The best seasons for trekking in Nepal are the relatively dry and warm months, March till June and September till November. During these times, the temperature is not too hot and the air is usually clear. It is also possible to trek out of season, but you can expect rainy days and annoying leeches during the summer monsoon season. During the winter months many passes can be closed due to a lot of snow.
In the monsoon time, Nepal is quite empty of trekkers. The north side of the Himalayas is in rain shadow, what means that the high mountains block the rain. Annapurna Circuit is in rain shadow after Pisang. But the sky could still be cloudy and it can rain occasionally. In mornings the air may be clearer, but the views are still nowhere as good as during the trekking season. Landslides may block the trekking routes in the monsoon time.
Seasons in Nepal:
Spring (March-May): The temperature is warm in lower parts while moderate in higher altitudes. You can enjoy plenty of opportunities to have clear mountain views. It is also the time for flowers to blossom. The national flower of Nepal – rhododendron paints the ascending forests with its magnificent colour and beauty.
Summer (June-August): This is also the monsoon season in Nepal. The weather is hot and wet. The skies are cloudy. It rains almost every day with l thunderstorms in the evening. The vegetation is deeply green.
Autumn (September-November): This is the best tourist season in Nepal. The weather is pleasant and the skies clear with amazing mountain views. This is the peak season for trekking so better book your flight in advance. This is also the season of two biggest and colourful festivals in Nepal: Dashain and Tihar.
Winter (December-February): The weather is cool and the sky is clear with occasional snowfalls at higher altitudes. This season is good for trekking in lower elevations. The morning and night is cold and the days are warm when sunny.