In general the best season for trekking is from May through October. The weather during these month is surprisingly mild,with warm days and cool to chilly nights in the mountains: not very different from summer conditions in many mountain ranges of America and Europe. July and August can be very rainy, receiving over half of Tibet’s annual precipitation. India’s monsoon manages to push over the Himalaya into much of West, Central, and Eastern Tibet, bringing rain (and sometimes snow) most nights and occasionally during the day. At this time of year even the most barren hills don a fuzzy green coating of grass, while up in the alpine areas the valleys become lush and speckled with wildflowers and blooming shrubs.
By September, autumn is knocking on the door. The days are cooler and the willow and poplar leaves begin turning yellow. Indian summers are not uncommon and may last into late October or early November, bringing sunny days and clear, crisp nights with temperatures often near freezing. But September and October can also bring surprise snowstorms to the mountains.
The coldest time of the year is December through February, but this is also a time when Tibet snowfall is generally too great to consider winter treks. However, the regions around Lhasa, Shigatse, and Dingri in Central in Central Tibet, and the Dege and Labrang/ Rebkong region in Eastern Tibet receive relatively little snow, while it is generally cold, the trekking can still be enjoyable.
In March the lower-altitude valleys start to warm and show signs of spring. A jacket is no longer need on sunny afternoons and the winds start losing their chill. May and June are usually the hottest and most windy months in Central Tibet. Wind is common throughout the year in most ares of Tibet, but during these months it can be particularly bad. Billowing afternoon dust storms occasionally engulf the larger Central Tibetan valleys such as the Lhasa and Tsangpo River basins. A cotton surgical-type mask is recommended for dusty road conditions.
As in any mountainous region, the weather can change quickly and snow is possible any month of the year. Even in summer it is advisable to carry winter clothing, particularly if you plan to ascend above 16,000 (4880m). But more often than not the weather in Tibet is fine and wonderfully clear.