The climate in Nepal depends on the area. In the Kathmandu Valley and its environs summer temperatures range from 67-81 degrees F, and in winter temperatures are between 36 and 68 degrees F. The monsoon season is from June through mid-September and if traveling during these months you will encounter heavy rainfall.
The best seasons for trekking in the Himalayas are early spring (March and April) and late autumn (October and November) for the clearest mountain views and best weather. From December to February there is snow on the trails and freezing temperatures at high altitudes. The Terai lowlands have a humid, subtropical climate, that is warm in the winter and very hot in the summer.