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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??



Replies:
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



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