Ground fog is a collection of water droplets in the air that forms at ground surface. Fog at ground surface can reduce visibility to about 1 km (62 miles). Most ground fog forms near a body of water (lake, ocean, river, etc.), humid ground or swamps. Fog forms when the temperature of air is reduced to its dew point or when moisture is added to air at ground level. The following explains how ground fog forms.
The dew point is the temperature at which water vapor suspended in the atmosphere condenses. Like clouds, fog is made of tiny water droplets which condense as the result of the air being cooled down to the dew point. For water vapor to condense in the air, the dew point must be equal to the air temperature. Ground fog usually forms at night, when air near the surface of the ground cools to its dew point, reaching a relative humidity of 100%.
Ground fog usually forms inland and can be caused by a number of ways, depending on how the cooling of air occurred. Radiation fog occurs when the ground is cooled overnight by thermal radiation. The next day, the cool temperature of the ground produces condensation of the passing air in contact with it through rapid heat loss. Ground fog is usually less than a meter deep; however, turbulence can cause it to become thicker. Ground fog disappears just a little after sunrise, although it may persist in the cold winter months.
Advection fog is a type of fog which can form at ground level or on a body of water (sea, lake, etc.). At ground level, advection fog forms when humid air comes in contact with a cool ground surface and is cooled to its dew point, causing the water vapor contained within the mass of air to condense into fog. Evaporation fog is created when a cool air mass moves over warmer humid land, producing freezing fog.
Precipitation fog occurs when water drops fall over drier air, causing the droplets to evaporate. When the water vapor reaches its dew point, it condenses and turns into fog. This type of fog can form anywhere below the cloud, including at ground level. Hill fog usually forms when a mass of air is carried up a slope, causing moisture to condense and creating freezing fog on mountain tops.
Hail fog, which is another type of ground fog, occurs where hail accumulations have caused both a decrease in temperature and an increase in moisture leading to saturation. This type of ground fog is most commonly localized; however, it can be extremely dense and appear unexpectedly, forming shortly after the fall of hail.