Under fog ( Old High German nebul , Germanic * nebula m. Fog, darkness and represented in all Germanic languages, related for example with Latin nebula.. Mist, fog and Greek nephele. Cloud, clouds ) are understood in the meteorology part of the atmosphere , in which water droplets are finely distributed and which is in contact with the ground, the water droplets being created by condensation of the water in the humid and oversaturated air . From a technical point of view, fog is an aerosol , but in the meteorological system it becomes theHydrometeors counted.
Fog is therefore visible because light due to the Mie scattering scattered is causing the Tyndall effect occurs and the actually colorless droplets are visible. Fog is only spoken of when the visibility is less than one kilometer. Visibility of one to about four kilometers is considered to be haze . A fog in spatially very limited areas is referred to as a fog bank and a day on which a fog has occurred at least once is referred to as a fog day .
Fog and haze differ from clouds only in their contact with the ground, but are otherwise almost identical to them. In rising terrain, a layer of cloud can therefore turn into fog at higher altitudes. In aviation, such cases are referred to as overlying clouds .
With a visibility of 500 to 1000 meters one speaks of a light fog , of 200 to 500 meters of a moderate and of less than 200 meters of a strong fog. Laypeople usually only perceive a visibility of less than 300 meters as fog.
General conditions and properties
Fog is created in a mostly stable atmosphere , when water-saturated air reaches the dew point for various reasons . The differentiation of fog into certain types such as cooling, evaporation or mixing fog refers to these different causes and is presented in the section fog types .
The saturation amount of the air, i.e. the maximum amount of water vapor that the air can contain without condensation occurring, depends on many factors. They are discussed in the article Saturation Vapor Pressure. A drop in temperature or an increase in the absolute water content above the saturation concentration ideally results in immediate condensation , i.e. small water droplets are formed. The extent to which this condensation actually takes place immediately, or whether supersaturation occurs first, depends essentially on the condensation nuclei . The condensing water vapor can attach itself to them and thus changes into the liquid state much more easily than would be the case without condensation nuclei. The formation of mist droplets is therefore a heterogeneous nucleation , which is not possible without interfaces. In this way, a mixture of fog, smoke, soot and other particles can arise , especially with corresponding air pollution , and lead to an above-average fog density, which is referred to as smog . The surface properties of these particles, in particular their hygroscopicity, are also of particular importance .
Essential factors that determine the formation of fog are, on the one hand, the availability of water vapor and, on the other hand, a broad spectrum of factors such as aerosol particle concentration , temperature distribution, orography and, above all, the thermal surface properties of the respective terrain.
Droplet size and fog density
At a few hundredths of a millimeter, the droplet diameters within a nebula are much smaller than in a typical cloud, but due to the different condensation nuclei they also vary greatly between the individual droplets . Their size determines whether a fog is oozing or not. If it is slightly wet, it is a droplet size that does not exceed 10 to 20 μm on average, and 20 to 40 μm in the case of dense fog. In individual cases, droplet sizes of 100 μm were found, but this is an exception. Smaller droplet radii indicate maritime conditions, while larger radii indicate continental conditions. In fog, one cubic meter of air contains around 0.01 to 0.3 grams of condensed water in the form of droplets.
Because the mist droplets are very small in diameter, they have a very small Re number of less than 0.1. There is therefore a laminar flow when sinking. According to Stokes' law, the rate of descent increases with the square of the diameter. According to Hoerner, a droplet with a diameter of 20 µm has a sinking speed of 10 mm / s.
Place and appearance
Most of the fog arises in the winter half of the year near bodies of water , because during this time of year water evaporates due to solar radiation , but the air cools down so much in the evening that the water condenses again. The air also absorbs water through the sublimation of ice and snow. If there is a sudden ingress of cold air in summer or after rain, fog can also occur during this time, but this does not happen too often. Fog deposits on plants and other solid surfaces at temperatures above 0 ° C. If the temperature is below freezing point , frost forms .
