Nepal's climate varies with its topography and altitude. It ranges from the tropical to the arctic. The low-land Terai region with its maximum altitude to approximately 305m, which lies in the tropical southern part of the country, for instance, has a hot and humid climate that can rise above 45 Degree Celsius (113 Degree Fahrenheit) during summer. The mid-land regions are pleasant almost all the year round, although winter nights are cool. The northern mountain region, around an altitude above 3,300m has an alpine climate with considerably lower temperature in winter as can be expected.
September, October and November (Autumn):
This is the best trekking season in Nepal which offers you excellent weather and wonderful mountain views. The trekking trails will be full of trekkers.
March April and May (Spring):
The second premium trekking season in Nepal. The temperature is quite moderates, especially the rhododendrons (National flower of Nepal) are in complete blossom and the mountain views are excellent.
June, July and August (Summer):
This is the most preferable season to trek in the rain shadow areas like upper Mustang, around Annapurna, Upper Manag, Manaslu and Upper Dolpo etc. these regions are out of reach of the rain clouds because of the huge mountains, and unaffected by the monsoon. This season is also good for forest researchers and botanist, during this time for trek raincoat and insect repellents are strictly required.
December, January and February (Winter):
This is winter in Nepal will be cold for high mountain trekking but still suitable time for trekking and hiking at lower altitudes normally below 3500m.