Weather Watch:  Advection (Sea) Fog

December's issue of <Weather Watch> discussed steam fog, a fog formed
when the air temperature drops below the water temperature in light
winds.  In contrast, <advection fog> or <sea fog> forms when moist air
is transported, or advected, over a cold surface, like the ocean or a
lake.  If the water temperature is less than the dewpoint temperature
of the air, moisture in the air can condense, forming a cloud on the
ground (fog).  Advection fog is also distinguished from steam fog in
that the wind is usually blowing, unlike the calm conditions during
steam fog.  Advection fogs are common in the spring and early summer
because the post-winter water warms much more slowly than the air
does.  Very thick advection fogs can also form when warm moist air
blows across a melting snowpack.

-David Schultz, NOAA/National Severe Storms Laboratory