The polar climate regions are characterized by a lack of warm summers. Every month in a polar climate has an average temperature of less than 10 °C (50 °F). Regions with polar climate cover more than 20% of the Earth. The sun shines for long hours in the summer, and for many fewer hours in the winter. A polar climate results in treeless tundra, glaciers, or a permanent or semi-permanent layer of ice. It has cool summers and very cold winters.