Climate of Bhutan:
Bhutan's climate ranges from tropical in the south, to temperate in the center of the country, to cold in the north... and like much of your adventure in the Himalayas it will be quite unpredictable. The weather can vary dramatically from place to place and can vary equally dramatically from day to day or within the same day.
In Thimphu and Paro valleys, the winter daytime temperature averages 60 degrees Fahrenheit during clear winter days but drops well below freezing during the night. Mid December to early January can be a beautifully clear and dry time in Western Bhutan. The fluctuations are not quite so great during the summer and daytime temperature often rises to the mid-eighties Fahrenheit.
Punakha and the central valleys are lower than their Western neighbours and tend to always be a few degrees warmer. The higher peaks will be snow-covered all year. The higher passes, particularly Thrumsing Labetween Bumthang and Mongar, can be treacherous during the winter as snow falls frequently and ices up the road. Light snow will often dust Thimphu and Paro in winter but infrequently will there be heavy snowstorms despite their location in the Central Himalayas.
The Summer monsoon from the Bay of Bengal affects Bhutan from late May to early October. Views over the Himalayas from the higher passes are usually obscured from May to August. There are notable advantages to visiting Bhutan during the wet season including the spectacular rhododendron blossom from March through May and the deep green valleys.
The Spring season in Bhutan can only be compared to a master artist's palette. Colors that, until now, have only existed in the imagination. Truly a spectacular time.
The Autumn season, October through November, is usually very mild and clear. The Fall colors surround and embrace your senses.