WEATHER WATCH COLUMN:  MARCH 2002 ISSUE
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Weather Watch: Radiation Fog

The Earth's surface is always cooling by radiating away energy.  At
night with no incoming solar energy to offset the cooling, the
temperature of the Earth and the air near the surface drops.  If the
air temperature drops enough to reach the dewpoint temperature, then
moisture in the air may condense to form <radiation fog>.  If the
ground is wet from a previous rain or melting snow, the opaqueness and
depth of radiation fog can be enhanced.  Clear skies and light winds
also help fog formation.  Radiation fog is typically a few hundred
feet thick and is common in low-lying areas where the cooler air
pools.  In strong radiation-fog events, an inversion may occur in
which temperature increases with height, preventing mixing and
possibly leading to air-pollution episodes.

<-David Schultz, NOAA/National Severe Storms Laboratory>