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Curiosities: What are clouds made of?

April 14, 2007

Submitted by Jimmy Andruss, 7th Grade, Sennett Middle School.

Clouds are made of uncountable tiny particles, either water droplets or ice crystals, or a mixture of the two, says Grant Petty, a professor of atmospheric and oceanic sciences at the University of Wisconsin–Madison.

“The water droplets are usually about 10-20 micrometers across, or about one-twentieth of the diameter of the period at the end of a sentence,” Petty says.  “A volume of air the size of a sugar cube may contain many thousands of droplets.”

Clouds are important for many reasons. They reflect sunlight and trap heat, and thus influence global warming. They are also the source of rain and snow, and the site of thunderstorms, hurricanes and tornadoes.

Clouds can suddenly appear in the sky, Petty says. “When humid air rises, it cools until the invisible vapor it carries is forced to begin condensing as droplets, just like vapor from your shower condenses on your cold bathroom mirror.”

At extremely high altitudes and extremely cold (-40 F and below) temperatures, clouds may contain only ice crystals.  These clouds usually look wispy or hairy. The clouds that produce rain and snow are warmer, and commonly contain a combination of water droplets and ice crystals, Petty says.