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. In winter, from December to Freburary, it is cold at night and can be foggy in the early morning, but afternoon are usually clear and pleasant, though there is occasional snow in the mountains. The weather is warmer and winter is much milder at lower elevations, including Kathmandu at 1400m. It rarely snows below 2000m.
The monsoon creates a rainy season from mid june to mid september. It is hot during the monsoon and it rains almost every day, but it is a considerate rain, limiting itself mostly to the night. During this season Trekking in middle hills is difficult but it is the best season for trekking in Tibet Kailash Manasarover holy Yatra and other Tibet tours. Trekking in Trans-Himalayan regions
such as Mustang and Tibet via Simikot of Nepal is possible during moonsoon as well. Most of the pprecipitation in the Himalaya occurs during the summer monsoon. It usually doesn't rain for more than a few days during the entire autumn season from mid-October to mid -December. During winter and spring there may be a week or so of rainy days, and occasional thunderstroms blanket the hills with snow. The Himalaya makes its own located weather, which varies significantly over a distance of a few km. Expect cloudes in the afternoon and be prepared for occasional rain.
There is less snow on the mountains and on many of the high trails during winter. Winter snowstroms in December and January may make an early spring pass crossing difficult and can present an avalanche danger, especially on the apporach to the Annapurna Sanctuary. During the spring , sudden violent thunderstroms, accompanied by high winds blow down trees and tents asn cause streams to swell.