Nepal’s climate is influenced by maritime and continental factors and has four distinct seasons. Spring lasts from March to May and is warm with rain showers, and temperatures around 22°C. Summer, from June to August, is the monsoon season when the hills turn lush and green. Temperatures can get quite warm, up to 30°C and more during heat waves. During this season, trekking in most of Nepal is difficult and uncomfortable, the trails were very muddy.
Autumn, from September to November, is cool with clear skies and is the most popular season for trekking. Temperatures are not too warm, with daily maxima about 25°C and cool nights with minima of 10°C, it usually does not rain for more than one or two days during the entire autumn and the winter season. In winter, from December to February, it is cold at night with temperatures sometimes below zero. However, the maximum temperatures can still reach up to 20°C. Then the mountains are covered with snow including some high hills.
Clothing depends on place and time, however, it is recommended to have both light and warm clothing, because the evenings can be quite cool. In the mountain areas, warm clothes are generally a must. (Source: weatheronline.co.uk)