Nepal has two main trekking seasons. From February to May and late August to December. Though Nepal remains an all year round destination for biking, sight seeing, meditation, paragliding and many other activities.
Feb and March are great times to trek as the days are bright and sunny and the rain fall is minimum. At night time it can get very cold and some higher places still experience snow. It’s a great time to see the villages preparing for the spring cropping season and to see the mountain areas in snow. A t this time the peaks have a lot of snow and make for some stunning scenery.
From March is coming the spring time bringing milder days and nights, though it can still be cold in the mountains. During spring Nepal is at its pretties with wild flowers and rhododendrons in full bloom and the rivers flow crystal clear from the snow melt. Its also an industrious time for the villages and a great chance to see the traditional way of farming.
In April and may the weather begins to get hotter and the days can be quite hazy, though on the mountain side the evenings remain cool. It’s a great time for trekking as by now all the trails are clear of snow. Occasional rain fall and persistent clouds make the weather a bit humid.
March, April and May are also the high season for peak climbing.
June, July to mid August, is the monsoon season in which the Nepal experiences its most heavy rain fall. Trekking at this time is possible in some low altitude areas, if you don’t mind getting wet and in dry areas like upper mustang and the Dolpa. Road travel can be quite difficult in remote areas during this time due to landslides and road closers esp. if you plan on coming into Nepal from Sikkim of Ladhak.
From late August the monsoon begins to dwindle and the sun comes out again. The days are warm and clear. September onwards offers the best and clearest time for viewing the mountains. Its ideal for trekking as the weather is warm and during the days T-shirts and shorts is adequate. In the lower area expect the weather to be quite hot.
In October the weather begins to cool esp. in the evening. Anywhere from the end of October you can expect the snows. Still it’s the best time for trekking because the air is clean and clear and its generally sunny and the clear views are mostly guaranteed.
In November and December the trekking is still very good though in the high passes and altitude areas you should be prepared for snow.
Dec and January are the coldest parts of the year, the mountains down to about 2500 meters get heavy snow and the high passes are mostly closed for trekking, as well as routes like Annapurna base camp and Everest base camp. It is possible in these times to do lower altitude trekking around Kathmandu or Pohkara, Dhading district and Ghorka. In fact it’s a good time of year to do these kinds of treks as the days are dry and sunny without being hot, though the evening are quite cold and normally the views are good.