Fog can occur in almost all climatic zones and, depending on its character, occur sporadically as well as regularly or long-lived or short-lived. The highest frequency of fog can be seen in humid areas and with large temperature fluctuations or strong cooling. This is particularly the case when cold and warm ocean currents meet and in areas of upwelling . However, the perceived frequency of fog is linked to the observation, which is why it often appears to be higher than the actual frequency of fog in the vicinity of settlements and which is why it is estimated to be more foggy without an empirical basis. The spatial scale ranges can also fluctuate strongly, so a fog can have a horizontal spread of a few hundred meters, but also sometimes hundreds of kilometers. The vertical spread varies from a few decimeters to several hundred meters.
Precipitation on surfaces
When there is no wind, the mist droplets sink and deposit on solid surfaces. With a slight air movement, the degree of separation is higher because the air movement is still much higher than the sinking speed of the droplets. The degree of separation is particularly high on large surfaces such as conifers or grain. Corresponding systems are used to extract water from fog.
The fog is a form of precipitation. There are droplets of different sizes in the fog. The larger droplets sink faster than the small ones. When a larger droplet meets a small one, it becomes an even larger one. This new droplet accelerates to the final speed of its size. Therefore, larger droplets hit the ground than corresponds to the distribution in height. The amount of precipitation is very low at up to 0.12 l / m² per hour.
Types of fog
In meteorology, nebulae are usually differentiated according to the conditions in which they arise, but this also means that many nebulae cannot be assigned to a certain type of fog based on their external appearance alone. There is also a large number of fog terms that are often very vaguely defined or at least very unclearly used, especially when these refer to the place or time of occurrence and not the cause of the fog. According to the cause, a distinction is essentially made between radiation, advection, evaporation, mixed and orographic nebula, as well as ice nebula, which is often viewed separately as a special form. In addition, there are also a number of other well-known terms such as morning fog, mountain fog or sea fog, which in many cases are difficult to match with specific conditions of origin and often lead to misunderstandings as to which name stands for which generic type of fog.
A distinction between ground fog and high fog is also possible, whereby the top of the ground fog according to the meteorological definition must be below the observer's eye level with a visibility of more than one kilometer. It is also possible to define the ground fog as a fog with ground contact, but this is redundant to the definition of a fog itself. The widespread understanding of a high fog as a fog with no ground contact is therefore also misleading, since it is usually a low cloud of the Stratus type, i.e. not fog in the actual sense. Only in the case of some intermediate stages of nebulae, which have been dissolved at their base or are about to lower to ground level, are also referred to as high fog in meteorology.
Radiation fog arising out of the nightly broadcast of the earth's surface and therefore occur mainly in autumn and winter at low-wind weather conditions , where they are mostly a radiation inversion are connected. Since they are based on a cooling of the air with constant or negligible fluctuations in the absolute humidity, they are also counted as cooling mists .
The layers of air close to the ground can cool down significantly, especially on cloudy nights. As a result, the water vapor condenses in the air and a weak, often multi-layered fog that hardly reaches a height of 100 meters with a comparatively small droplet size is formed. In the morning, this mist usually dissolves quickly, as the high specific surface of its droplets enables rapid evaporation due to the increased saturation vapor pressure . Only in winter is the sun's radiation sometimes not strong enough to break up the fog. The foggy, cloudy weather often lasts for days in the lowlands.
Radiation nebulae are very unstable structures and usually dissolve as quickly as they came. They mostly appear as early morning or morning fog , but their beginnings may well be in the late afternoon of the previous day. Whether or not a radiation fog is created is often a question of a few tenths of a degree Celsius. The frequency, density and thickness of this type of fog are therefore subject to great fluctuations. The predictability of the phenomenon is comparatively low when radiation fog is also so frequent that a daily rhythm can develop. The occurrence of a radiation fog is a signal for low temperatures, in particular with cold air inclusions in lowlands , for example in basins , typically cut-off fog carpets with sharp contours, which are then also referred to as valley fog or, if very pronounced, a sea of fog .
