Summer mornings can bring an interesting form of fog: Ground fog. According the American Meteorological Dictionary, a ground fog is defined as a fog that hides less than 0.6 of the sky and does not extend to the base of any clouds that may lie above it. A ground fog typically forms at night, when air in a shallow layer near the surface cools sharply to its dew point, making the relative humidity 100%. It then persists for a short time after sunrise, until sunlight heats the air enough to lower the relative humidity. As an obstruction to vision in an aviation weather observation, ground fog is encoded GF.
Mary Knapp, Weather Data Library