Refer to figure.

RADIATION FOG. Caused by radiation of the earth’s heat at night, and the conductive cooling below dew point of the air in contact with the ground, in conditions of calm or very light wind and clear skies. Radiation fog occurs at night and early morning and usually does not last long after sunrise (about 30 minutes). Predominantly autumn and winter.

Average extent of radiation fog is approximately 500 feet but may vary from few feet to 1000 feet.