Why are tall mornings so foggy in Hanover?

...for Professor Eric Posmentier, who teaches Dartmouth's introductory meteorology class:

Fall mornings in Hanover come in two varieties, explains Posrhentier, frosty and foggy. Thanks to Hanover's rivervalley location, an ample supply of moisture and cooler mountain air sinks into the valley and causes condensation. Fall is the best season for fog formation because, "the river water is still warm, and evaporation occurs at a rapid rate, keeping moisture in the air. The nights are also lengthening so the air has more time to cool. Consequently it takes more time to burn off the water vapor in the morning, and we see more fog." Some of us do anyway. Not many students are up early enough to see the fog on the Green. (Though many late-risers cross the Green in a fog of their own making.)