Southern Bhutan has a hot and humid subtropical climate that is fairly unchanging throughout the year. Temperatures can vary between 15-30 degrees Celsius (59- 86 degrees Fahrenheit). In the Central parts of the country which consists of temperate and deciduous forests, the climate is more seasonal with warm summers and cool and dry winters. In the far Northern reaches of the kingdom, the weather is much colder during winter. Mountain peaks are perpetually covered in snow and lower parts are still cool in summer owing to the high altitude terrain.
For more information on the temperature for the different month, check out https://www.bhutan.travel/page/climate-weather.