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Effects of Expansion in the Air

المؤلف:  GEORGE A. HOADLEY

المصدر:  ESSENTIALS OF PHYSICS

الجزء والصفحة:  p-266

2025-12-27

395

 The effect of heat upon air may be seen in the currents that are set up near a heated surface. The air next the surface becomes warmer and lighter, and cooler air displaces it. This sets up an ascending current over the heated area, and a horizontal current at near-by places. If this heated surface is an extensive tract of the earth, the result will be the setting up of violent winds toward it.

Just as the temperature of air is raised by compression, so also it is lowered by expansion against reduced pressure. The ascending current over the heated surface rises to an altitude where the atmospheric pressure is considerably less. Because of the resulting expansion of this air, the temperature is likely to fall so low as to cause the condensation of its water vapor into clouds and rain.

The beginning of a thunderstorm exemplifies both these effects. Use is made of the second effect in liquefying air and other gases. The pressure on highly compressed cold air is suddenly reduced, and the resulting expansion of part of the air withdraws so much heat from the rest that the residue is liquefied.