The bog fog also has a special shape, i.e. fog that occurs over bogs and therefore has its own name because the fog frequency is particularly high here. The cause is the very rapid cooling of the soil surface due to its high soil moisture and the resulting poor heat conduction properties, not increased evaporation due to the large amount of water available. A bog mist is therefore not an evaporation mist either, because the humidity is mostly dissipated here by winds before the mist is created. The fog itself, however, is linked to calm and rarely reaches thicknesses that exceed the visual range of an observer. This example shows the major role the soil heat balance plays in the formation of a radiation fog. The same effect can also be observed in a weaker form in meadows , which is why they are also referred to as meadow fog .
With an albedo of up to 0.90, fog generally exhibits an extraordinary ability to reflect incident sunlight. This is usually in sharp contrast to the surroundings with an albedo of typically around 0.2 to 0.3. The result is a tendency towards self-preservation, especially in the case of radiation nebulae, because the low temperatures that first led to its formation are further reduced or prevented from increasing by the now rapidly falling global radiation . The radiation of the water droplets themselves is particularly large, which results in a nighttime temperature minimum on the top of the fog.
With a stable stratification of the atmosphere on the ground and an inversion in height, i.e. a fumigation , more and more particles of various types accumulate at the inversion limit. This high-altitude haze, with its high albedo, can now not only have a fog-sustaining effect, but even create a fog. The fog, initially actually a cloud and sometimes referred to as high fog , gradually sinks from the height of the inversion to the ground and often lasts for days.
An advection fog or contact fog is another form of cooling fog that usually occurs in Central Europe in winter and is based on advection (approach) of air masses . The differentiation from the mixed mist can be difficult under certain circumstances, but here all mist forms that are decisively characterized by advection and partly also by mixing processes should be counted among the advection nebulae.
Advection mists are created by the fact that moist warm air flows from the south into the colder areas in the north, creating a layer of cold air close to the ground. The warm air is cooled down, which is why condensation and thus the formation of droplets occur. When a high pressure situation arises, this fog can last for days or weeks without being able to be dissolved by the sun. It only disappears when there is a further exchange of air, because it is not only the longest lasting form of fog, thicknesses of several hundred meters are not uncommon.
A special case of the advection fog is the coastal or sea fog . The water surfaces are usually significantly cooler than the land surfaces, especially in spring. If there is then an advection of the warm air masses located over the land, they cool down quickly over the water. The water droplets formed after reaching the dew point are then deposited as a thin layer of fog over the surface of the water, which is also referred to as cold water fog . In Germany, this type of nebula is found mainly in late spring on the Baltic Sea and is caused by the advection of warm air from southern Europe. This form of fog is particularly serious when it comes to sea winds during the day due to warming in the interior of the country . The fog, which is actually stored over the water, is then advanced to the coast and can reach several kilometers inland. Such an ingress of coastal fog is characterized by a considerable change in visibility and temperature conditions and also occurs extremely suddenly, so it can lead to considerable dangers, especially in road traffic. In addition, due to the comparatively small droplet sizes of the coastal fog, a considerable reduction in brightness is to be expected. The situation of very warm water temperatures and comparatively cold air, especially in autumn, leads to hot water mist , in which mixing processes usually dominate, which is why it is more likely to be assigned to mixed mist here.
Likewise, differently tempered ocean currents can lead to an advection fog, provided that the air flows from the warm to the cold water surface. This phenomenon, known as sea mist , can be seen in Newfoundland , for example , when the Labrador Current comes into contact with the Gulf Stream . The well-known Newfoundland Nebula is one of the most permanent and dense nebulae ever. In the Aleutian Islands , this type of fog also occurs more frequently due to the contact of the ocean currents Oyashio and Kuroshio .
