Fog, in essence, is a cloud touching or near to Earth's surface that reduces visibility to less than 1 kilometer (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 2,000–1,000 meters (6,520–3,280 feet). When visibility drops below 1,000 m, the mist is referred to as a fog.