Re: What keeps clouds up?

[PHRP@FHSUVM.FHSU.EDU (Roger A. Pruitt)]

> If the air is saturated all the way to the ground then a cloud may form at
> ground level where it is called fog.
>
> David Bowman
> dbowman@gtc.georgetown.ky.us

Air does need to be saturated (at or below the dew point temperature) for
fog to form. However, the region of saturation may only be a thin layer near
the surface of the earth. Radiation fog forms this way. Radiational cooling
near the earth's surface can lower the temperature of the air near the
surface to below the dew point temperature and the result is a cloud at the
surface.

Advection fog caused by warmer humid air moving over a cold surface is
commonly seen off both coasts. Several years ago my wife and I were on a
whale watch out of Bar Harbor, Maine, and the boat we were on went through
very thick fog produced by the cold ocean currents near the coast. About 15
miles out the air was clear with great visiblility, and be could look back
to the coast and see that the fog layer was maybe only a few tens of meters
thick. 

Roger
Roger A. Pruitt, PhD
Professor of Physics
Fort Hays State University
Hays, KS  67601