September 04, 2005
On the morning of 23 October 2004, I climbed a small mountain overlooking the historic Rockville Bridge on the eastern shore of the Susquehanna River in Pennsylvania. At 09:38, most of the railway bridge and virtually the entire river was obscured by thick fog. However, by 10:02, when this photo was taken, much of the fog had dissipated, and a westbound Norfolk-Southern train could then easily be seen crossing the bridge.
Evaporation fog is formed when cool air moves over a warm water surface. As this layer of air is warmed by the water, it rises and mixes with the still cooler air above, where it's chilled. If it's chilled sufficiently, the water vapor will condense into fog once the air is saturated. This type of fog typically occurs over rivers and lakes in autumn when early morning air temperatures are often colder than water temperatures -- the thermal inertia of water is considerably greater than that of air.Related Links: