Weather Facts

Why is fog more common during the fall?

Foggy mornings are much more common as we head into the autumn months and the reasons are quite simple. The nights are getting longer and longer which allows the temperatures to fall to the dew point. As the air cools, it cannot hold as much water vapor so the water vapor condenses into tiny moisture droplets that stay suspended in the air. This is what we call "fog". (See my weather fact on the dew point and why it is so important). The month of October can be especially foggy across Southeast Virginia, especially during the morning hours. October usually is the foggiest month of the year throughout most of the Mid-Atlantic region. Humidity levels can sometimes still be quite high during October. Once we get into the latter part of fall, humidity levels drop and there is a tendency for higher wind velocities. Therefore, the frequency of foggy mornings starts to decline on average. In Southeast Virginia, fog can occur at any time of year but it is far more likely in the fall. Note that during the early spring, coastal areas experience fog on occasion which is caused by the cold ocean waters cooling the air above the surface to the dew point. This phenomenon usually occurs later in the day through the mid to late morning hours, when the sun is less intense. This fog normally dissipates as it heads inland.