The best seasons to visit for trekking in Nepal, tours, adventure, and mountaineering in Nepal are autumn (Sept, Oct, Nov) and spring (March, April, May). These are the two most favorable and recommended seasons. In these seasons you will be rewarded with good weather, sunny and warm with clear blue sky and magnificent views. The spring, however, might get some occasional showers. During the monsoon (June, July, Aug) the issue could be less visibility, rain, muddy trails, and leech-infested trails. The mountains are hidden by clouds. However, during the monsoon the hills turn lush and green; the higher valleys, mountains, and meadows blossom with flowers and abundant vegetation, so for a keen botanist, the monsoon is a blessing to trek in Nepal. You can also trek in winter (Dec, Jan, Feb), but the problem in winter is cold weather with snowfall at higher elevations. Over the past few years, peak climbers and trekkers have found early winter (end of Nov until mid-Dec) quite a favorable time as the snow gets hard and wind condition remains stabilized. Read here for some ideas on how many days to spend in Nepal.
Nepal has some trans-Himalayan regions such as Mustang and Dolpo (Upper Mustang, Lower Mustang, Upper Dolpo, Lower Dolpo) towards the northwest of the Dhaulagiri Mountain range. These regions lie in a rain shadow and get less precipitation as the cloud flowing from the south is blocked by the Himalayan range. So, trekking can be organized in this area even in summer/monsoon.
The trekking routes are crowded during the main seasons, i.e. spring and autumn but during monsoon and winter, the trails are not packed, and you can enjoy rather the best of nature. If you want to avoid the crowd trekking still at the best time, starting your trek towards the starting or end of each best season is a good idea, i.e. starting the end of Feb to early March, starting towards the end of May, starting mid-September, starting towards the end of November. Or you can choose to do remote area treks to avoid crowds on popular trails.
Nevertheless, because of the effects of global warming, the climate is changing worldwide, and there could always be a likelihood of exception in these weather patterns. In the best seasons also, there could be swindling of bad weather, and sometimes even in adverse months; weather turns out to be outstanding for trekkers.
Nepal has a diverse climate and topography. The altitude ranges from 80m from sea level to 8,848m. While the southern plains are hot most of the year, temperature decreases gradually towards the North as the altitude increases.
Autumn is the most favorable time to trek. Generally during autumn, the weather is clear with mild to warm days and cold nights. However, in the higher altitude, the nights drop into freezing temperature. In this season, the mountains views are surprisingly clear. This is also the season of festivities as Nepal celebrates the biggest Hindu festivals Dashain followed by Tihar.
The weather is cool and the sky is clear with occasional snowfalls at higher elevations. Daytime temperatures will be cool; however, the nights obviously very chilly. Days can be warm when sunny. It is also possible to trek during winter, from December until the end of February. The days are generally clear, but occasional winter storms bring snow as low as 2500m. This season is suitable for trekking in lower elevations. Trekking in high altitudes is more challenging due to weather during this time because of semi-regular snowfall followed by more winter storms.
As the snow gets hard and wind condition remains stabilized in early winter (until mid-Dec), climbing some of the trekking peaks is possible. Climbing Mera peak, Island peak, Chulu, and trekking in Annapurna, Everest, and Langtang in early winter have been quite popular over the past few years.
The mornings are usually clear in the spring, but afternoon cloud build-up brings occasional showers. The days are mixed up with warmth and rain, which displays wildflowers like rhododendrons. The whole country is lush and abundant green during this season. This period instigates the second most popular and pleasant trekking season as this is rice-planting time. Late march into April is especially beautiful. It is also a good time for climbing as the high passes are usually snow free, and the mountain views are still evident in April. During April and May, there is an expectation of thunderstorms, hail showers, and strong winds among the fine periods. In May, the weather occasionally becomes hazy and disturbed by the clouds.
From June to September is the monsoon season. Generally, the morning is cloudy, and cloud wisps form on random ridges and peaks. Trekking at this time of year is generally difficult and uncomfortable as the weather is hot and it rains almost every day. The trails become muddy and are often leech-infested and the mountains are usually obscured by clouds. There are, however, possibilities for summer trekking in the trans-Himalayan regions of mustang, Dolpo, etc. These regions lie in a rain shadow and therefore receive significantly less precipitation than the more southerly areas.