north of the Arctic Circle, where the sun does not set on the June Solstice, is used in astronomical and some geographical contexts. However the two most widely used definitions in the context of climate are the area north of the northern tree line, and the area in which the average summer temperature is less than , which are nearly coincident over most land areas (NSIDC). This definition of the Arctic can be further divided into four different regions: The Arctic Basin includes the Arctic Ocean within the average minimum extent of sea ice . The Canadian Arctic Archipelago includes the large and small islands, except Greenland, on the Canadian side of the Arctic, and the waters between them. The entire island of Greenland, although its ice sheet and ice-free coastal regions have different climatic conditions. The Arctic waters that are not sea ice in late summer, including Hudson Bay, Baffin Bay, Ungava Bay, the Davis, Denmark, Hudson and Bering Straits, and the Labrador, Norwegian, (ice-free all year), Greenland, Baltic, Barents (southern part ice-free all year), Kara, Laptev, Chukchi, Okhotsk, sometimes Beaufort and Bering Seas. Moving inland from the coast over mainland North America and Eurasia, the moderating influence of the Arctic Ocean quickly diminishes, and the climate transitions from the