Fog. Haze

Fog and haze are the result of condensation of water vapor (i.e., the transition of water vapor contained in the air into a liquid state) in close proximity to the earth's surface. Fog are a collection of water droplets or ice crystals suspended in the air that degrade the meteorological visibility range to less than 1 km. With visibility from 1 to 10 km, this set is called haze.

At positive temperatures, fog consists of water droplets with a radius of 2 ... 5 microns, and at negative temperatures it consists of supercooled water droplets, ice crystals or frozen droplets. The water droplets that form the haze have a radius of less than 1 micron. When the droplets become larger, the haze can turn into fog, and when evaporation occurs, the fog can turn into haze.

Visibility in fog depends on the size of the droplets or crystals that form it and on the water content of the fog. The water content of the fog is the amount of liquid or solid water contained in 1m³. Observations show that the water content of fog is 0.02-1.0 g/m³. The water content of fog is influenced by temperature - at positive temperatures it is significantly greater than at negative temperatures.

The number of drops per unit volume in a weak fog is 1-10 pcs/cm³, in a strong fog – 400... 600 pcs/cm³.

Fogs usually form when the partial pressure of water vapor in the layer of atmosphere adjacent to earth's surface, exceeds the saturated vapor pressure, and the relative humidity is 100% or slightly less.

According to the physical conditions of formation, fogs can be divided into three groups:

1. cooling mists;

2. fogs not associated with cooling;

3. fogs caused by human activity.

Cooling mists are formed as a result of a decrease in the temperature of the air adjacent to the earth's surface below the dew point (dew point is the temperature at which relative humidity reaches 100%). Cooling fogs, in turn, are divided into radiative and advective.

Radiation mists are formed as a result of radiative cooling of the soil, which then cools the air adjacent to it. The following conditions favor the formation of such fogs: sufficient relative humidity; clear or partly cloudy weather and light wind.

Advective fogs are formed when warm air moves over a cold active surface. The following reasons for their occurrence are possible:

1. Movement of tropical air to higher latitudes;

2. The movement of warm continental air to the colder surface of the sea; such fogs are observed over the seas in summer. When the wind direction changes, they can move to the coast;

3. The movement of warm sea air to the colder surface of the continent. These fogs are called coastal fogs;

4. Movement of air from a warm surface to a cold one. Such fogs form where cold and warm sea currents meet. They are called marine;

Marine fogs also include fogs that form over the ice of the Arctic basin. Such fogs form at any time of the day and can exist at significant wind speeds.

To fogs not associated with cooling , include evaporation fogs and displacement fogs.

Mists of evaporation observed in cases where the temperature of the water surface is higher than the temperature of the adjacent air. Their formation is due to the cooling and condensation of steam coming from the water surface into the air. Such fogs often form in autumn over rivers and lakes. In cold times, they appear over ice-free bays of the seas, as well as over polynyas among the ice.

Displacement fogs are formed by mixing two air masses that have different temperatures and contain water vapor close to the saturation state. Most often, such fogs form on the shores of seas and lakes when there is a large contrast in air temperature over land and over the water surface.

To fogs caused by human activity, These include urban and frosty (furnace) fogs, as well as specially created artificial fogs, for example, to combat frost.

City fogs are formed in large cities, where large quantities of waste are released into the air industrial production, they are also condensation nuclei, and in this case condensation (fog formation) begins already at relative humidity from 75%. Urban fogs sometimes have a dark color due to the presence of smoke particles, soot and other impurities in the droplets.

Frosty (furnace) mists are formed in winter at low air temperatures and in the presence of a ground inversion (a retaining layer that prevents the transfer of heat, water vapor and various impurities. The temperature in this layer increases with height). They usually occur in the morning over small populated areas, when a large number of condensation nuclei begin to enter the air along with smoke from the furnace fires, which is why the name fogs is associated.

In Magadan, advective fogs are mainly observed.

Below is the average number of days with fog, greatest number days and the average duration of fog in hours according to the Magadan station.

VIII

Average number of days with fog

0.03

0.05

Most days with fog

Average duration of fogs (hours)

0.03

When water vapor saturates and then condenses or sublimates, tiny droplets of water and ice crystals form in the surface layer of the atmosphere. The accumulation of such particles causes deterioration in horizontal visibility. Air turbidity caused by the accumulation of condensation or sublimation products in the ground layer, with visibility less than 1000 m, is called fog. If, under the same conditions, visibility is more than 1000m, but less than 10 km, then the phenomenon is called haze. In international aviation codes, haze is a phenomenon with visibility from 1000m to 5 km.

The following conditions are necessary for fog to form:

Air saturation with water vapor at the surface of the earth is up to 100%;

Presence of condensation nuclei.

