Nepal has four distinct seasons. Spring, from March to May, is warm and dusty with rain showers. Summer, from June to August, is the monsoon season when the hills turn lush and green. Autumn, from September to November, is cool with clear skies, and is the most popular trekking season. In winter, from December to February, it is cold at night and can be foggy in the early morning, but afternoons are usually clear and pleasant, though there is occasional snow in the mountains.
Because Nepal is quite far south (at the same latitude as Miami and Cairo) the weather is warmer and winter is much milder at lower elevations, including Kathmandu at 1400 metres. It rarely snows below 2000 metres.
The monsoon in the Bay of Bengal governs the weather pattern. The monsoon creates a rainy season from the middle of June to the middle of 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 most of Nepal is difficult and uncomfortable. Clouds usually hide the mountains and the trails are muddy and infested with leeches.
It usually does not rain for more than one or two 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 evenings and occasional thunderstorms blanket the hills with snow. The Himalaya makes its own localised weather, which varies significantly over a distance of a few km. Despite the sanguine assurances of Radio Nepal that the weather will be "...mainly fair throughout the kingdom", always expect clouds in the afternoon and be prepared for occasional rain.
Most of the precipitation in the Himalaya occurs during the summer monsoon. There is less snow on the mountains and on many of the high trails during winter. Everest itself is black rock during the trekking season, becoming snow-covered only during summer. There are always exceptions to this weather pattern, so be prepared for extremes. Winter snowstorms in December and January may make an early spring pass crossing difficult and can present an avalanche danger, especially on the approach to the Annapurna Sanctuary.
In Kathmandu, spring and autumn days are comfortable and the evenings are cool, usually requiring a light jacket or pullover. Winter in Kathmandu brings cold foggy mornings and clear evenings, but pleasant day temperatures with brilliant sunshine most days after the morning fog has lifted. It never snows in Kathmandu, though there is frost on cold nights in January and February. The hottest month is May, just before the rains start.