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Fog, in essence, is a cloud touching or near to the Earth's surface that reduces visibility to less than 1km (0.6 mile). A fog is a suspension of tiny water (sometimes ice) particles in the air. Fogs are a result of the air's humidity being high enough that condensation occurs around suitable nuclei; and are found most often near coasts and large inland bodies of water. A light fog is called a mist or haze. In industrial areas, fog and smoke may mix to give smog.

There are four main types of fog: advection fog develops from air flowing over a surface of different temperature, such as steam fog that results from cold air passing warm water; frontal fog forms when warm rain falls through cold air near the ground; radiation fog occurs when the ground cools on a clear, still night, and is most common in valleys; upslope fog when air cools as it ascends a slope.

By standard definition, visibility in a mist is 2,000–1,000m (6,520–3,280ft). When visibility drops below 1,000m, the mist is referred to as a fog.

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