Fog is an accumulation of water droplets or ice crystals suspended in the ground layer of air, impairing horizontal visibility to 1000 m or less. In essence, it is a cloud lying on the surface of the earth or water.
Based on the main physical reasons, causing the formation of fogs, they can be divided into two main classes: cooling fogs and evaporation fogs. The first of these classes absolutely predominates.
Cooling fogs are formed by condensation or water vapor when air cools from the earth's or water's surface. They are divided into two types: radiative and advective.
Radiation fogs appear in conditions of stagnant air due to its nighttime cooling from the cooling underlying surface. Observed on clear, quiet nights and in the morning before sunrise. As soon as the sun's rays begin to warm up, these fogs quickly disappear.
Advective fogs are associated with the cooling of warm air as it moves to a cold surface (the same process is associated with the formation of low stratus clouds, which can turn into fog as they descend). These are the most powerful and lasting fogs. They are observed at any time of the day and can be transported over long distances.
Evaporation fogs (vapor) are formed due to the evaporation of water vapor from an underlying surface warmer than the air. Such fogs are most intense in winter over non-freezing bays and polynyas; over rivers and lakes they appear more often in autumn, and on land - after heavy rains in the evening and at night in the summer - autumn period.
Most common feature distribution of fogs over the earth's surface is an increase in their frequency high latitudes. Depending on the conditions of formation, fogs in Russia have different continuous durations. In continental areas, short-term fogs prevail, lasting no more than 2–4 hours, and on the coasts of the northern seas they can not stop for several days.
In the European part of Russia, the greatest number of days with fog is observed on the sea coast, in high mountain areas, on the slopes of hills facing moisture-carrying streams. Particular attention should be paid to (), where the frequency of fogs is extremely high throughout the year. In some places, the average annual number of days with fog can reach 230–280, and their duration per year is 2050 hours. One fog lasts on average 9 hours.
Under monsoon circulation conditions in the Far East a large number of fogs are observed in the warm period of the year. On and on individual years the number of days with fogs can exceed 160–180 per year with a duration of 1000–1400 hours.
The annual cycle depends on geographical conditions: over continents, fogs most often form in autumn, over seas and oceans - in spring, when the water surface is coldest.
Fogs with visibility of 500–200 meters are already seriously impeding traffic. Fogs with visibility less than 50 m and duration of 12 hours or more are classified as special fogs. hazardous phenomena weather conditions and can completely paralyze the work of air and sea ports and ground transport. The average continuous duration of fog with visibility of 500 m or less is usually 2–4 hours, but in some cases they can persist for more than a day.
In large industrial centers, due to the large number of active condensation nuclei emitted by industrial enterprises, fogs can form even in unsaturated air and are observed 1.5–2 times more often than in the surrounding area. In Yakutsk, for example, the duration of fogs in the city center is 1300 hours, and on the outskirts (near the airport) - 475 hours.
In large cities of the North-West, the number of days with visibility less than 500 m can vary from 6 to 65. The longest fogs with such visibility are observed here in the autumn months. Their continuous duration is on average 3 hours. The maximum duration of fogs per year in some cities is close to 200–300 hours.
In the cities of Western Siberia, fogs with visibility less than 500 m are observed in 50–70% of cases from total number, and in the Cis-Baikal region - about 10%.
Large populated areas of the Republic of Sakha (Yakutia) are characterized by frosty fogs that form at temperatures of –42°C and below, in which visibility during the day decreases to 40–50 meters. The most dangerous fogs form in December – January.
In coastal cities Far East, Primorye, Kamchatka Peninsula, Sakhalin Island, summer fogs are the most intense.
Finding yourself in the middle of a continuous white cloud, so dense that it is practically impossible to distinguish anything at arm's length, you often ask yourself the question: why such a thick fog formed, why did it white and you begin to wonder how long this phenomenon usually lasts, and also why any fog dissipates.
Fogs are formed when drops or ice crystals accumulate in the air in the lower layers of the atmosphere, due to which a cloud-like veil is formed along the earth's surface, limiting visibility so much that space beyond one kilometer is not visible, and in some cases objects become difficult to distinguish even at a distance several meters.
If the temperature environment exceeds -10°C, the vapor veil consists only of droplets. If the temperature fluctuates from -10 to -15°C, it is made up of water droplets and ice crystals, and when it is -15°C outside, the fog consists of small ice crystals, shimmering in the light of night lamps.
