Monday morning satellite images over the Tennessee Valley revealed a wonderful outline of the valley and river systems that make our landscape so unique:
Fog often shows up on satellite imagery looking like stationary clouds. This is also true of snow cover, but fog can be picked out by the fact that it dissipates just after sunrise (and in this case, the way it parallels valleys and rivers).
The reason fog follows waterways and valleys has to do with how types of fog form.
Most fog develops as radiational fog, named because it’s started by the radiational cooling that occurs at night. At night the Earth’s surface emits long-wave radiation and cools off the ground and the air near the ground. The cooling of the air near the surface causes moisture in the air to condense into fog.
Valley fog forms the same way, but cooler air can ‘slide’ down into the valleys and collect there through the night. This additional cooling can enhance fog formation, which contributes to the thick fog that covers valleys during the early morning hours.
Fog near rivers can form the same way, or can form through evaporation of moisture from the river itself. As the ground cools, that cooler air can slip over bodies of water (which are slower to cool at night than the Earth’s surface). As the cooler air slips over the warmer and moist air over the river, the warm air will cool and condensation will form fog.