South Sikkim lies at an altitude of 400 m to 2000 metres and hence enjoys a temperate climate for most of the year. The climate ranges from sub-tropical in the south to tundra in the northern parts. The tundra-type region in the north is clad by snow for four months a year though the temperature drops below 0 °C (32 °F) almost every night.[24] The peaks of north-western Sikkim are perpetually frozen.[25] Most of the inhabited regions of Sikkim, however, witness a temperate climate, with the temperatures seldom exceeding 28 °C (82 °F) in summer or dropping below 0 °C (32 °F) in winter. The mean monthly temperature in summer is 15 °C.[26] The state has five seasons: winter, summer, spring, and autumn, and a monsoon season between June and September. The average annual temperature for most of Sikkim is around 18 °C (64 °F). South Sikkim  receive regular rainfall. .[27] During the monsoon, heavy rains increase the possibility of landslides. The record for the longest period of continuous rain is 11 days. In the northern region, because of high altitude, temperatures drop below −40 °C (−40 °F) in winter. Fog also affects many parts of the state during winter and the monsoons, making transportation perilous.