Even in buoyant areas it often to mist formation, for example with the California Current , the Humboldt current or the Benguela current . Finally, a final form is an air flow directed in the direction of inland ice, mostly from the sea. Here, too, the air masses cool down and, for example, the Greenland Fog is created. To a lesser extent, this effect is also evident in less extreme contrasts, for example in the case of uneven snowmelt .
A mountain fog, or in its meteorologically exact name orographic fog, is formed when humid air rises on slopes with adiabatic cooling . It is therefore also counted among the cooling nebulae, but the cooling takes place here due to the lowering of the air pressure and not via the radiation or advection. This form of mist only occurs when the condensation level is below the summit or ridge. Stable orographic nebulae exist wherever an equally stable wind flow constantly leads air masses to a mountain range, whereby one can also speak of an advection nebula. This is especially the case in regions influenced by the trade winds , for example in the southern Andes or in Madagascar . They also occur in the Alps and German low mountain ranges , but then mostly only in individual weather conditions over short periods of time.
The formation of an orographic fog is in principle identical to a cloud created by uplift and one could therefore also speak of an uplifting fog. Elevation processes do not only occur at orographic obstacles, but only there does the surface of the earth rise with it and thus enable a consistently surface-bound condensation. Nevertheless, this definition is more general and in special cases a different kind of fog formation can occur. This is the case, for example, with small-scale convections as they play a role in mixed mists. Elevation processes when a warm front passes through can also cause short-term fog events.
In contrast to the previous fog forms, which were all associated with cooling, the evaporation fog is a type of fog that is caused by the increase in the absolute humidity and thus the dew point. This is achieved through increased evaporation , while the temperature of the air parcel remains constant or changes only insignificantly.
In nature, this occurs above all in warm autumn lakes, where one speaks of a steam mist (also river - or on the sea sea mist , or sea smoke or sea smoke ). Such a type of fog can also arise when humid air at moderate temperatures sweeps over a blanket of snow or frozen ground and, when heated, increases the evaporation rate. This special form is known as thaw fog .
The front fog is a special form , which predominantly forms as narrow strips of fog in front of a warm front or after a cold front , and more rarely directly when the front passes through. The first two types are caused by rain falling into colder air masses and partially evaporating in the process. However, the fog itself when the front passes through is more characterized by mixing or cooling processes, so it usually does not represent an evaporation fog.
Mixing processes play a role in many types of fog and can therefore not be clearly delimited in the classification chosen here. The mixture of two quantities of air is calculated using the Mollier hx diagram . Because the fog line is a curve, the mixture of two unsaturated amounts of air can also form fog. The basic principle is always the same: Air quantities with different moisture content and / or different temperatures mix and thus equalize their temperatures, which can lead to falling below the dew point under certain circumstances. However, such a shortfall usually occurs through a combination of the mixing effect with other processes, not through the mixing alone. Since the mixture itself is not associated with radiation, adiabatic cooling or additional evaporation, it still has to be considered as a separate aspect. It is essential that the air mixes very slowly and is a poor conductor of heat. This is also the reason why mixing processes are usually associated with advection or convection of air masses and almost always play a role here.
A mixed fog in the narrower sense occurs mainly on cool autumn nights over waters that are warmer than the surrounding area and which then seem to "steam". Its typical vortex-like shapes are created through a multi-stage process.
First of all, colder air penetrates the water from outside and warms up over it. This results in a reduction in the relative humidity, since warm air can absorb more water vapor than cold air. However, this also leads to an increase or at least a stabilization of the relative humidity again via evaporation. The meanwhile high temperatures of the air near the water surface are in contrast to the surrounding air located further up and not heated by the water surface, so there is an unstable stratification of the atmosphere .
Because of the convection that sets in , the air begins to rise. As a result, the two air layers are mixed, with their temperatures equalizing and the air that was originally close to the surface cools. The relative humidity rises rapidly and condensation occurs very quickly. Since the resulting water droplets are subject to strong movements in the air turbulence, the effect of sea or sea smoking arises for the observer . The heated layer of air is usually very thin and the effects can therefore only be observed up to a height of a few meters. This is shown over a very large area in warm ocean currents that extend into colder areas, for example the Gulf Stream on the Scandinavian coast.
The same effect can also be seen in other contexts, mostly with strong solar radiation and the associated high rate of evaporation following a rain shower. Here roofs, streets and the surface of the earth can form steam. A related effect is the lake effect snow .
When ice fog water droplets, but small float, unlike regular smoke ice crystals in the air. An ice fog occurs when water vapor in very cold air of usually below -20 ° C directly into ice crystals resublimated , that is, without going through the condensation to liquid water. The colder it is, the more frequently ice fog occurs, which is almost mandatory at temperatures below −45 ° C and the presence of a source of water vapor.
In nature, ice fog fields occur wherever its formation conditions meet, i.e. low temperatures on the one hand and a water supply on the other. Since the water quantities do not have to be particularly large for this due to the extremely low amount of saturation, anthropogenic emissions, volcanic activities or even herds of animals are possible in addition to open water surfaces. On a larger scale, ice fog can mainly be observed over the polar sea , but it is also quite common in the fjords of Norway and Alaska .
Ice fog is a special case because, as stated, it is not linked to condensation processes. They are therefore either assigned to the nebula as a special form or separated from the nebula as a separate form. Depending on which definition is used, it is therefore possible to include the existence of condensation processes in the definition of the term fog or not. Ice fog can be clearly differentiated from normal fog, since it is the only place where halos occur and the reduction in visibility does not normally lead to the sun being covered.
Strong turbulence usually dissolves the fog. However, they can also produce fog. This is the case when the turbulence transports the moist air from deep clouds to the ground. If the temperature increase downwards is not too great, the clouds can become turbulent fog.
Because air pollutants are more concentrated in the much smaller amount of water compared to acid rain, the pH value in the fog is much lower. The acids contained in the mist therefore have a much stronger effect when it falls on plants and objects. A pH of 2 was measured in a light mist (something like vinegar).
The observation of fog can relate to a large number of parameters and can also be carried out by a large number of methods, but is essentially aimed at the following variables: fog frequency, time and duration of occurrence, fog density and vertical and horizontal extent of the fog. These variables can be determined locally for a measuring station or regionally on the basis of several individual measurements.
As a measure of the fog density and thus the most important criterion of a fog, which also results in fog frequency and duration in the case of continuous observation, the observer's visual impairment when looking towards the azimuth is generally used . However, especially for regular measurements at airports and seaports, automated or electronic measuring methods are used, for example transmissiometers , ASOS (automated surface observing system) and remote sensing data . The latter can also record the extent of the nebula and include satellite , radar and lidar measurements. Satellite data in particular are becoming more and more important as the resolution improves. However, they are also not without problems, since a minimum of temperature differences is necessary in the infrared range and the fog in the visible range can be covered by clouds.
In synoptics, the following symbols, defined by the World Meteorological Organization , are used to code a foggy weather condition within a weather map . The assigned number key below the symbol applies to both the SYNOP code and the METAR .
For all symbols see: Weather map # More detailed weather maps and weather map symbols
|40||Fog at some distance, but has not reached the observer in the last hour. The altitude of the nebula is higher than that of the observer.|
|47||Sky obscured by fog or ice fog, which has become thicker in the last hour.|
|45||Sky obscured by fog or ice fog, with no changes in the last hour.|
|43||The sky is obscured by fog or ice fog, which has become thinner in the last hour.|
|46||Sky visible in spite of fog or ice fog, which became thicker in the last hour.|
|44||Sky visible despite fog or ice fog, with no changes in the last hour.|
|42||Sky visible in spite of fog or ice fog, although it got thinner in the last hour.|
|28||Fog cleared an hour ago.|
|12||Coherent layer of fog with an elevation of less than two meters at the weather station.|
|11||Fog with a height of less than two meters in individual swaths or banks at the weather station.|
|41||Fog or ice fog in swaths, therefore strongly fluctuating visibility.|
|48||Fog or ice fog with frost or clear ice with a visible sky.|
|49||Fog or ice fog with frost or clear ice formation when the sky is overcast.|
Importance and uses
In meteorology, the nebula has quite different meanings. Depending on its origin, it can be interpreted as an indication of a certain weather situation and is therefore an important aid in weather observation. Due to its high albedo , however, it also has a local influence on the radiation balance , which is important in connection with frost , for example .
