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".
In the visible satellte image below, the red arrows show where some of the valley fog is located.
Mainly look for "white vein" like features. The areas outlined in
yellow are clouds.
Below is an animation from 6:01 AM EDT to 9:45 AM EDT. The loop starts out dark, but as
the sun rises, the valley fog and clouds are illuminated. Note that the valley fog does not
move will the clouds do. By the end of the loop most of the valley fog has dissipated.