Denoting regions within the Arctic Circle, i.e. north of 66° 32′ N (often taken as 66½° N). Within these regions the sun does not set on 21 June (the summer solstice in the Northern Hemisphere) nor rise on 22 December (the winter solstice in the Northern Hemisphere). In climatology the Arctic is defined in terms of the treeless zone of tundra and of the regions of permafrost in the Northern Hemisphere. Arctic air masses are exceedingly cold, with the Arctic Ocean as their source region. During winter, they are very dry, and little different from a continental polar (cP) air mass. In summer, the air is more maritime. A maritime Arctic (mA) air mass is generally similar to maritime polar air. Initially it has continental characteristics, but when it crosses warmer water, it is warmed from below, and in consequence becomes unstable.