Fog itself is not precipitation . However, there are different types of precipitation that are directly linked to fog.
A distinction is made between the fog eaves as liquid precipitation and solid precipitation in the form of frost or clear ice . They all belong to the group of intercepted precipitation , which cannot be measured adequately in terms of quantity. Under certain circumstances, this represents a significant problem in drawing up a detailed water balance .
More recently, techniques have been developed that make it possible to extract water from fog. For this purpose, nets or foils are stretched over a large area, on which fog accumulates and the fabric runs down. The yield of water per unit of area spanned is astonishingly high. In South America, there are coastal cities that have literally blossomed as a result of the development of this resource by plants on nearby mountain ranges. The natural precipitation in these regions is rather poor. The constant wind from the sea, which constantly supplies new air humidity, contributed to the success.
Cloud forests and fog deserts
Cloud forests are forests in which fog often occurs due to their location. This can be, for example, on the slopes of large mountains at altitudes over 2000 m in South America, where there are many epiphytes that haveaccess to water all year round,regardless of the rainy season . This includes many mosses, ferns and higher plants such as orchids . There are also some endemic animal species, such as the quetzal , Guatemala's national bird .
Other areas strongly influenced by fog are the fog deserts , especially the Namib . This desert is one of the driest places with an average rainfall of 20 mm per year. However, there is morning fog up to 100 km inland on up to 200 days a year and so plants and animals can be found here that can use the fog as a source of water. The best known are black beetles , which do a headstand on high dune ridges and thus collect condensation. The Welwitschia also benefits from the dew due to its extensive roots. The Atacama in South America is also a foggy desert and here, too, there are plant specialists such as some nettle plants, on whose thick hair coat mist condenses from the air and runs down the plant to the roots.
Movement in the fog
First of all, it is important to note the general lack of orientation that results in a very dense fog. This danger exists in general for any kind of movement in fog, but especially for hikers and mountaineers in open terrain. Due to the low speed, apart from a few special cases, there is no direct risk of overlooked obstacles and collisions, as is the case when using means of transport, but the often very low temperatures in combination with the fog leave one particularly in winter under other circumstances harmless disorientation can quickly turn into a life-threatening situation. In sparsely populated regions in particular, and particularly in mountains, moors and marshes , it is therefore advisable to remain in place when there is fog. Fog in the mudflats represents a particular danger . Here the disoriented mudflat hiker can quickly become a victim of the rising tide. Therefore, a mudflat hike should only be undertaken in stable, fog-free weather conditions and with an experienced guide.
In traffic meteorology, fog plays a role, above all on road traffic , by restricting visibility and the associated effect. Fog banks that appear suddenly are a frequent cause of car accidents and pile-ups , which is why the use of rear fog lights is advisable in thick fog . However, their use is prohibited when the visibility is greater than 50 m because they have a dazzling effect on the following traffic. Visibility ranges of 150 m and less mean that speeds are restricted on motorways. From 100 m you have to drive more slowly on country roads. If the visibility is less than 50 m, traffic is generally severely impaired. According to the rule of thumb, the recommended maximum speeds are 100, 80 and 50 km / h in the three cases. If the visibility continues to decrease, the speed must be adjusted or, in extreme cases, the vehicle must be stopped completely.
