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Weather in Action: Valley Fog
The visible satellite image below shows a classic dendritic pattern created by early
morning fog in the valleys of New York and Pennsylvania. Fogs such as this develop during
nights favorable for radiational cooling - conditions of clear skies and light winds.
When the earth radiates its heat after sunset, rapidly cooling air on the hilltops
becomes more dense than the surrounding air. The more dense coller air sinks and flows
down the hillsides, pooling in the valleys. As this process continues through the night,
the temperature of the air eventually cools down to the dew point and saturation occurs.
Initial fog development in the saturated air is shallow but as the process continues it
becomes thicker and more dense, reaching a peak around daybreak. Moisture evaporating from
streams or rivers at the bottom of the valleys adds to the fog formation, especially when
the air temperature falls below the river water temperature. Valley fog will evaoprate
within a few hours after sunrise, a process normally refered to as "burning
off".
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