What is fog and what causes the different types of fog?
The official definition of fog is a visibility of less than 1,000 m. This limit is appropriate for aviation purposes, but for the general public and motorists an upper limit of 200 m is more realistic. Severe disruption to transport occurs when the visibility falls below 50 m. Useful labels for these three categories are aviation fog, thick fog and dense fog.
Fog is caused by tiny water droplets suspended in the air. The thickest fogs tend to occur in industrial areas where there are many pollution particles on which water droplets can grow.
Fogs which are composed entirely or mainly of water droplets are generally classified according to the physical process which produces saturation or near-saturation of the air. The main types of fog are:
Radiation fog usually occurs in the winter, aided by clear skies and calm conditions. The cooling of land overnight by thermal radiation cools the air close to the surface. This reduces the ability of the air to hold moisture, allowing condensation and fog to occur. Radiation fogs usually dissipate soon after sunrise as the ground warms. An exception to this can be in high elevation areas where the sun has little influence in heating the surface.
Valley fog forms where cold dense air settles into the lower parts of a valley condensing and forming fog. It is often the result of a temperature inversion with warmer air passing above the valley. Valley fog is confined by local topography and can last for several days in calm conditions during the winter.
Advection fog occurs when moist air passes over a cool surface and is cooled. A common example of this is when a warm front passes over an area with snow cover. It is also common at sea when moist tropical air moves over cooler waters. If the wind blows in the right direction then sea fog can become transported over coastal land areas.
Upslope fog or hill fog forms when winds blow air up a slope (called orographic uplift). The air cools as it rises, allowing moisture in it to condense.
Evaporation fog is caused by cold air passing over warmer water or moist land. It often causes freezing fog, or sometimes Types of frost. When some of the relatively warm water evaporates into low air layers, it warms the air causing it to rise and mix with the cooler air that has passed over the surface. The warm, moist air cools as it mixes with the colder air, allowing condensation and fog to occur.
Evaporation fog can be one of the most localised forms of fog. It can happen when:
Freezing fog is composed of supercooled water droplets - which remain liquid even though the temperature is below freezing-point. One of the characteristics of freezing fog is that rime - composed of feathery ice crystals - is deposited on the windward side of vertical surfaces such as lamp posts, fence posts, overhead wires, pylons and transmitting masts.
Fog is closely related to visibility, forecasts for visibility are provided for 5000 locations across the UK.
The Met Office also provides severe weather warnings for dense fog, as part of the National Severe Weather Warning Service.
Last updated: 6 December 2013