Fog continues to influence the driver's perception of speed, so that he has the impression that he is driving more slowly than is actually the case. If the speedometer is not used sufficiently, the result is too high a speed in relation to the range of vision. The dampness of the road caused by fog can also lead to dangerous situations, as the braking deceleration is reduced. In the case of serious accidents, all factors usually occur together, often with negative effects that are not dependent on the fog, such as fatigue, time pressure or the influence of alcohol, which increases the risk of accidents accordingly. For this reason, above all, a strong reduction in speed and thus the stopping distance is necessary, whereby this should be less than half of the visual range for safety.
In order to reduce the dangers, sensors have now also been developed and put into series production that can use radar to detect obstacles in the direction of travel, even through smoke screens. The speed of the journey should therefore not be increased, but there is a certain probability that the driver can already rely on being warned in good time so that he can come to a standstill without damage by braking or at least suffer a much gentler impact. In urban environments or in tunnels, however, these sensors only function to a limited extent, since there are generally too many interfering radio reflections.
Radar systems have proven their worth in ships , with fog leading to serious collisions several times in the past. This meant that shipping traffic in particular often came to a complete standstill. However, the risk of collision caused by fog still exists, especially with ships and aircraft without the appropriate technology.
Fog near the ground poses a particular danger for pleasure craft. While small pleasure craft are shrouded in fog, large ships sometimes protrude above the layer of fog, do not perceive it as a major hindrance and therefore travel at undiminished speed. Together with the low radar echo of sport boats, this results in a considerable risk, especially in busy fairways (e.g. Great Belt ).
A major catastrophe, arguably the main cause of fog, was the collision of the ships Andrea Doria and Stockholm in 1956, in which 52 people died. In addition to these rather rare events, however, the focus is on the economic damage, since the cessation or at least slowing down of shipping traffic results in considerable financial burdens. In order to enable the safest possible navigation even when visibility is limited, fog horns and fog buckets are used as navigation signs . The latter include Howl Onnen and Gong tons and formerly bells tons . Important coastal shipping routes are also monitored by land-based radar systems.
Aviation has been relying heavily on radar since the second quarter of the 20th century and was therefore able to provide pilots with assistance from the ground at a very early stage. In the meantime, the number of radar units for controlling fog, clouds and other effects that obstruct the view in today's long-haul aircraft has reached the limit of two digits. However, the majority of small aircraft continue to fly purely according to visual flight conditions and must therefore continue to take fog into account.
In air traffic, fog makes air traffic itself less difficult today, but it has significant consequences if it falls below a visibility of around 1.2 kilometers at airfields. Starts are possible in principle, but the risk is greater. Although it is possible at all commercial airports to bring an aircraft to the ground using the instrument landing system, the pilot reserves the right to visually check the lights and runway as a last resort . If heavy fog occurs, this can lead to the temporary failure of the entire airport. Aircraft have to be rerouted and, if necessary, inevitable landings, for example due to a lack of fuel, are then risked. The Franz-Josef-Strauss Airport in Erdinger Moos , a former moor and potential fog area near Munich, is an example of an airfield that is more frequently affected by fog. Even if the actual downtimes are still tolerably short, if only due to the size of the system, this shows the importance of the structural planning of such fog-sensitive systems.
Space travel and miscellaneous
Fog can also be important in similar situations, for example in military operations, rescue missions or for the operation of a spaceport . For example, the start of shuttle missions from Cap Canaveral often had to be postponed due to fog. The landing in Normandy in 1944 and the deployment of UN troops in Tuzla in 1995 were also delayed by foggy weather.
Environmental pollution and the spread of pollutants
Fog is made up of water droplets, but it is by no means pure water. A large number of substances can be dissolved in it, for which the mist or its droplets represent a diffusion medium. Fog combined with heavy air pollution played an important role in smog disasters such as 1930 in Belgium, 1948 in Pennsylvania and especially in 1952 in London . However, it also poses a problem regardless of such extreme events, for example in combination with oil and forest fires. Often these pollutants provide the condensation nuclei on which the fog can form, which is the main reason why London was particularly foggy in the past. The transition to dry haze is fluid in many cases.