According to the synoptic conditions of formation, fogs are divided into

Intra-mass;

Frontal.

Intramass fogs, depending on the process leading to saturation of air with water vapor, are divided into:

Cooling mists;

Mists of evaporation.

Cooling fogs arise due to cooling of the earth's surface (radiation), or when moving relative warm air along the cold underlying surface (advective). This group also includes fogs associated with atmospheric fronts (frontal).

Radiation fogs are formed due to radiation cooling of the earth's surface and due to this cooling of the ground layer of air. In the warm half of the year, radiation fogs are formed at night in clear or partly cloudy weather with a weak wind not exceeding 3 m/s. They occur mainly over lowlands and wetlands. The vertical thickness (thickness) of such fogs can range from several meters to several tens of meters. They are especially dense in the lowest ground layer, where the greatest air cooling occurs; with height, their density quickly decreases. While flying through the fog, you can clearly see rivers, large landmarks and lights, and the airfield. And horizontal visibility near the ground can be up to 100m or less. Oblique visibility sharply deteriorates when an aircraft enters a layer of fog on landing.

Flight above the radiation fog does not present any particular difficulties, because... This fog is usually located in patches and allows for visual orientation. The indicated visibility conditions during radiation fogs sometimes lead to an incorrect assessment of the meteorological situation.

Radiation fogs of the warm half of the year usually dissipate with sunrise, and sometimes rise above the ground, forming a thin layer of scattered stratus (St fr) clouds, the height of which does not exceed 100-200 m. Fog dissipation can occur when the wind increases to 4-5 m/s or more.

In the cold half of the year, radiation fog can be more dangerous than in the warm half. During this period, with established clear weather, cooling of the air may spread to greater height. The resulting radiation fog has a vertical thickness from several hundred meters to 1.5-2 km and persists for a long time (sometimes up to several days).

Advective fogs arise during the movement (advection) of relatively warm, moist air masses along a cold underlying surface. Through turbulent mixing, cooling extends to a height of several hundred meters, where an inversion layer is usually observed. Fog appears in the cooled ground layer, which is often accompanied by drizzling precipitation. Under the influence of the retarding inversion layer underneath, the greatest accumulation of water vapor is observed. As a result, the density of advective fog increases as it rises. With this fog, horizontal visibility is somewhat better near the ground, but at a height (above several tens of meters) it deteriorates sharply.

Unlike radiation fogs, advective fogs can be observed with wind speeds of 5-10 m/s or more. These fogs can occur at any time of the day and persist for a long time and spread over a large area.

Advective fogs pose a great danger to aviation, especially at low altitudes. Moving at significant speeds (20-40 km/h), they can close existing and reserve airfields over a large area within a short period of time and hold out for a long time. Flight above advective fog is only possible using instruments and under favorable weather conditions at the landing airfield. Advective fogs can cause icing, sleet, and drizzle.

Frontal fogs are associated with atmospheric fronts that separate warm and cold air masses. Most often, frontal fog occurs on a warm front in a wedge of cold air located in the front part, in the zone of precipitation.

The reason for the formation of this type of fog is the decrease in pressure ahead of the front. It leads to adiabatic expansion of surface air and its cooling. Water vapor, which is in the air in a state close to saturation (due to the evaporation of precipitation), condenses when the air is cooled to the dew point and below. The result of condensation is frontal fog. This fog occupies a strip up to 200 km wide. Sometimes it can merge with overlying clouds or join advective fog that has arisen in the post-frontal air. Frontal fog is especially dangerous for flights when it merges with frontal clouds. If the frontal fog merges with the advective fog of the transfrontal warm sector, then the weather conditions are extremely unfavorable and dangerous for flights over large areas.

Evaporation fogs arise due to the influx of water vapor from a warm water surface into cooled air. For the formation of such fogs, a difference between air and water surface temperatures of more than 10°C is required.

Evaporation fogs occur at sea over ice-free bays in winter time and autumn - over rivers and lakes in autumn months when the surface of the water in rivers and lakes turns out to be significantly warmer than the air. These fogs can reach great intensity and heights of several meters, sometimes tens of meters.

Frosty fogs are formed due to the entry of water vapor into the surface layers of air with combustion products. In populated areas and at airfields in Siberia, such fogs occur during the heating of furnaces and during the operation of aircraft engines at air temperatures below -40°C. In big cities, where there is 24/7 air a large number of water vapor generated during fuel combustion, such mists can also occur at more high temperature, about -16°C and below.

In the presence of a weak wind and its slight increase with height, such fogs form above the ground at an altitude of 50-200 m, significantly worsening oblique visibility from an airplane.

The summer fog by the river is incredibly beautiful. Only in such moments do you understand how good it is to live! And the distant shores, hidden by a foggy haze, evoke lyrical memories and dreams.

