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Q & A: Making Clouds - What keeps them together?

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Most recent answer: 10/22/2007
Just two questions about clouds: 1. What holds them together? There is a lot of wind up where they are, and they are just collections of tiny drops of water. So, why don’t they just blow apart? 2. What holds them up? Again, they are just collections of tiny drops of water. So, why don’t they fall to earth? I know they eventually fall in the form of rain. But what takes them so long?
- Richard Treptow
Chicago State University, Chicago, IL, USA
Richard -

The answer to both of your questions is actually the same. Clouds aren't just random droplets of water. What they are is tiny droplets of water that have condensed from water vapor in areas where the conditions (temperature, air pressure and humidity) are exactly right. Anywhere where you have the correct combination of these conditions, you'll get a cloud. Anywhere that you don't, you don't.

Areas with this kind of conditions don't occur in random tiny places. Instead, there will be a large area of one condition that gradually shifts into another. (If this doesn't make sense, think of just temperature. You don't get a different temperature at every spot on the globe. Instead, you get a large area of warm air that gradually shifts into a large area of cooler air. The same is true for air pressure and humidity.) This is why you get a large area of 'cloud' in one place and nothing at all in another. You can think of clouds as being physical maps of the weather conditions in the sky.

This is also what holds the clouds up. In general, if a cloud is high up in the sky, the conditions just below it aren't right for the cloud to form. As the water droplets fall, they reach areas with the wrong temperature or pressure and evaporate again. Only if the clouds become densely packed enough for the water to form large enough droplets can the droplets fall to the ground. Not all clouds form high up, either... sometimes the right conditions can occur at groundlevel, giving us fog.


(published on 10/22/2007)

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