Artificial fog control
The fog, which is undesirable in terms of traffic safety but also in relation to open-air events, is removed in special cases by technical and chemical processes, provided that the effort required for this appears proportionate. Although the processes are diverse, they are mostly characterized by high costs and low effectiveness. The removal of fog is therefore generally a very complex and expensive undertaking, which is why it is only used in special cases and is also becoming increasingly rare in these.
A process that is rarely used today is runway heating, i.e. the simple warming of the runways of an airport in order to dissolve the fog through the then higher temperatures of the air near the ground. This is only promising if the fog is not as thick and at the same time as low wind speeds, but is rarely used today due to the high energy costs. Another possibility goes exactly the opposite way, by trying to increase the droplet sizes within the mist so that it rains out. For this purpose, liquid or solid propane or carbon dioxide are used , which, through their heat of evaporation, cause a reduction in air temperatures and thus increased condensation or resublimation. Again, this is only possible at temperatures below about 0 ° C with reasonable effort. Another possibility is to mix the layers of air and thereby dissolve the inversion, which is usually done using a helicopter. However, their range of action is very small and the effectiveness of such a method is therefore usually limited to short periods of time.
Basically, it has been shown that measures to eliminate fog can only be successful to a limited extent and only make sense where extremely high costs or dangers arise from fog. A comprehensive removal of fog on roads or even just highways is therefore disproportionate from the outset. In addition to an adapted driving style on the part of the road users, only correspondingly sensitive traffic route planning and long-term water management measures can help, i.e. avoiding fog. Therefore, properties characterized by inclusions of cold air, such as sinks, are not very suitable for fog-sensitive systems, as are locations with frequent advection mists.
Fog can be created artificially either through targeted oversaturation of air with water or directly through a fine spray of liquid (spray mist). Most artificially created fog, or better still, haze, is a by-product with a short survival time. Especially in winter, the mostly warm exhaust gases from factories and vehicles lead to the formation of small amounts of fog, which, however, usually evaporate again very quickly. On the other hand, if you want to intentionally create fog , you usually use fog machines . Such artificial fog is generated in various ways in theater and event technology. Depending on the desired nature of the fog, different techniques and machines are used:
- fine haze to make the cones of light from headlights more visible, so-called haze ;
- dense effect fog, mostly used selectively and for a short time;
- Ground fog for a dense carpet of fog close to the ground.
A scientific application of artificial fog is the cloud chamber . This takes advantage of the fact that ionizing radiation forms condensation nuclei on which a particularly large number of water droplets form. Fast-flying particles create a strip of dense fog along their trajectory. The particles are deflected differently by a magnetic field so that they can be identified by their trajectory.
During the Second World War , so-called mist acids were used to create artificial fog to protect factories or military installations against air raids. They also served as warfare agents with a fog effect .
In the military there are so-called 'smoke candles'; these are smoke grenades that produce a strong smoke or fog effect.
Fog is also important in winemaking . For example, the warmer Ciron river flows southeast of Bordeaux into the cooler Garonne , creating a fog in October and November that promotes the growth of the Botrytis cinerea fungus . This punctures the grapes' berry skins, causing water to escape from them and thus concentrate their sweetness. This is important for the wines from Sauternes , the most prominent representative of which is the wine from Château d'Yquem .
Fog also reduces the climate by increasing the heat capacity of the air, which can protect grapes from freezing. There is even a real heat transport from river plains to the exposed heights of the vineyards. In the presence of fog, a great deal of energy is released through condensation and frost formation from fog, so that the temperatures inside the grapes remain above or at the zero degree limit at which water freezes for longer.
Classification in the scheme of chemical substances
|Schematic classification of the substances|
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