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.

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