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 May 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.