Why this phenomenon occurs is not difficult to answer: its appearance is due either to the evaporation of water from a warm surface into cold air, or to the cooling of warm air currents saturated with moisture. For example, the appearance of ground clouds can often be observed in the evening or in the morning after the temperature of the soil and vegetation (grass) drops; the lower layers of the atmosphere cool so much that they begin to release excess moisture in the form of water droplets.
Another example, this time in winter, is fog over a river, lake or other body of water, on the ice of which an ice hole has formed: in cold weather there is always a veil over it, spreading over the water surface. This happens because the temperature of the water during frost is warmer than the ice surrounding it and the air in contact with it (because of this, the air above the water is always warmer than the rest and there is almost always fog over the river in the area of the ice hole).
After warm air mixes with cold air currents, it begins to cool, releasing steam and forming a cloud at the very surface of the Earth. Therefore, the fog over the river and other bodies of water is usually stable and long-lasting: cold and warm air currents and currents constantly mix here.
A striking example of this phenomenon is considered to be located in Atlantic Ocean Canadian island of Newfoundland. Due to the fact that two currents collide with each other here – the warm Gulf Stream and the cold Labrador Stream, local residents are forced to spend about one hundred and twenty foggy days a year among the haze.
Formation of terrestrial clouds
When air saturated with water vapor cools or mixes with colder air currents, droplets begin to be released into the atmosphere. After this, if there are tiny particles of dust above the earth’s surface, they begin to stick to them, layering on top of each other and forming larger droplets (the more dust in the air, the faster a cloud forms, so big cities almost always shrouded in a weak, almost imperceptible veil).
IN warm time year, the size of such a drop ranges from 5 to 15 microns, during frosts - from 2 to 5 microns, so the winter cold fog not as thick as summer. As soon as the drops reach the required volumes, objects turn out to be blurry and difficult to distinguish: the air becomes whitish in heavy fog and bluish in light fog.
The answer to the question of why this phenomenon comes in different colors is simple: smaller droplets scatter short blue rays better, while in dense ground clouds larger droplets and light waves scatter all rays equally, regardless of their length.
The water content of such clouds usually does not exceed 0.5 g/m3, but sometimes dense fog can contain up to 1.5 g/m3 (this water is enough for plants to receive the necessary moisture, this is especially important for vegetation in the arid regions of the planet). How impenetrable the shroud will be depends largely on the humidity of the air, which is usually between 85 and 100% during the occurrence of ground clouds:
- if visibility does not exceed 50 meters, thick fog is observed, and the number of drops is 1200 per cubic centimeter;
- if the space is visible at a distance of 50 to 500 meters - moderate (water drops in this case from 100 to 600);
- if visibility is a kilometer - weak (drops - from 50 to 100).
Fogs are also common during frosts, and the phenomenon can be seen even when the humidity does not exceed fifty percent. They can usually be observed in cities, especially at railway and bus stations, where the haze is formed by steam that appears during the combustion of fuel and is released into the air through chimneys and exhaust pipes.
Kinds
Terrestrial clouds do not always owe their origin only to nature: big number fogs occur in cities, and therefore they consist not only of drops and dust, but also smoke, soot, which are emitted by factory or chimneys, or occur after or during fires when forests, peat or steppe burn. Based on their origin, meteorologists divide fogs into dry (smoke, soot, etc. are to blame for their formation) and wet (only water and dust are involved), and often the second form flows into the first.
In turn, wet fogs, the formation of which is directly influenced by nature - this is evening, night or morning fog (this period is optimal for the formation of clouds creeping along the ground), meteorologists are also divided into groups:
- Underground. Evening or morning fog that spreads low over the earth's surface or body of water (for example, fog over a river). The shroud may be continuous, or it may be in separate wisps, and visibility will not exceed a kilometer.
- Translucent. Despite the fact that visibility along the surface is low and in some cases does not exceed several meters, clouds can be clearly distinguished in the sky. This type includes night, evening, and morning fog.
- Solid. Visibility of dense fog is very limited and often does not exceed fifty meters. The sky is almost invisible, so it is almost impossible to distinguish clouds. This is mainly evening, night and morning fog, and during cold weather when temperatures rise, cold fog can be seen during the day.
Why do fogs disappear?
The duration of this phenomenon varies and can range from half an hour to several days (especially during cold weather or when warm and cold air and water currents collide, for example, fog over a river). The main reason why any fog dissipates is the air warming up. Since the veil forms near the surface, after the sun's rays warm it up, the air also heats up, as a result of which the droplets evaporate and turn into steam.
