Nepal's climate varies with its topography. It ranges from sub-tropical to arctic depending upon the altitude. The Terai region has a hot and humid climate. The mid-land region are pleasant almost all the year round, although winter nights are cool. The northern mountain region, around an altitude above 3,353 m. has an alpine climate with a considerably lower temperature in winter as expected. Please find the Climate-chart below.
Monsoon: The Monsoon of Nepal has made two clear wet and dry seasons. Wet days of summer are Monsoon days. Most of the rainfalls in Nepal are due to Monsoon in summer. The rest are dry days. Spring and Autumn are in between seasons. In general the monsoon begins from mid June and ends at beginning of September. The months of seasons vary with altitude and slope of the mountain. Northern slopes and higher altitudes are less warm than those of southern slopes and lower altitude. The raining season depends on monsoon activities. Mostly rains at night, making the following days crispy clean and fresh. The length of summer and autumn also depend on the Monsoon. Most of the northern belt of the Trans-Himalayan zone is monsoon-shadowed and ideal for trekking. The climate of Kathmandu valley can be taken as a standard.
Summer: (May-August) is hottest season of Nepal. Hot but dry days are more comfortable than those of Monsoon days which are hot and humid. Autumn: (September to November) begins with the end of Monsoon and ends with beginning of winter in November. It is also a festival season. You can enjoy lots of green and blue sky. Best season for out door activities like trekking, rafting, and wild life jungle safari.
Winter: Cold months are November till February. Winter days are also dry days with a few rains. Typical day of Kathmandu valley in winter season is as warm as 20 degree Celsius in a sunny days but night temperature falls below freezing. The little rains in winter are due to Mediterranean activities. Hence the winter rainfall in western Nepal has more rain than those of eastern Nepal.
Spring: (February till April): Spring begins with occasional shower and rains. The days are mild days with the new vegetation. The sky is not as blue as Autumn. It is a little hazy if no showers for a long period.
The climate of Nepal is best assessed with the climate at 4 distinct places. These are Kathmandu valley, Pokhara valley (West), Chitwan (Terai), and Namche Bazaar (East & High mountain).