The Arctic regions experience an annual cycle of winter ‘night’ and summer ‘day’. Most weather results from the intensely cold ground air which is chilled by contact with land, which loses heat from strong terrestrial radiation—winter clouds are scarce. Only infrequently do depressions penetrate the inversions so formed. Winter temperatures are close to –40 °C. Snowfall is slight, but winds cause frequent blizzards and drifting. In spring, days are longer and sunny, but temperatures remain low because of the high albedo of the snow surface. In summer, some depressions bring thicker cloud and light rain, and snow- and ice-melt in June and July keep air temperatures low.

Since the early 1970s, sea-level pressure over the Arctic has decreased; the stratospheric polar vortex has become colder and has been persisting longer into spring; and water mass characteristics of the Arctic Ocean have changed. Ostermeier and Wallace (2003) J. Climate 16, 2 link these changes to a shift in the North Atlantic Oscillation.