In northern Nepal summers are cool and winters are severe, while in the south, summers are very hot while winters are mild to cold. Nepal has five seasons: spring, summer, monsoon, autumn and winter.
In the Tarai (southern Nepal), summer temperatures exceed 40° C and above 45 in some areas, while winter temperatures range from 7°C to 23°C. In mountainous regions, hills and valleys, summers are temperate while winter temperatures can plummet to sub zero. The Kathmandu Valley has a pleasant climate with average summer temperatures of 20°C – 35°C and 2°C – 12°C in winter.
Average temperatures in Nepal drop 6°C for every 1,000 m you gain in altitude.
The Himalayas act as a barrier to the cold winds blowing from Central Asia in winter, and form the northern boundary for monsoon rains. Some places like Manang and Mustang fall in the rain-shadow behind the mountains and are mostly dry. Eighty percent of all the rain in Nepal is received during the monsoon (June-September). Winter rains are more pronounced in the western hills. The average annual rainfall is 1,600 mm, but it varies by eco-climatic zones, such as 3,345 mm in Pokhara and below 300 mm in Mustang.
There is no seasonal constraint on traveling in and through Nepal. Even in December and January, when winter is at its severest, there are compensating bright sun and brilliant views. As with most of the trekking areas in Nepal, the best time to visit are during spring and autumn. Spring is the time for rhododendrons to bloom while the clearest skies are found after the monsoon in October and November. However, Nepal can be visited the whole year round.