Firstly, the weather in Tibet is not as hash as many people imaged. It gets warmer and warmer in recent year and most of the places you can visit all around the year, the temperature is about -15 to 20 degree. The big feature of Tibet climate is that it varies greatly within a single day. So, you may don't need the Air conditioner in Summer here. And another big feature is the different area, the climate is big different. In the same winter, the east of Tibet is quite warm in daytime but in west tibet (Ngari prefecture) is terrible cold and even down to -30degree in daytime.