Generally speaking, the best season is from April to October with the peak season from May to September. In between July and August are rainy months. Situated on the vast and dry Qinghai-Tibetan Plateau, it has a short cool summer and long freezing winters, with wide variations in temperature from day to night.
Although the Tibetan climate is not as harsh as many people imagine, be prepared for sudden drops of temperature at night, particularly in Western Tibet. The best time of year to be in Tibet is between April and early November, after which temperatures start to plummet.
However, in May and June there is a wind factor to consider and dust storms are not unusual. These are not pleasant if you're hitching or trekking but usually come in squalls and can be seen coming. Lhasa and Shigatse experience very mild weather between May and November although July and August can be rainy. These two months usually see around half of Tibet's annual rainfall.
The best season for touring in Tibet is from April to early November. But various routes have their own golden times:
Lhasa, Tsedang, and Shigatse- As the weather and tourism facilities in these places are quite good, so you can visit them in anytime for enjoying the breathtaking scenery!
Lhasa -the border of Nepal/China- This area is not available for tourists from Dec-Jan because of the sowed land; every August, the rain season, landslide will be occurred!
Mt. Everest area - it is too cold to the tourists visiting this area from Oct to April! But the beauty of the roof of the world will move you in anytime!
Ali (Mt.Kailash) - You can enjoy the wonder in this area at the time of every May, June, July, Sep, Oct without the trouble of the landslide by the big rain & snow.
East Tibet -Don't touch this area in winter and in Jul, Aug, as there will be a terrible landslide in summer and frozen load in winter!
North Tibet -Since it is too high with the average altitude at 4,500M, you only can appreciate her beauty in summer!