"A summer fog for fair, a winter fog for rain; a fact most everywhere, in valley or on plain."
Above is an old lore; but is it true? Why would fog mean different things in the summer and in the winter? I wondered that myself, so I did some research to try to find out...
How Fog Forms
Fog is typically formed in two different ways.
(1) Warm, moist air moves over a cold surface and causes fog to form.
(2) The dew point reaches 100%, and fog forms.
While the end result of these two things is different, you can see that they both form one thing: fog! But, depending on how and when the fog forms, it can mean two very different things: clear summer skies or winter rain!
Winter fog is caused by (1) - warm air going over a cool surface. When this happens in winter, its a sign of a looming rainstorm. This type of fog is very common in winter months.
Summer fog is usually because of (2) - the dew point reaches 100% and thus it forms. This can only happen on clear summer nights. Because if it were cloudy, the clouds would hold the heat to the earth and keep it arm, which would prevent cold air from entering, so fog is out of the question if it's going to rain!