Environmental Science
An important reason for the large daily temperature range over deserts is:

Dry air is a very poor heat conductor
The light-colored sand radiates heat very rapidly at night
The ozone content of desert air is very low
Free convection cells are unable to form above the hot desert ground
There is little water vapor in the air to absorb and re-radiate infrared radiation

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Environmental Science
Much of Tibet lies at altitudes over 18,000 feet where the pressure is about 500 mb. At such altitudes, the Tibetans are above roughly:

75% of the air molecules in the atmosphere
10% of the air molecules in the atmosphere
25% of the air molecules in the atmosphere
50% of the air molecules in the atmosphere

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