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 temperate climate, with monsoon rains in the summer and snow-fall in winter. Winter in Bhutan is from mid-November until mid-March, and at this time of year the climate is dry, with daytime temperature of 16-19 degree centigrade (with sunshine and clear skies) and nighttime temperature falling below zero. The monsoon usually arrives mid-June, with the rain falling mainly in the afternoons and evenings. At the end of September, after the last of the big rains, autumn suddenly arrives, and is a magnificent season for trekking-lasting until mid-November.