The climate in highland Tibet belongs to the typical downy special climate. Climates are much different in different areas in Tibet and temperatures vary greatly within a single day.
Climate in southeastern Tibet (e.g. Nyingchi) is gentle and temperate with the average temperature of eight degrees. In western Tibet (e.g. Nagqu), the average temperature is below zero degree. While in Lhasa and the central part of Tibet, the climate is rather normal and seemingly nice for travelers. Visitors would not feel too cold in winter or hot in summer, especially from March to October, the best seasons for traveling. Most annual rainfall comes in the rainy season that starts from May to September, when the precipitation covers 90 percent of the whole year. Usually it rains at night in Lhasa, Shigatse and Chamdo area. The precipitations gradually decrease from 5000 millimeters in the lower part of southeast to just 50 in the northwest.
So generally speaking, the best time to travel Tibet is between April and October, and peak season is from May to September. The best months are May, June, September or October as July and August are rainy months. Those who travel in their own car or on foot should avoid the rainy season especially when entering Tibet along the Sichuan-Tibet Highway, and the sections between Lhasa and Nyingchi and between Lhasa and Ngari. There will be mudslides, cave-ins and mire on certain sections of the road, blocking the passage of vehicles. However, if you are only planning several days in and around Lhasa and getting there by air, there will be no problem for you to travel at any time from April to October.