However, even the most inveterate esthete will not always have an answer to the question of what fog is and what is the mechanism of its formation. If you don’t know this either, we invite you to read our article.

We should start with the fact that this natural phenomenon occurs when air heated during the day comes into contact with the cold surface of water or soil.

So what is fog? This is condensation in the form of tiny droplets (aerosol), which, collected in one place, sometimes reduce visibility to zero.

Note that the formation of fogs is impossible without solid or liquid particles called condensation nuclei. It is on them that water begins to precipitate, forming drops. It goes without saying that classic water fogs are formed only when the ambient temperature is not lower than -20 degrees Celsius. Otherwise, their ice form will form.

By the way, what is ice fog? In fact, their formation begins with the condensation of the same water on particles in the air, but due to the low temperature, these droplets instantly turn into the solid fraction. Considering that the refractive index of ice is higher, visibility in this case drops even more.

This will be confirmed to you by all drivers who have at least once worked in conditions Far North. In such conditions it is extremely difficult to drive a car, since there is almost no help. And the glass freezes over in a couple of minutes, so it’s simply impossible to see the road.

Most often, fog (the nature of which we have discussed) forms in the autumn, since the air during this period cools more slowly than the water or earth's surface. In the place where this natural phenomenon occurs, humidity atmospheric air strives for 100%.

As we have already said, the structure of the fog can be very different. The formation can be represented only by water droplets, water and ice, and also exclusively by ice crystals.

As you can see, fog is a multifaceted natural phenomenon, and therefore it is not surprising that there are several types of it:

Solid type. Visibility is limited to almost zero, road traffic and aircraft flights are suspended.
Smoky variety. Visibility is moderate, and the danger at low speeds is low.
“Underground” - fog spreads at ground level.

On the shores of Canadian Newfoundland, all local residents are familiar with this natural phenomenon. The fact is that in these parts the Gulf Stream connects with the Labrador Current, which causes a strong temperature difference. For six months everything here is shrouded in a gloomy haze, and therefore pilots and sailors really do not like this area.

But there are places on our planet where fogs have never been seen. For example, this is the Indian city of Bombay. Well, the Chilean region hasn’t even seen rain in the last few hundred (or even thousands) years, so this natural phenomenon There's definitely nowhere to get it from.

So you have learned what fog is and where it comes from.

Fog- an 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° - a mixture of water droplets and ice crystals, at temperatures below −15° - ice crystals sparkling in the sun 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, fogs can be observed at any relative air humidity (even less than 50%) - due to the condensation of water vapor formed during fuel combustion (in engines, furnaces 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.

The following types of fog are noted at weather stations:

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, V chemical industry, heating engineering and other areas.

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, the following are considered: haze - visibility more/equal to 1000 m, but less than 10 km, and fog - visibility less than 1000 m. Heavy fog is 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 are fogs that appear as a result of radiation cooling of the earth's surface and the mass of moist 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 when there is a temperature inversion that prevents ascent 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 means total weight 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.

Fogs occur at any time of the year, but most often they occur in late summer or autumn, when the air cools faster than the ground cools. As a result, cool air falls to the ground or water, which still retains heat, condensation occurs, and many water droplets hang in the air. It looks like a huge cloud is hanging right above the ground or a body of water. In the place where the fog formed, the air humidity is 100%. Fogs have different structures. If the air temperature is not very cold, above 10 degrees below zero, then the foggy cloud consists of water droplets. At a temperature of 10-15 degrees below zero, the cloud consists of a mixture of water droplets with ice crystals. If the temperature drops below 15 degrees below zero, then ice fog forms when the entire cloud consists of ice crystals. In cities and populated areas fogs are denser due to the fact that condensation mixes with exhaust gases and dust.

What kind of fogs are there?

Fogs are different. It depends on how good the visibility is in the foggy area.

Haze is the most weak appearance fog.

Ground fog is fog that spreads over the ground or body of water in a thin layer. This fog does not have much effect on visibility.

Translucent fog, visibility in which ranges from several tens to several hundred meters. Through such fog the sun and clouds are visible.

Continuous fog, when a whitish cloud envelops the earth, through which it is impossible to see literally anything at a distance of several meters, and sometimes even at arm's length. With such fog, traffic becomes impossible. If a driver finds himself in a cloud of continuous fog, it is better for him to wait until the fog clears.

There are not only natural fogs, but also artificial ones. Artificial fogs arise from human industrial activities. Artificial fog consists of dust, smoke, exhaust gases, chemical substances, other combustion products. Otherwise it is called smog. Smog– one of the most important problems of modern cities, as it causes irreparable harm to human health and pollutes the environment.