Prague has an oceanic climate (Köppen: ''Cfb'') with humid continental (''Dfb'') influences, defined as such by the  isotherm. The winters are relatively cold with average temperatures at about freezing point, and with very little sunshine. Snow cover can be common between mid-November and late March although snow accumulations of more than  are infrequent. There are also a few periods of mild temperatures in winter. Summers usually bring plenty of sunshine and the average high temperature of . Nights can be quite cool even in summer, though. Precipitation in Prague (and most of the Bohemian lowland) is rather low (just over  per year) since it is located in the rain shadow of the Sudetes and other mountain ranges. The driest season is usually winter while late spring and summer can bring quite heavy rain, especially in form of thundershowers. Temperature inversions are relatively common between mid-October and mid-March bringing foggy, cold days and sometimes moderate air pollution. Prague is also a windy city with common sustained western winds and an average wind speed of  that often help break temperature inversions and clear the air in cold months. 