Fog appears when water vapour (water in its gaseous form) condenses. Mist is a phenomenon caused by small droplets of water suspended in the cold air.
Fog
Fog is a visible aerosol made up of microscopic water droplets or ice crystals hanging in the air near or at the surface of the Earth. Fog is a low-lying cloud that looks a lot like a stratus and is greatly impacted by neighbouring bodies of water, geography, and wind conditions. Fog, in turn, has an impact on a variety of human activities, including shipping, travel, and warfare.
When water vapour (water in its gaseous state) condenses, fog forms. Water vapour molecules unite during condensation to form small liquid water droplets that float in the air. Sea fog is generated when water vapour condenses on salt particles near bodies of saline water. Fog is comparable to mist, however, it is less translucent.
Formation
Fog arises when the temperature difference between the air and the dew point is less than 2.5 degrees Celsius (4.5 degrees Fahrenheit). When water vapour condenses into small water droplets hanging in the air, fog is formed. Wind convergence into upward motion areas; precipitation or virga falling from above; daytime heating evaporating water from the surface of oceans, water bodies, or wetlands; transpiration from plants; cool or dry air moving over warmer water; and lifting air over mountains are some examples of how water vapour is added to the air. To generate clouds, water vapour condenses on condensation nuclei such as dust, ice, and salt. Fog is a stable cloud deck that forms when a cool, stable air mass is confined beneath a warm air mass, similar to its elevated cousin stratus.
Fog is most common when the relative humidity is near 100 per cent. This is caused by either an increase in air moisture or a decrease in ambient air temperature. Fog, on the other hand, can form at lower humidity levels and can sometimes fail to form at 100% relative humidity. Because the air cannot store any more moisture at 100 per cent relative humidity, it will become supersaturated if further moisture is supplied.
Types
Fog can form in a variety of ways, depending on how the condensation was created by cooling. In calm conditions with a clear sky, radiation fog is created by the cooling of land by infrared thermal radiation after sunset. By conduction, the cooling earth cools neighbouring air, causing the air temperature to fall and meet the dew point, resulting in fog. The fog layer can be less than a metre thick under perfect calm, although turbulence can promote a thicker layer.
The fog that covers less than 60% of the sky and does not reach the base of any overhead clouds is referred to as ground fog. However, the word is often used interchangeably with shallow radiation fog; in some situations, the fog can reach tens of centimetres in depth over specific types of terrain in the absence of wind.
Advection fog develops when moist air is cooled as it travels over a cool surface by advection (wind). It happens frequently when a warm front passes through an area with a lot of snow. It occurs most frequently at sea, where moist air collides with cooler waters, especially areas of cold water upwelling, such as off the coast of California (see San Francisco fog). Advection fog can also be caused by a significant temperature differential over water or bare ground.
Raindrops falling from relatively warm air above a frontal surface evaporate into cooler air close to the Earth’s surface, causing it to become saturated, similar to how stratus cloud forms near a front. In the absence of any lifting agent, this sort of fog might emerge from a very low frontal stratus cloud sinking to surface level after the front passes.
Ice fog arises at very low temperatures and can be caused by the various mechanisms outlined below, as well as herds of animals exhaling wet warm air. It’s linked to the diamond dust kind of precipitation, in which every minute ice crystals form and fall slowly. This is common in blue sky circumstances, which can result in a variety of halos and other effects from sunlight refraction by airborne crystals.
Freezing fog, which forms rime, is made up of supercooled water droplets that freeze on surfaces when they come into contact. As precipitation falls into drier air behind the cloud, the liquid droplets evaporate into water vapour, forming precipitation fog (or frontal fog). The water vapour cools, condenses at the dewpoint, and fog forms.
Hail fog can form in the area of large hail accumulations when the temperature drops and the moisture rises, causing saturation in a relatively shallow layer near the surface. It usually happens when there’s a warm, humid layer on top of the hail and the wind isn’t too strong. This ground fog is usually limited, but it can be intense and abrupt.
Freezing conditions
When liquid fog droplets freeze to surfaces, forming a white soft or hard rime, this is known as freezing fog. This is quite typical on mountain tops where low clouds are present. It’s similar to freezing rain, and it’s practically the same as the ice that forms inside a freezer that isn’t “frostless” or “free.” The term “freezing fog” can also apply to a fog in which water vapour has been super-cooled, causing little ice crystals to form in the air, akin to very light snow. It gives the fog a “tangible” quality as if one could “grab a handful” of it.
Topographical influences
Up-slope fog or hill fog forms when winds blow air up a slope (called orographic lift), adiabatically cooling it as it rises, and causing the moisture in it to condense. This often causes freezing fog on mountaintops, where the cloud ceiling would not otherwise be low enough.
Valley fog forms in mountain valleys, often during winter. It is essentially a radiation fog confined by local topography and can last for several days in calm conditions. In California’s Central Valley, valley fog is often referred to as tule fog.
Visibility effects
According to the number of droplets in the air, visibility in fog can range from the appearance of haze to a virtually complete absence of visibility. Every year, thousands of lives are lost in traffic accidents involving fog conditions on highways, including multiple-vehicle crashes, all over the world.
Artificial fog
Artificial fog is a man-made fog that is usually created by vaporizing water- and glycol- or glycerine-based fluid. The fluid is injected into a heated metal block and evaporates quickly. The resulting pressure forces the vapour out of a vent. Upon coming into contact with cool outside air, the vapour condenses into microscopic droplets and appears as fog. Such fog machines are primarily used for entertainment applications.
Historical references
The presence of fog has often played a key role in historical events, such as strategic battles. One example is the Battle of Long Island (27 August 1776), when American general George Washington and his command were able to evade imminent capture by the British Army, using fog to conceal their escape. Another example is D-Day (6 June 1944) during World War II, when the Allies landed on the beaches of Normandy, France during fog conditions. Both positive and negative results were reported from both sides during that battle, due to impaired visibility.
Mist
Mist is a phenomenon caused by small droplets of water suspended in the cold air. Physically, it is an example of dispersion. It is most commonly seen where warm, moist air meets sudden cooling, such as in exhaled air in the winter, or when throwing water onto the hot stove of a sauna. It can be created artificially with aerosol canisters if the humidity and temperature conditions are right. It can also occur as part of natural weather, when humid air cools rapidly, notably when the air comes into contact with surfaces that are much cooler than the air.
Description
Cloud cover is often referred to as “mist” when encountered on mountains, whereas moisture suspended above a body of water or marsh area is usually called “fog”. One difference between mist and fog is visibility. The phenomenon is called fog if the visibility is 1 km (1,100 yds) or less. In the United Kingdom, the definition of fog is visibility less than 100 m (330 ft) for driving purposes, UK Highway Code rule 226, while for pilots the distance is 1 km. Otherwise, it is known as mist.
Snow
Freezing mist is similar to freezing fog, only the density is less and the visibility greater (when fog falls below 0 °C (32 °F), it is known as freezing fog).
Conclusion
When microscopic droplets restrict horizontal visibility to less than 1 km at the Earth’s surface, the term “fog” is used, while “mist” is used when the droplets do not reduce horizontal visibility to less than 1 km. In general, the mist is used equally with “light fog.” The term “smog” (derived from the words “smoke” and “fog”) is commonly applied to situations including fog and heavy air pollution, as well as chemical reactions between fog droplets and other contaminants.
The number density and size distribution of the droplets, as well as the structure of the fog, all influence visibility reduction. In terms of time and space, this structure might change a lot. In “fog,” the air is typically damp, moist, or wet. Individual fog droplets can often be seen with the naked eye when lighted, and they appear to be moving in a turbulent manner.