Fog is a cloud in contact with the ground. Fog differs from other clouds only in that fog touches the surface of the Earth. The same cloud that is not fog on lower ground may be fog where it contacts higher ground such as hilltops or mountain ridges. Fog is distinct from mist only in its density. Fog is defined as cloud which reduces visibility to less than 1 km, whereas mist is that which reduces visibility to less than 2 km.

The foggiest place in the world is the Grand Banks off the island of Newfoundland, Canada. Fog is frequent there as the Grand Banks is the meeting place of the cold Labrador Current from the north and the much warmer Gulf Stream from the south. The foggiest land areas in the world are Point Reyes, California, and Argentia, Newfoundland and Labrador, both with over 200 foggy days a year.

Fog forms when the difference (Δ) between temperature and dewpoint is (5 °F) 3 °C, or less.

Fog begins to form when water vapor (a colorless gas) condenses into tiny liquid water droplets in the air. Conversely, water vapor is formed by the evaporation of liquid water or by the sublimation of ice. Since water vapor is colorless, it is actually the small liquid water droplets that are condensed from it that make water suspended in the atmosphere visible in the form of fog or any other type of cloud. Fog normally occurs at a relative humidity around 100%. This can be achieved by either adding moisture to the air or dropping the ambient air temperature. Fog can form at lower humidities, and fog can sometimes not form with relative humidity at 100%. A reading of 100% relative humidity means that the air can hold no additional moisture and the air will then become supersaturated if additional moisture is added. Fog formation does require all of the elements that normal cloud formation requires with the most important being condensation nuclei. When the air is saturated, additional moisture tends to condense rather than staying in the air as vapor. Condensation nuclei must be present in the form of dust, aeresols, pollutants, etc. for the water to condense upon. When there are exceptional amounts of condensation nuclei present, especially hydroscopic (water seeking such as salt, see below) then the water vapor may condense below 100% relative humidity.

Fog can form suddenly, and can dissipate just as rapidly, depending what side of the dewpoint the temperature is on. This phenomenon is known as flash fog.

Another type of formation also common is sea fog (also knows as salt fog or salty Fog). This is due to the peculiar effect of salt. Clouds of all types require minute hygroscopic particles upon which water vapor can condense. Over the ocean surface, the most common particles are salt from salt spray produced by breaking waves. Except in areas of storminess, the most common areas of breaking waves are located near coastlines, hence the greatest densities of airborne salt particles are there. Condensation on salt particles has been observed to occur at humidities as low as 70%, thus fog can occur even in relatively dry air in suitable locations such as the California coast. Typically, such lower humidity fog is preceded by a transparent mistiness along the coastline as condensation competes with evaporation, a phenomenon that is typically noticeable by beachgoers in the afternoon.
Fog occasionally produces precipitation in the form of drizzle. Drizzle occurs when the humidity of fog attains 100% and the minute cloud droplets begin to coalesce into larger droplets. This can occur when the fog layer is lifted and cooled sufficiently, or when it is forcibly compressed from above. Drizzle becomes freezing drizzle when the temperature at the surface drops below the freezing point.

The thickness of fog is largely determined by the altitude of the inversion boundary, which in coastal or oceanic locales is also the top of the marine layer, above which the airmass is warmer and drier. The inversion boundary varies its altitude primarily in response to the weight of the air above it which is measured in terms of atmospheric pressure. The marine layer and any fogbank it may contain will be “squashed” when the pressure is high, and conversely, may expand upwards when the pressure above it is lowering.

Fog as a visibility hazard

Fog reduces visibility. Although most sea vessels can penetrate fog using radar, road vehicles have to travel slowly and use low-beam headlights. Localised fog is especially dangerous, as drivers can be caught by surprise.

At airports, some attempts have been made to develop methods (such as using heating or spraying salt particles) to aid fog dispersal. These methods enjoy some success at temperatures below freezing.

Accidents

Fog contributes to accidents, particularly with modes of transportation. Ships, trains, cars and planes cannot see each other and collide. Notable examples of accidents due to fog include the July 28, 1945 crash of a B-25 Mitchell into the Empire State Building, and the July 25, 1956 collision of the ocean liners the SS Andrea Doria and MS Stockholm.

The worst accident in aviation history occurred in the fog when 2 Boeing 747s collided in 1977 in Tenerife. One 747 had clearance to taxi down a foggy runway and the other could not see any distance down the runway when the captain decided to take off without proper clearance.

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