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Dear Tom, What is the difference between fog and...

November 20, 1999|By Tom Skilling, chief meteorologist at WGN-TV.

Dear Tom,

What is the difference between fog and clouds? We have a huge debate going on and would like to know the answer.

Mary Kate Graf

Dear Mary Kate,

Altitude of occurrence is the primary difference between fog and clouds. Fog is defined as a cloud in contact with the ground. One twist is that fog doesn't officially exist--it's just haze or mist--until it reduces visibility to less than one kilometer (5/8 mile). A cloud faces no such visibility criterion.

Both form when the moisture in a layer of air reaches a level which allows minute suspended droplets, often just 1/100th the size of a raindrop, to reach an equilibrium with their surroundings. When this happens, water molecules condensing on the droplet equal those evaporating from it. At that point, droplets can't disappear through evaporation, instead floating and becoming visible to us by scattering or dispersing available light. This process produces the cloud's white or gray color and, at the same time, is responsible for the visibility reduction we experience in a fog bank or as we fly in an airplane through a cloud.



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