Fog is a cloud in contact with the ground. It occurs when moisture from the surface of the Earth evaporates; as this evaporated moisture moves upward, it cools and condenses into the familiar phenomenon of fog. Fog differs from cloud only in that fog touches the surface of the Earth, while clouds do not. All types of fog form when the relative humidity reaches 100% and the air temperature drops below the dewpoint, pushing it lower by forcing the water vapor to condense. 
Fog reduces visibility. Road vehicles have to travel slowly and use more lights. Localised fog is especially dangerous, as drivers can be caught by surprise.

Fog can occur at any time of year and in any location. Its presence can be influenced by nearby water, the local topography, wind conditions at the time, and even the activities of humans. The official visibility distance in fog is less than 1km. If visibility is further than 1km, it is not fog, but mist. Fog can appear just as suddenly as it can disappear. When it appears suddenly like this, it is called ‘flash fog’.

There are a number of types of fog, named according to how the fog forms. Radiation fog results from land cooling after sunset. Advection fog or ground fog results from wind passing moist air over cool ground. Precipitation fog or frontal fog is the result of precipitation falling into the drier air below a cloud. Hail fog is sometimes seen after a hail storm due to the increased moisture from the hail.
Freezing fog is found when liquid fog freezes onto cold surfaces, and frozen fog or ice fog is when very small droplets freeze into midair crystal. Sea fog appears thanks to the spray and salt crystals in the air. Sea smoke or steam fog is a localized fog created as cold air passes over warmer water.

Fascinatingly, some plants and animals depend largely or entirely on fog as their moisture source. Californian Redwoods receive 30–40% from the fog coming in off the coast, while some animals and insect gather 100% of their water from fogginess. There are even communities of people who construct fog nets to gather the moisture in the air where other sources of water are scarce.

Probably the most famous fog is that which descends on San Francisco during its summer months. Amusingly, there is a Twitter account for the San Francisco fog, which has been named Karl. His most active period is in August (or ‘Fogust’).

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