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THURSDAY MARCH 8 - FOG OVER SNOWCOVER
Fog can form in many different weather situations. It's most common after dark,
often during the fall when nights are lengthening. That gives plenty of time for
air near the ground to cool off, and if there's not much wind, moisture in the air
can condense into a nearly stationary cloud at the ground, that we call fog.
For fog to form when there's a good breeze, you typically need warm moist air blowing
over a cooler surface. The surface chills the air moving over it, leading to the fog.
In winter, that cooler surface is often snow-covered ground. We've seen this happen
a few times this winter as southerly winds brought in moist, relatively mild air over
the snowpack. The fog literally was blowing with the wind.
As an aside, the famous fogs that creep past the
Golden Gate Bridge into San Francisco
Bay form in a similar way, but there the cooler surface underneath is the Pacific
Ocean, which at San Francisco's latitude, stays chilly all year round.