Fog. Haze

Fogs. Types of fogs. Impact on flights.

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.

To form fog you need following conditions:

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 are caused by cooling earth's surface(radiative), or when relatively warm air moves along a 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 long time(sometimes up to several days).

Advective fogs arise during the movement (advection) of relatively warm, humid air masses on 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 represent great danger for aviation, especially at low altitudes. Moving at significant speeds (20-40 km/h), they can close existing and reserve airfields over a large area for a short period of time and hold on long time. Flight above advective fog is only possible using instruments and favorable conditions weather 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 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 in this case 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 firing of furnaces and during the operation of aircraft engines at air temperatures below -40°C. In big cities, where air enters the air around the clock 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.

Fog, which is especially common on the coasts, is actually a stratus cloud located directly at the earth's surface. This dense blanket of condensed water vapor results from the cooling of air containing saturated vapor.

The fog represents stratus on the earth's surface, consisting of microscopic drops of water or ice crystals. A car driving in supercooled fog may develop a layer of ice.

Fog is formed when water vapor enters warm air or when cooling humid air to a temperature lower than the dew point. Air can cool when entering this area new, even colder air. In this case, the vapor contained in the air condenses and a fog of more or less significant vertical extent appears above the earth's surface.

Exist different kinds fog. They are divided depending on the processes of their formation and the place of occurrence. All classifications of fogs are more or less descriptive.

We will highlight three main types of fog here. Each of them is possible under slightly different conditions. These three types are: radiation fog, advective fog and frontal fog.

Radiation fog. Radiation fog is also known as ground fog. This type of fog occurs when the lower layer of air quickly cools through heat exchange with the earth's surface. In this case, the surface air cools, and warmer air appears above it. If the air is still, little or no fog will form. But if there is a light wind, fog formation is very intense. However, with more strong wind The fog dissipates due to air mixing. A light wind in the area of ​​fog formation leads to the spread of condensation to a higher surface layer of the atmosphere. At the same time, the fog becomes denser. In the morning when Sun rays the air begins to warm up, the fog droplets evaporate, i.e., they turn into water vapor again, and the fog dissipates.

In areas located at a significant altitude above sea level, radiation fog may appear on mountain slopes. Denser air flows down the slopes and forms dense and high fog in the valleys lying between the hills.

Radiation fog occurs more often in autumn and winter, when there is relatively high relative humidity air, as well as a significant length of night. Radiation fog also appears in the centers of areas high blood pressure, usually characterized by light winds and cloudless skies. In relatively stable air, radiation fog that occurs in the evening or at night can persist throughout the day.

Advective fog. Advection fog forms in air moving over an area in which the surface temperature is lower than the temperature of the advancing air. When relatively warm air moves over colder land or body of water, it quickly cools and the water vapor in it begins to condense. A thick low fog appears. In other words, vapor in the lower layer of the atmosphere quickly becomes saturated near the earth's surface, having a significant vertical extent. This type of fog can be observed at any time of the day, not just during the cold night hours.

Advective fog most often appears on the coasts, as well as in areas partially covered with snow and interspersed with snow-free areas. Such fogs are formed in cases when warm southerly winds transport air to the north, i.e., to colder climatic regions.

IN northern regions Atlantic coast The United States often experiences high advective fog. They are extremely dangerous for shipping between the US, Canada and Europe. IN winter months Major shipping lines are moved to more southern areas so that ships can avoid the fog that forms off the coast of Newfoundland.

If advective fog occurs over open sea, it is called sea fog. This occurs when warm air moves over a colder sea surface and is cooled by conduction. It can also appear over areas of the ocean where currents that differ noticeably in temperature pass close to each other, since the air masses above them are mixed. Sea fogs can be very long. Sometimes they do not dissipate for several weeks.

In mountainous areas, slope fog may occur. This occurs when, due to adiabatic cooling, warm air encountering a mountain on its way is forced to rise along its slopes. Rapidly expanding, the rising air lowers its temperature to the dew point, which causes the appearance of thick mountain fog.

Frontal fog. Frontal fogs arise at the points of contact between two various properties. Such places are called frontal zones or proto-fronts. Fronts are very common in the atmosphere, but not all of them are necessarily accompanied by fog.

Most often, frontal fog occurs ahead of a warm front. This fog can be very long lasting. It is usually accompanied by precipitation. Frontal fogs are frequent east coast USA.

Frontal fogs also form on some areas, but less frequently than on warm ones, and not on such vast areas, which is explained by the smaller width of the zone occupied compared to warm ones.

Frontal fogs are a constant threat to all types of transport. They cause great damage when airline flights have to be canceled or flight routes have to be rerouted. Even with modern means For instrument landings, fog still remains a problem for military and civil aviation.

Fogs at sea have caused many ship collisions. They are especially dangerous on lines passing through the North Atlantic, where icebergs occur. The Titanic sank due to a collision with an iceberg.

In such large industrial centers as Los Angeles, New York, London, fog often mixes with smoke to form the so-called smog. Smog causes enormous harm to people's health. The mixing of thick fog with industrial smoke in industrial centers is very dangerous, especially for people with heart disease and respiratory diseases. The additional stress on the breathing and blood circulation of such patients during prolonged smog often leads to death.

