Location: Flying from D.C. to Chicago over the Appalachians
Date: 12 July 2014
During the day, the sun heats the surface of the Earth, but at night, this heat can be reradiated back to space, cooling the ground and the air near the surface. Usually temperature decreases as you go up in the atmosphere, but this localized cooling near the surface creates what’s called an inversion, where the temperature actually increases with height within a small layer. Any air that rises from the surface therefore remains cooler than its environment, which is a stable scenario where the air will then return to the surface. If the beneath that inversion can cool to its dewpoint (the temperature at which saturation occurs), condensation can occur, leading to a cloud. In this case, it’s so close to the surface that we refer to this as fog. This diagram shows this process (LW means longwave radiation coming from the ground at night and the T/Td lines on the left are just showing that the temperature decreases to the dewpoint temperature with time):
Valley fog is a common type of this “radiation” fog (named for the cold air being radiated from the surface) that occurs in the valleys of mountains. During the day, the sun heats the mountain slopes, creating a circulation where air rises along the slopes. At night, however, the cold air drains from the slopes down into the valley. On a clear, calm night, this drainage can increase the cooling at the surface such that the temperature in the valley drops to its dewpoint, creating fog.
So in the picture above submitted by Richard, you can clearly see where the valley is located in this stretch of the Appalachians as the fog persists in the morning hours. As the day goes on and the sun heats the slopes, this fog will dissipate.