Altitude sickness is the effect of the thin air at great heights. Travelers to Tibet may suffer some discomfort like breathlessness and headaches before becoming acclimatized.
Many hotels keep bottled oxygen for emergencies.
Lhasa is also connected to Beijing, Xian, Chengdu and Chongqing by air. March through October is the best time to visit. Layered clothing is recommended as day and night-time temperatures vary greatly. The rainy season is June-September. Since Tibet is in the zone of rain-shed only trace of rainfall takes place.