The higher above the earth's surface, the weaker the fog dissipates, since in upper layers atmosphere, the air temperature begins to drop again, the steam transforms into water droplets and forms clouds.
Fog is a cloud near the surface of the earth. There is no difference between fog and cloud in the sky. When a cloud is near the surface of the earth or sea, we call it "fog".
Fog usually forms at night and early in the morning in lowlands and over water bodies. It is associated with a cold air flow that descends to warm surfaces sushi or water.
Fogs are more common in autumn, when the air cools faster than the ground or water. In calm weather, with the onset of darkness, thin layers of fog form in low places above the ground. As the ground cools at night, the lower layers of air also become colder. When such cool air comes into contact with warm air, fog is formed.
As a rule, urban fogs are denser than rural ones. City air is full of dust and soot, which, when combined with water particles, form a dense blanket.
The foggiest area on Earth is the Atlantic coast of Newfoundland (Canada), where fogs are formed when moist, warm air passes over cold waters that move south from the Arctic Circle. The coldness of the water condenses the moisture in the air into small droplets of water. These droplets are not large enough to form rain. They are in the air in the form of fog.
But fogs in the San Francisco area form completely differently. Here, the cool morning breeze blows towards the warm sand dunes, and if rain the day before has soaked the sand, a dense layer of fog forms from the evaporating moisture.
Often the fog appears denser than the clouds. This is because the mist droplets are smaller in size.
Many tiny droplets absorb more light than the larger (but smaller) droplets that form a cloud. So it seems to us that the fogs are thicker than the clouds.
Similar articles
Fog is formed as a result of collision of air flows with different temperature. It is created when heated air comes over cold water or meets with a mass of more humid and cold air.Fog - atmospheric phenomenon, the accumulation of water in the air when tiny condensation products of water vapor are formed (at air temperatures above −10° these are tiny droplets of water, at −10…-15° they are a mixture of water droplets and ice crystals, at temperatures below −15° they are ice crystals sparkling in sun rays or in the light of the moon and lanterns).
Relative air humidity during fogs is usually close to 100% (at least exceeds 85-90%). However, in severe frosts (-30° and below) in populated areas, at railway stations and airfields, fog can be observed at any time. relative humidity air (even less than 50%) - due to the condensation of water vapor formed during the combustion of fuel (in engines, stoves, etc.) and released into the atmosphere through exhaust pipes and chimneys.
The continuous duration of fogs usually ranges from several hours (and sometimes half an hour to an hour) to several days, especially in the cold season.
At weather stations it is noted the following types fog:
- Ground fog is fog that spreads low over the earth's surface (or body of water) in a continuous thin layer or in the form of separate wisps, so that in the fog layer the horizontal visibility is less than 1000 m, and at a level of 2 m it exceeds 1000 m (usually the same as for haze, from 1 to 9 km, and sometimes 10 km or more). It is observed, as a rule, in the evening, night and morning hours. Separately, ground ice fog is noted - observed at air temperatures below −10 ... -15 ° and consisting of ice crystals sparkling in the sun's rays or in the light of the moon and lanterns.
- Translucent fog - fog with horizontal visibility at a level of 2 m less than 1000 m (usually it is several hundred meters, and in some cases it drops even to several tens of meters), poorly developed vertically, so that it is possible to determine the state of the sky (the number and shape of clouds ). It is most often observed in the evening, at night and in the morning, but can also be observed during the day, especially in the cold half of the year when the air temperature rises. Separately, translucent ice fog is noted - observed at air temperatures below −10 ... -15 ° and consisting of ice crystals sparkling in the sun's rays or in the light of the moon and lanterns.
- Fog is a continuous fog with horizontal visibility at a level of 2 m less than 1000 m (usually it is several hundred meters, and in some cases it drops even to several tens of meters), sufficiently developed vertically so that it is impossible to determine the state of the sky (the number and shape of clouds ). It is most often observed in the evening, at night and in the morning, but can also be observed during the day, especially in the cold half of the year when the air temperature rises. Separately, ice fog is noted - observed at air temperatures below −10 ... -15 ° and consisting of ice crystals sparkling in the sun's rays or in the light of the moon and lanterns.
The highest number of foggy days at sea level - an average of more than 120 per year - is observed on the Canadian island of Newfoundland in the Atlantic Ocean.
