With the notable exception of the occasional tropical rainstorm, autumn usually features the most tranquil weather of any season. As a result, it's typically the foggiest time of the year.

Fog forms most readily at night when skies are clear and winds are light. These conditions allow the air near the ground to cool rapidly, and if enough cooling occurs, fog will form. Colder air tends to be heavier and thus has a tendency to seek out the lowest elevations, such as valleys, so that's where fog is most prevalent. And if there happens to be water in the valley - a stream or river - chances for fog are even greater.

These satellite images of Pennsylvania and nearby states taken a few weeks ago dramatically illustrate this tendency for fog to form in valleys. The Susquehanna River and its tributaries in central and northeastern Pennsylvania and southern New York are easiest to find, while you can even see the faint outline of the Allegheny, Monongehela, and Ohio Rivers in western Pennsylvania.