Winter (Dec–Jan) is for the most part dry and settled, albeit colder. When precipitation does fall, the snow line drops to 2500m and sometimes lower. Passes over 4000m (including the highest on the Annapurna Sanctuary and Circuit treks) may be uncrossable due to snow and ice, and some settlements described in trekking guidebooks may be uninhabited. High-altitude treks, such as Everest, require good gear and experience in cold-weather conditions, as temperatures at 5000m can drop below minus 20°C and heavy snow can fall; if you’re up to it, however, this can be a magical time to trek. Some treks see something like twenty or thirty times the numbers of trekkers in autumn as in midwinter, and below 2000m temperatures can be quite spring-like, though valleys are often filled with fog or haze.
During May and June it gets hotter, hazier and increasingly unsettled. The warming Asian landmass has begun drawing air up from the south, ushering in the pre-monsoon – a season of erratic weather and increasingly frequent afternoon storms. The trails and lodges again begin to empty out. This is a time for going high, but be prepared for rain, especially in traditionally wet areas such as Annapurna and far eastern Nepal.
Few foreigners trek during the monsoon (mid-June to early Sept), because of the rain, mud, leeches, travel difficulties and general lack of mountain views. (The leeches along the middle-elevation trails won’t hurt you, but are not for the squeamish.) However, treks in the Himalayan rain shadow and in Nepal’s far west are sheltered from the brunt of the monsoon. Even in wet areas, mornings are often clear, wildflowers and butterflies can be seen in abundance, the terraces and forests are a luscious green and the soundscape – dripping leaves, roaring rivers – magical. Authentic Nepali culture is more in evidence, too, as the summer off-season is when locals return to their farming and other traditional activities. Note also that the monsoon isn’t consistently rainy: it builds up to a peak in July and August, then tapers off again.
Conditions in the post-monsoon harvest season (roughly mid-Sept to early Oct) are hard to predict: it all depends on the timing of the rains. If you’re lucky, you can enjoy clear, warm weather and gloriously empty trails. At lower altitudes it can be hot and sticky, however, and you may face an extended monsoon tail, with clouds obscuring the peaks and heavy afternoon showers or snow flurries higher up.