Average annual number of days with fog in some Russian cities:
| Arkhangelsk | 31 | Astrakhan | 36 | Vladivostok | 116 | Voronezh | 32 | Ekaterinburg | 12 |
| Murmansk | 24 | Naryan-Mar | 40 | Omsk | 27 | Orenburg | 22 | Petropavlovsk-Kamchatsky | 94 |
| Syktyvkar | 21 | Tomsk | 19 | Khabarovsk | 16 | Khanty-Mansiysk | 15 | Yuzhno-Kurilsk | 118 |
| Irkutsk | 52 | Kazan | 16 | Moscow | 9 | Saint Petersburg | 13 | ||
| Rostov-on-Don | 36 | Samara | 41 |
Pier in the fog. Vancouver Island, Sydney
Mountain road in fog (D81 highway in Corsica)
Fogs prevent the normal operation of all types of transport (especially aviation), so fog forecasts are of great economic importance.
Artificial fog creation is used for scientific research, in the chemical industry, heating engineering and other fields.
Classification
Sea fog in the Oresund Strait
Country road in the fog (Moscow region, Naro-Fominsk)
Fog in San Francisco (Golden Gate)
Fog on the Volga near Nizhny Novgorod
According to the method of occurrence, fogs are divided into two types:
- Cooling fogs are formed due to the condensation of water vapor when the air is cooled below the dew point.
- Evaporation fogs are evaporation from a warmer evaporating surface into cold air over bodies of water and wet land areas.
In addition, fogs differ in the synoptic conditions of formation:
- Intramass - formed in homogeneous air masses.
- Frontal - formed at the boundaries atmospheric fronts.
Haze is a very faint fog. In haze, the visibility range is several kilometers. In the practice of meteorological forecasting, it is considered: haze - visibility more/equal to 1000 m, but less than 10 km, and fog - visibility less than 1000 m. Heavy fog considered when visibility is less than or equal to 500 m.
Intramass fogs
Intramass fogs predominate in nature; as a rule, they are cooling fogs. They are also usually divided into several types:
- Radiation fogs - fogs that appear as a result of radiation cooling of the earth's surface and the mass of moisture surface air to the dew point. Typically, radiation fog occurs at night in anticyclone conditions with cloudless weather and a light breeze. Radiation fog often occurs in conditions temperature inversion, preventing the rise air mass. After sunrise, radiation fogs usually dissipate quickly. However, in the cold season, in stable anticyclones they can persist during the day, sometimes for many days in a row. An extreme form of radiation fog, smog, can occur in industrial areas.
- Advective fogs - formed due to cooling of warm humid air when it moves over a colder surface of land or water. Their intensity depends on the temperature difference between the air and the underlying surface and on the moisture content of the air. These fogs can develop both over the sea and over land and cover vast areas, in some cases up to hundreds of thousands of km². Advective fogs usually occur in cloudy weather and most often in the warm sectors of cyclones. Advection fogs are more persistent than radiation fogs and often do not dissipate during the day.
Sea fog is advective fog that arises over the sea during the transfer of cold air to warm water. This fog is evaporation fog. Fogs of this type are frequent, for example, in the Arctic, when air flows from the ice cover to the open surface of the sea.
Frontal fogs
Frontal fogs form near atmospheric fronts and move with them. Air saturation with water vapor occurs due to the evaporation of precipitation falling in the front zone. The decline observed here plays some role in the intensification of fogs ahead of fronts. atmospheric pressure, which creates a slight pdiabatic decrease in air temperature.
Dry fogs
To the fogs in colloquial speech and in fiction sometimes referred to as so-called dry fogs (haze, haze) - a significant deterioration in visibility due to smoke from forest, peat or steppe fires, or due to loess dust or part of the sand, raised and carried by the wind sometimes over considerable distances, as well as due to emissions from industrial enterprises .
The transitional stage between dry and wet fogs is not uncommon - such fogs consist of water particles along with fairly large masses of dust, smoke and soot. These are the so-called dirty urban fogs, which are a consequence of the presence in the air of large cities of a mass of solid particles emitted during combustion by chimneys, and even more so by factory chimneys.
Characteristics of fogs
View of Bragino (Yaroslavl)
Fog in the Izborskaya Valley (Pskov region)
The fog water content indicator is used to characterize fogs; it denotes the total mass of water droplets per unit volume of fog. The water content of fogs usually does not exceed 0.05-0.1 g/m³, but in some dense fogs it can reach 1-1.5 g/m³.
In addition to water content, the transparency of fog is affected by the size of the particles that form it. The radius of fog droplets typically ranges from 1 to 60 µm. Most drops have a radius of 5-15 microns at positive air temperatures and 2-5 microns at negative temperatures.