Bhutan has four seasons. Bhutan's climate ranges from tropical in the south to temperate in the upper regions 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 valley to valley, from day to day or within same day. For example: in the Thimphu and Paro valleys, the winter daytime temperature averages 60 degrees Fahrenheit during winter days but drops well below freezing point during the night.
The fluctuations are not quite so great during the summer and daytime temperature often rises to the mid 80 Fahrenheit. Punakha, Wangduephodrang and central valley's are lower than valleys in Western region and tend to be always warmer. The higher peaks will be snow-covered all year. Light snow will often blanket Thimphu and Paro valleys in winter. The higher passes, particularly, Thrumshing La between Bumthang and Mongar can be treacherous during the winter as snow falls frequent and ices up the road.
The summer monsoon from the Bay of Bengal affects Bhutan from late May to later September. Views over the Himalayas from the higher passes are usually obscured from June to August. There are notable advantages to visiting Bhutan during the wet season including the spectacular rhododendron blossom from March through June and the deep green valleys. Many species of wild orchids are in full bloom during spring to late summer season.
The spring season in Bhutan can only be compared to a master artist's palette, truly a spectacular time. The autumn season, late September through November, is usually very mild and clear. The Fall colors surround and embrace your senses. The sky is usually at its clearest, affording magnificent views of the Himalaya range. The spring and Fall seasons are traditionally the most popular times to visit the country.