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Floating Clouds
Mon Sep 12, 2005
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Did you know that a typical small cumulus cloud weighs as much as 500 automobiles? So why do they float on the air so effortlessly? Hi, I’m Bryan Yeaton for The Weather Notebook.
Every cloud you see in the atmosphere is comprised mostly of air with water droplets and ice crystals mixed in. We know the tiny water droplets and ice crystals are heavier than air molecules, so why doesn’t gravity pull the clouds to ground?
Actually, they do. All the water in a cloud falls downward at a speed known as its terminal velocity, which results from a balance between gravity and the aerodynamic resistance of the falling object. This resistance, dependent on the weight and diameter of the drop, can be quite slow because cloud droplets are very small — as are ice crystals.
The typical cloud droplet is 470 millionths of an inch in diameter; it takes about 15 million to form an average raindrop. As a result, its terminal fall velocity is a pokey 30 feet per hour.
To fall from 9,000 feet, an average altitude for a cloud center, would take 300 hours in calm air.
Cloud formation requires rising air. Even a weak updraft ascends at 300 feet per hour, and as a result, cloud droplets falling within the updraft ascend relative to the ground rather than fall.
Also, any droplet descending below the cloud base quickly evaporates so the cloud base appears constant to us on the ground. The combination of slow descent and quick evaporation below cloud base gives the appearance that the cloud is floating on thin air.
Of course when cloud droplets combine into water drops too large and heavy for the ascending air to hold them up, rain begins to fall.
Thanks to our contributing writer, meteorologist Keith Heidorn. The Weather Notebook is supported by Subaru of America.
Today's Links
Scientific American: Why do clouds float when they have tons of water in them?
http://www.sciam.com/askexpert_question.cfm?articleID=00057690-D38E-1C71-9EB7809EC588F2D7
Why Doesn\'t Gravity Push Clouds to the Ground?
http://www.theweatherprediction.com/habyhints/228/
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