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Desert Diary


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With a little imagination, fluffy summer-time clouds can depict almost anything your heart desires—castles, animals, even your favorite cartoon character. But where do these clouds come from? A clear-blue morning sky becomes dotted with white, seemingly out of nowhere, as the day wears on. We know clouds are formed as water vapor, a gas, condenses into minute droplets of liquid water. But why in the form of these cumulus clouds and why as the day warms?

The answer lies in the powerful desert sun heating the ground unevenly—the temperature of bare soil rises rapidly, while vegetated and shadowed areas remain cooler. The hotter spots produce columns of hot air, known as thermals, lifting through the cooler, heavier surrounding air, much as a hot-air balloon rises. Expanding rapidly in the thinner air above, they drop to a temperature where the air's water vapor condenses and—voila!—a fluffy cloud!

Even when conditions are not right for clouds, you may see evidence of invisible thermals in the form of soaring birds using the rising currents for a free ride!
pen and ink

Listen to the Audio (mp3 format) as recorded by KTEP, Public Radio for the Southwest.


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, University of Texas at El Paso.


Clouds sitting on top of rising columns of air. University of Texas at El Paso, 4 July 2001. Franklin Mountains in background at right. Photograph by A.H. Harris.