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The Way in Which Ground Fog Appears on the Surface of the Ground
Cloud of Water Droplets
Ground fog is a cloud of water droplets in the air that forms at ground level. Fog is produced at ground level when the air is cooled enough to reach saturation. Most fog at ground level can reduce visibility to approximately 1km (62 miles). Most ground fog usually forms near a body of water, such as a lake, river, swamp, or humid ground. Fog will form on the surface of the ground when the temperature of air is lowered down to its dew point or when moisture accumulates into a parcel of air at ground level.
The dew point is described as the temperature needed for water vapor to condense in the atmosphere. Fog, in the same way as clouds, is made of tiny droplets of water which condense around tiny particles of dust, in a process known as nucleation, when the air is cooled down to the dew point. In order for water to condense in the air, the dew point must be the same as the air temperature. Ground fog most commonly forms during the night, when the air over the surface of the ground cools down, attaining a 100% relative humidity.
Fog in the Early Morning
Ground fog is produced in a number of ways, depending on the process by which the air was cooled down. Radiation fog most commonly is formed when the ground is cooled overnight by longwave radiation emission. At night, heat radiation cools the ground air until it reaches saturation, and fog begins to form.
This type of fog usually develops during the evening hours, and disappears shortly after the first morning hours due to absorption of solar radiation, although, it may remain in the cold winter months. Radiation fog is usually a little less than a meter in depth; however, strong turbulence can cause it to become thicker.
Mass of Cool Air Over a Body of Water
Advection fog occurs when a mass of air flows over a surface with a different temperature. This type of fog can form either on a body of water, such as the sea, lake, river, etc., or at the surface of the ground. At ground level, advection fog is formed when a mass of humid air makes contact with a cool surface, which causes it to reach its dew point.
One example of advection fog is the fog that forms over the surface of a glass of cool water. When you pour cool water into a glass, the outside of the glass begins to form a thin layer of condensed droplets due to the different temperatures interacting. The cool water inside the glass causes the atmosphere around it to cool down, reaching saturation
Evaporation fog, which is a type of advection fog, is created when a mass of cool air moves over warmer, humid land, or warmer water surfaces. In this instance, fog is formed when water from lakes, rivers, sea or swamps evaporates when coming in contact with the mass of cool air above them, reaching saturation. This type of fog is also known as steam fog.
Hail storm decreases Temperature and Increases Moisture
Precipitation fog forms when water droplets or ice crystals precipitate into drier air below the rainy cloud, causing evaporation or sublimation directly into water vapor. Water vapor cools and condenses, thus, creating fog. Precipitation fog can form anywhere below the cloud, including the surface of the ground.
Upslope fog is created when a mass of air is transported up a slope, while being cooled by adiabatic expansion. This type of fog is usually found on the mountain tops or slopes of hills.
Hail fog. another type of ground fog, forms immediately after a hail storm. After a hail storm, accumulations of hail on the ground cause a decrease of temperature and an increase in relative humidity in a thin layer on the surface of the ground, leading to the saturation of the air. Hail fog can be very dense and show up abruptly.