The width of Nepal is only about 200 km on average, but within this short distance the altitude of the land rises from lowly 60m to all the way up to above 8000m.Hence the weather depends upon the altitude of the place in Nepal. However, in general Nepal has four climatic seasons: Spring, Summer, Autumn and Winter. Spring starts from March to May. The temperature of this season fluctuates between 20* C to 30* C(68* F to 86* F). Summer starts from June to August. These are also the pre-monsoon months with occasional evening-thunderstorms and hot temperature. Autumn starts from September and ends by November. During this period, the climate is dry and mild with temperature fluctuating between 20* C to 30* C (68* F to 86* F). Winter starts from December to February. The morning and evening are very cold while the afternoon is pretty sunny. The temperature during these months rises from 15* C to 20* C (59* F to 68* F).
For detail information on temperature and rainfall, check the Weather Chart.
Clothing depends on place and time. Medium-weight and easy to wash cottons can be a good choice year-round in the Kathmandu valley. It is recommended that between October to February, woolen sweaters, jackets or similar other warm outfits are necessary. From March through May, light clothing such as short and long-sleeved shorts will do perfectly fine at Kathmandu, Pokhara and most other towns. For mornings and evenings, a jacket or heavy woolen sweater (you can find beautiful ones in reasonable price in Kathmandu) will be essential. For months from June to August, it is recommended that you bring an umbrella or raincoat and a pair of sandals with you as these months are the rainy months of Nepal. Expect lot of walking even if you don't plan to trek. So it's recommended that you bring comfortable footwear: sneakers and sandals are the best
If you intend to go for trekking, a pair of hiking boots will be great. Bring plenty of woolen socks too. Specialized trekking gear is easily available and can be rented in fairly inexpensive charge in Kathmandu and Pokhara: see FAQ on Trekking for details on proper clothing during treks.