Given the variations in altitude there are terrific variations in temperature. Very basically Nepal can be split into a winter, from December to February, a spring, from March to May, the rainy monsoon season, from June to early September, and an autumn during the second ½ of September, October and November. On the whole it’s both wetter in the east and colder in the mountains. Autumns are more reliably dry, the views better and with the most moderate temperatures. The lowland Terai remains hot throughout the year.
Lower and Upper Dolpo, Upper Mustang and Rara Lake, are in the Annapurna/Dhaulagiri rain shadow and get very little of the monsoon. They should be considered extensions of the Tibetan Plateau, colder from November to April, but dry and warmer from May to October.