Ask A Scientist General Science Archive

Author:     esquared
Text:       First, does fog form more readily in hilly areas or flat areas and 
why?  Second, if fog does form more readily in hilly areas (which is the 
hypothesis), does the fog form in the valleys, on the crests, or on the slopes 
and why?

Response #:  1 of 2
Author:      Jim Allsopp
Text:        Assuming we are talking about radiation fog...Radiation fog forms 
on clear calm nights.  The earths heat is radiated out to space after the sun 
goes down.  The layer of air near the surface of the earth also cools down.  
If the air cools enough, the moisture in the air condenses out forming fog.  A 
blanket of clouds will trap the earths heat and prevent fog from forming.  
Strong winds will mix up the air and keep that shallow layer of cool air from 
forming near the ground.  That is why radiation fog forms on clear cool 
nights.  The air near the ground cools rapidly and the cooling gradually 
spreads upward.  Since cool air is more dense than warm air, on a night with 
no wind, gravity will pull the coolest air down.  In a hilly area, the cool 
air settles into the valleys and therefore fog forms in the valleys while 
hilltops may be clear.  It does not really matter if the terrain is flat or 
hilly.  If the right weather conditions are present (clear skies, light winds 
and sufficient moisture) the fog will form.  Although you could argue that if 
there are light winds over both a flat and hilly area, the winds could prevent 
fog formation in the flat area, but fog could still form in  one of the 
valleys of the hilly area that are sheltered from the wind.

Response #:  2 of 2
Author:      Eric Peterson
Text:        Then there is Upslope Fog.  This type of fog forms exclusively in 
areas of sharp upslopes, such as the area around Los Angeles, or the eastern 
slope of the Rockies.  This type of fog requires a fairly brisk wind, with 
warm and humid conditions at the surface.  As the wind (Westerly in LA and 
Easterly in Denver) is forced up the slope, it is cooled beyond the dew point, 
and therefore produces fog.  This fog will disappear when the wind dies out, 
because radiation cooling takes effect.