Desert Diary
May 20, 2004

Physics/Head for the Hills

     

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"Head for the hills!" isn't only the cry when the bad guys come to town. It's also the time-honored motto of the desert dweller as the summer heat builds up. The reason, of course, is that the high country is notably cooler than the lowlands. Why this is so, however, is a mystery to many people. After all, on the mountain top, you're even closer to the sun.

The increasing coolness as you climb higher is mostly due to the thinning of the air. The amount of heat in a given quantity of air is determined by both the rapidity with which the air molecules move and the density of the air. At low elevations, those molecules are tightly packed, and a cubic inch holds a lot of heat. As the air becomes thinner higher in the atmosphere, there are fewer molecules of air in that cubic inch and thus less heat, even if the molecules are moving with the same speed as in the lowlands. The result? "Head for the hills!" makes darn good sense!

Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.

Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.

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