The three relief zones also define three climatic regions tropical, temperate with monsoon and alpine with monsoon. The southern part of Bhutan is tropical, and in general the east of Bhutan is warmer than the west of the country. The central valleys of Punakha, Wangduephodrang, Mongar, Trashigang and Lhuentse enjoy a semi tropical climate with very cool winters while Thimphu, Trongsa and Bumthang have a much harsher climate with heavy monsoon rains in the summer and heavy snowfall in the winter. Winter is from mid-November until mid-March and at this time of the year the climate is dry with with day temperatures falling to 16o to 18oand night temperatures falling below zero. The monsoon usually arrives in mid-June with the rain falling mainly in the afternoons and evenings. At the end of September, after the rainy season, autumn arrives suddenly which makes it the best season for trekking lasting till mid-November.