Ask A Scientist General Science Archive

Question:
Two questions for the weather experts out there.  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, again, why??
 esquared

Answer 1:
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 doesn't 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  ome of the valleys of the hilly area that
are sheltered from the wind.
 allsopp

Answer 2:  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 dewpoint,
and therefore produces fog.  This fog will disapear when the wind
dies out, bucause radiation cooling takes effect.
-- dipper