It's hard to generalize about the climate of Nepal ranging in elevation from near sea level to the 8848m peak of mount Everest.Nepal has mainly four seasons which are below.
Winter (December to February) in winter season weather is very cold, sky clean and clear, some time snowfalls at higher elevations. days are warm if sunny day but morning and night is very cold. Winter brings cold, foggy inmornings and clear evenings Spring (March to May) spring bring warmer weather and longer days Rhododendrons bloom in the hills. this is the best time to view wildlife and to travel in Nepal. Summer (June to August) these three month are monsoon and hot season in Nepal. it rain must of time.
The best time to visit Nepal for trekking is Spring and Autumn. The monsoon season affects domestic flights regularly and obscures the views. Winter is also a pleasant time to come to Nepal if you can brave the cold, although temperatures in the mountains will be almost definitely sub zero at night.