The southern part of Bhutan is tropical, and in general, the east of Bhutan is warmer than the west of the country. The central valley of Punakha, Wangdiphodrang, Mongar, Tashigang and Lhuntshi enjoy a semi tropical climate with very cool winters, while Thimphu , Tongsa and Bumthang have a much harsher climate, with heavy monsoon rains in the summer and heavy snow fall in winter. Winter in Bhutan starts from mid-November till the mid-March, and at this time of the year the climate is dry with daytime temperature of 16-18° C and nighttime temperature falling below zero. The monsoon usually arrives in mid-June, with the rain falling mainly in the afternoons and evenings. Autumn starts from the end of September, after the last of the big rains, and it is a magnificent season for trekking-lasting till mid-November.