The climate is not as harsh as many people imagine it to be. Tibet has a dry, cold climate with an average annual temperature ranging from 12 degrees below Celsius to 9 degrees above Celsius, depending on the region. Temperatures in the mountains and plateaus are especially cold, and strong winds are common year around. The river valleys experience a more moderate climate. Lhasa and central Tibet have average temperature of 0 degrees Celsius in December and an average of 17 degrees Celsius in June. The temperature can range widely on any given day. On a typical summer day, the temperature can rise from 3 degrees Celsius before sunrise to 27 degrees Celsius by midday. In general, temperatures in Tibet frequently drop suddenly after sunset. The average annual precipitation is 381 mm (15 in), with the largest amount falling in the southeast.
The best time of year to be in Tibet is from April to the beginning of November. Central Tibet, including Lhasa, Gyantse, Shigatse and Tsedang, generally has very mild weather from April to November, though July and August can be rainy. October and November often bring some dazzlingly clear weather and daytime temperatures can be quite comfortable in Tibets lower altitudes. The coldest months are from December to February-March, but the low altitude valleys of Tibet (around Lhasa, Shigatse and Tsedang) see very little snow. Spring does not really begin until April, though March can have warm sunny days
Lhasa/Tsedang: The weather and tourism facilities in these cities are quite good, so you can visit them anytime to enjoy breathtaking scenery. But, due to the global warming weather change some time all weather and climate can be change with in a hour/day and weeks in Tibet.
Shigatze /Nepal & China Border: This area is not available for tourists from December to January because of heavy snows. In August during the rainy season, landslides will often occur.
Everest Region: From October to April, tourists are not advised to visit this area because of the low temperatures and heavy snow with Ice and cold wind but the weather will be clear sky.
Mt. Kailash, Mansarovar and Tholing Tsaprana Region: You can enjoy the wonder of this area from May to September, excluding August due to landslides.
East Tibet: Avoid this area in the winter and in July and August due to heavy snows and landslides.
North Tibet: Due to its high altitude at 4,500 meters above sea level, this region is best enjoyed during the warmer summer months.