Fog is the aggregation of water droplets or ice crystals immediately above the surface of the earth (i.e., a cloud resting on the ground). Fog becomes possible when the moisture content of the air is increased beyond the saturation point. Fog sometimes occurs when cold air streams over a warm water surface (steam fog) or when a warm rain falls through a layer of cold air near the ground (frontal fog). Fog may also be caused by radiation of heat from the ground during a windless, cloudless night (radiation fog). Radiation fog is often found in valleys and depressions in the morning, especially during autumn. In all types of fog, moisture condenses on the microscopic particles (condensation nuclei) in the atmosphere.
Image courtesy of Carlye Calvin/UCAR