Nepal has a Monsoonal climate with four main seasons - though traditionally a year was categorized into six distinct climate periods: Basanta spring, Grishma early summer, Barkha summer monsoon, Sharad early autumn, Hemanta late autumn and Shishir winter.
Below is a general guide to conditions at different seasons:
Heavy monsoonal rains from June to September - the rains are generally lighter high in the Himalayas than in Kathmandu, though the mountain peaks are often not visible due to clouds. In the Kathmandu Valley & Pokhara - monsoon rains typically consist of an hour or two of rain every two or three days. The rains clean the air, streets, & cool the air. If you come, bring an umbrella, expect lower lodging prices & fewer tourists.
Clear and cool weather from October to December - after the monsoon, there is little dust in the air so this is the best season to visit the hilly and mountainous regions.
Cold from January to March, with the temperature in Kathmandu often dropping as low as 0Â°C 32Â°F at night, with extreme cold at high elevations. It is possible to trek in places like the Everest region during the winter, but it is extremely cold and snow fall may prevent going above 4,000 - 4,500 metres 13,000 - 15,000 feet. The Jomosom trek is a reasonable alternative, staying below 3,000 metres 10,000 feet with expected minimum temperatures about -10Â°C 14Â°F and much better chances of avoiding heavy snow.
Dry and warm weather from April to June - there is an abundance of blooming flowers in the Himalayas at this time, with rhododendrons, in particular, adding a splash of colour to the landscape. Terai temperatures may reach or exceed 40Â°C 104Â°F while Kathmandu temperatures are about 30Â°C 86Â°F. This is the best time to undertake mountain expeditions.
The recording of temperatures and rainfall of the major locations across Nepal was started in 1962 and their averages (http://exoticbuddha.com/2...) provides a reference point for analyzing the climate trend.