Bhutan has a diverse climate and it generally depends on the altitude of the region. For every thousand meters of elevation the temperature drops by approximately 7 degrees. Southern Bhutan experiences a sub-tropical climate. It represents hot and humid summer and very cold winter. Over the mountainous northern half, the winters are harsh with regular snowfall.
For trekkers, the most critical factor to understand is the South Asian monsoon. Bhutan receives the bulk of its annual rainfall between June and September, making the trails muddy, leech-prone, and the skies cloudy. However, the country enjoys two distinct, crystal-clear trekking windows: the pre-monsoon spring (March to May) and the post-monsoon autumn (September to November). Spring brings vibrant rhododendron forests into full bloom, while autumn offers the sharpest, most reliable mountain views. Be prepared for significant swings-warm afternoon in the valleys can quickly turn into freezing nights at higher camps.