The best seasons for trekking are the dry and warm seasons, March-June and September-November. During these times, the temperature is bearable and skies are usually clear, although the skies are foggier and the rain begins in May-June. It is possible to trek out of season, but expect rain and leeches during the summer monsoon season and severe cold and closed passes during the winter months.
During the monsoon season, the treks are virtually empty of travellers. Most rain usually comes at night; days are cloudy with less rain. The north side of the Himalayas is in rain shadow, meaning that the mountains somewhat block the rain. Annapurna Circuit is in rain shadow after Pisang. The skies are still cloudy and it rains occasionally. In mornings the skies may be clearer, but the views are still nowhere as good as during the trekking season. Landslides may block the trekking routes.