Nepal Travel Trek

When is the best time to go to Nepal?
You can find good places to trek somewhere in the Himalayas at any time of year. Even though weather is probably the best guide for deciding when to plan your trip to Nepal.
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:
Autumn: September, October and November are considered the best times of the year. During this period, the climate is dry offer the clearest skies, least rain, the festivities of harvest season and mild with temperature fluctuating between 20* C to 30* C (68* F to 86* F). The monsoon will have just ended, and clear skies with optimal temperature will prevail. The main festivals of Dashain and Tihar (Hindu equivalent of Christmas in terms of festivity) fall during these months. However, this is also the busiest tourist season, and the main tourist centers and trekking trails tend to be crowded with travelers like you.
Winter: Winter starts from December to February are cool and mostly clear. 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). The tourist flow ebbs a little, but not significantly, between the winter months of December and mid-February. It catches up once again between mid-February and mid-April.
Spring: Spring starts from March to May are warm months, though it is often hazy. The temperature of this season fluctuates between 20* C to 30* C (68* F to 86* F). Rhododendrons blooms at ascending altitudes from February through May are also a very good time to trek as snow melts at the upper altitudes.
Summer: Summer starts from June to August. These are also the pre-monsoon and monsoon months with occasional evening-thunderstorms and hot temperature, which time it rains almost everyday and most of the Himalayas are hidden behind the clouds. June is warm and humid, though still a good time to trek to the Everest area. July and August are monsoon months with lots of rain, but still you have great chance to trekking behind the Himalayas Manang, Mustang and Dolpo known as the rain shadow areas of Nepal.
September sees the end of the monsoon. The whole country is beautifully green and the trekking season starts again. In short, plan to visit Nepal between September and May; keeping in mind that September - November and March - May are the best times (but crowded with other travelers).For detail information on temperature and rainfall, check the Weather Chart.
When to travel?
The best time to visit Nepal is September to May. Best time to visit Tibet and Bhutan is April to November. Best time to visit India is Dec - April and August to November.  Nepal Himalaya is governed by the Southerly Monsoon which brings heavy precipitation between the months of  Mid June and mid September, i.e. it rains a lot. There are great treks available in the rain shadow area during these months but if you are planning a more popular trek it is best to avoid the monsoon months. Autumn through to Spring is the best time to fully experience Nepal
CLIMATE IN NEPAL

Nepal has a great deal of variation in climate but is generally predictable and pleasant. Climate in Nepal range from freezing cold in high altitude regions in the Himalayas to subtropical in its lowlands or Terai region. Annual rainfall ranges from 70 and 75 inches in the eastern Terai and between 30 and 35 inches in western Nepal. Nepal has four distinct seasons. Spring from March to May, is warm and dusty with rain showers. Summer is from June to August, and much of the season is dominated by the monsoon, when the hills turn lush and green. Autumn from late September to November, is cool with clear skies, and is the most popular trekking season.
This retreat leads to an almost complete disappearance of moist air by mid-October, thus ushering in generally cool, clear, and dry weather, as well as the most relaxed and jovial period in Nepal. The presence of the east-west-trending Himalayan massifs to the north and the monsoon alteration of wet and dry seasons also greatly contribute to local variations in climate. Altitude also affects annual rainfall or precipitation patterns. Up to about 3,000 meters, annual rainfall totals increase as the altitude increases; thereafter, annual totals diminish with increasing altitude and latitude. There are certain pockets with heavy annual rainfall totals, for example, the Pokhara Valley in central Nepal. Eastern Nepal receives approximately 2,500 millimeters of rain annually, the Kathmandu area about 1,420 millimeters, and western Nepal about 1,000 millimeters.

. Climatology
  Mean values

annual average temperatures around the country(in degree Celsius)