Khamp -- the Tibetan Pheasant Credit: Matjaz Vrecko

Climate and vegetation
Tibet, the Roof of the World, is the highest plateau in the world. However, most of its land is arid or a semi-desert. The average annual precipitation is about 381 mm (about 15 inches) and is significantly less in some areas. Tibet experiences bitterly cold weather and is lashed by harsh winds throughout the year. The average annual temperature is around 1.1 degrees Celsius (34 degrees Fahrenheit).

The mountains are treeless, with only grasses and shrubs growing. Tibetan people live in the valleys of the Indus and the Brahmaputra, where they are able to grow some crops. The river valleys also support a few species of trees including oak, cypress, polar and maple. In addition, the people grow apple, peach, pear and apricot trees.

The rainshadow effect Credit: Mahabir Pun

Rain Shadow Effect
The rainshadow effect is responsible for the aridity of Tibet. The Himalayas mainly get rain from the monsoon winds blowing in from the south. The monsoon winds, full of moisture picked up from the Indian ocean, cross over the subcontinent of India and reach the Himalayas. As the winds are obstructed by the high Himalayas, they condense and give heavy rainfall to the southern slopes of the Himalayas, which lie on the windward side of the mountains.

Most of the moisture in these winds gets exhausted and the air that finally crosses the main Himalayan ranges is rather dry. As a result, the Trans Himalayan regions receive very little rainfall. Therefore, Tibet and other regions lying in the northern part of the Himalayan system are said to lie in the rainshadow area of the Himalayas and hence get little rain. The lack of rain is responsible for making Tibet a very dry land.