Environmental Science
Low clouds retard surface cooling at night better than clear skies because:

The formation of the clouds releases latent heat energy
The clouds absorb and radiate infrared energy back to earth
The water droplets in the clouds reflect infrared energy back to earth
The clouds are better conductors of heat than is the clear night air
The clouds start convection currents between them

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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:

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

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