Nepal can be subdivided into three different regions and all these three regions have different climates.
The south of Nepal (Terai), bordering India, can be very hot in April, May and June; the rest of the year is suitable to visit Terai and it is especially pleasant in the winter. In this region, jungle nature parks are surely worth visiting (safari).
The north of Nepal is the Himalaya range bordering Tibet. In this part it’s generally colder but the temperature in the valleys is typically agreeable. Trekking is possible in all seasons and depends on where you want to go. Ideal conditions for the majority of trekking routes are between September to November and March to May. During the rainy season in the summer, it’s ideal to trek in the “rain shadow” area, where the rain rarely falls.
Between the Terai and Himalayas lies the hilly region. The capital Kathmandu is located in this region. The temperatures here vary between 0°C during the coldest winter nights to 35°C during warm summer days. The sun shines in Kathmandu throughout the year and temperatures during the day are typically around 20°C, even in the winter.