When a gas is cooled, it contracts. In a sealed environment, the pressure would therefore reduce, as the molecules try to get closer together, that is, push outwards less. But in the atmosphere, the closer-packed molecules (or atoms, of course) make the air more dense, therefore heavier.
If you were at a higher elevation, the cooling is caused by the reduced pressure causing the molecules to move further apart. The temperature reduces about 1 degree Centigrade for every 100metres gain in elevation.
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