This image shows the atmospheric profile of temperature (red) and dewpoint (blue) taken at Wilmington around the time the picture was taken.  Notice in the lowest part of the atmosphere that the temperature cools off rapidly near the surface, then quickly warms up with height.  This is known as a temperature inversion.  Notice that near the surface, the temperature drops so low that it meets the dewpoint, a clear indicator that the air at the surface is saturated and fog may develop.