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

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

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

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