Fog

The World Meteorological Organization distinguishes fog from mist by visibility: fog is used when microscopic water droplets reduce horizontal visibility at Earth's surface to less than 1 kilometre. The Met Office describes fog as a cloud at ground level. If the droplets are ice rather than liquid water, the phenomenon is known as ice fog.

Air can become saturated when it cools, or when moisture is added and mixes into cooler air. Tiny suspended droplets then scatter light and limit visibility. The circumstances producing saturation distinguish several common kinds of fog; fog is not confined to one temperature or one weather pattern.

Radiation fog develops as the ground loses heat, usually overnight under clear skies and light winds, cooling moist air near the surface. It often occurs in sheltered valleys and may dissipate after sunrise. Advection fog forms when warm, moist air moves over a colder surface and is cooled to saturation; it can occur over land or water and may be carried by wind. Evaporation or mixing fog can form when water vapour from a warmer water surface enters colder air and mixes with it, as in steam fog. Upslope fog forms when air is forced up rising terrain and cools. Freezing fog consists of supercooled liquid droplets that freeze on contact with exposed surfaces, leaving ice deposits.

Fog and mist both involve suspended water droplets; the standard distinction is that visibility is below 1 kilometre in fog and at least 1 kilometre in mist. Haze differs because it consists mainly of very small dry particles rather than water droplets. Forecast services may use narrower visibility thresholds for public warnings or operational categories, so local definitions can vary by context.

Reduced visibility can make road, air and marine travel hazardous. Drivers entering dense fog should slow down and use low-beam headlights, as advised by the U.S. National Weather Service. Fog may also deposit liquid water or ice on surfaces; freezing fog can coat objects with ice.