Dear Tom,
What might be the relationship between clouds and fog? Is low-lying fog actually a cloud?
— Leon J. Hoffman, Chicago
Dear Leon,
Physically, clouds and fog are identical. Both are visible aggregates of water droplets (or ice crystals) so small that they are nearly suspended in the air. Clouds differ from fog only in that, by definition, fog is either at the ground or within several feet of it, whereas the bases of clouds are aloft.
It’s a convenient shorthand to say that cloud and fog droplets are “suspended” in the air, but that is technically incorrect: The droplets do fall. The fall rate is so slow — generally only 1 foot per minute, or less — that, for all practical purposes, clouds and fog appear to be suspended.
A final distinction: Meteorologists also consider a visible concentration of smoke or dust aloft to be a cloud.
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