Soot suspended in the air of cities and large industrial areas contributes to the formation of fog and smog. In such areas, a large number of condensation nuclei are released into the atmosphere. As a result, normal thermal conditions and air circulation are disrupted. Air pollution even affects the weather, for example, the temperature at night in industrial areas decreases more slowly than usual.

One of known cases smog a large number deaths occurred in London in December 1952. In the cold air, located above the city, an inversion formed, which slowed down mixing in the surface layer of the atmosphere. Industrial smoke, which continued to enter the atmosphere, mixed with still, moisture-saturated air and formed a thick cloud with a high content of sulfur oxides over the city. This cloud appeared main reason respiratory tract irritation. The number of deaths per day has increased. By the middle of the first week after the smog began, nearly 1,000 deaths caused by the smog were reported. The effects of the smog were noticeable for several weeks after it cleared. Throughout this time, the number of deaths remained above normal.

Almost any parent is once faced with the need to answer many questions from their child, revealing to him the structure of the world around us.

But how many of us are ready to answer, for example, such a simple question - what is fog? Before telling a child, adults should themselves have a good understanding of the topic of the issue; only in this case can they become an indisputable authority in everything for the child.

What is fog?

So, what is fog, why does it form and is it harmful to your health to breathe this air? To the first part of the question, most adults can answer the following: fog is small, almost invisible droplets of water that condense in cold air.

At the same time, the transparency of the air deteriorates: if the visibility limit is less than one kilometer, the phenomenon is called fog. The visibility limit within one to ten kilometers is called haze.

Just as steam appears above a pan of hot soup - the result of intense evaporation of water and its condensation upon contact with air at room temperature - fog appears when warm layers of air suddenly cool down with the formation of tiny droplets of moisture.

If the air cools to a temperature below zero, the moisture droplets immediately freeze, forming equally small ice crystals.

Types of fog

Meteorologists distinguish several types of fog, depending on the method of formation and geographical conditions terrain. They are divided into two main types: evaporation and cooling fogs.

Cooling fogs are as follows:

Radiation mists have nothing to do with radioactivity. They form in the summer in the evening and at night, mainly over lakes, rivers or low-lying areas. Due to solar radiation, water in reservoirs heats up during the day. At night, the lower layers of air cool down faster than water, which, evaporating and condensing again in the cold air, forms layers of fog.


Advective fogs most common in coastal areas. They are formed due to the penetration of warm air mass from the sea to the colder coastal land line. The width of the coastline, where active fog formation is observed, can reach several hundred kilometers.

Slope fogs are formed on mountain slopes due to the rise of warm air mass from the surface of the earth and its adiabatic cooling.

Types of evaporation mists:

Sea mists Most often they are formed in the cold season due to the evaporation of water from non-freezing areas of the sea. Entering layers of frosty air, the steam condenses to form fog.

Autumn mists are formed due to the evaporation of water from the surface of a river or lake, when these evaporations come into contact with cold land air, since water retains heat longer than land.

Mixing mists- as the name implies, the reason for their formation is the mixing of air currents with different humidity and temperature. Mixing fogs are most common in areas where warm and cold sea currents meet.

There is another variety - city ​​fogs, the cause of which may be any of the above reasons, enhanced big amount solid microparticles of dust, combustion products and other industrial emissions contained in urban air.

These particles serve as nuclei for moisture condensation, due to which fog not only forms in large cities more often than in suburban areas, but also has a number of negative qualities. This type of fog is called smog in Britain.

How does fog affect human health?

Ordinary fog formed in clean air is completely harmless to health, provided that a person is dressed appropriately for the weather.

Another thing is smog, which contains not only water droplets, but also car exhaust, emissions industrial enterprises, thermal power plants and other pollution.


It certainly harms the respiratory and cardiovascular systems human body, and also negatively affects the entire environment– plants, animals and even buildings and structures in the city.

Fog is an accumulation of small water droplets or ice crystals, or both, in the surface layer of the atmosphere, up to a height of several hundred meters, reducing horizontal visibility to 1 km or less.
Fog is formed as a result of condensation or sublimation of water vapor on aerosol (liquid or solid) particles contained in the air. Fog of water droplets is observed at air temperatures above -20 °C, but can occur even at temperatures below -40 °C. At temperatures below -20 °C, ice fogs predominate.
Visibility in fog depends on the size of the particles that form the fog and on its water content (the amount of condensed water per unit volume.). The radius of fog droplets ranges from 1 to 60 microns. Most drops have a radius of 5-15 microns at positive air temperatures and 2-5 microns at negative temperatures. The water content of fog usually does not exceed 0.05-0.1 g/m3, but in some dense fogs it can reach 1-1.5 g/m3. The number of drops per 1 cm3 ranges from 50-100 in light fogs to 500-600 in dense fogs. In very dense fogs, visibility can be reduced to a few meters.

Based on visibility range, the following types of fog are distinguished:
1) Haze is a very thin fog, a continuous more or less uniform gray or bluish clouding of the atmosphere with a horizontal visibility range (at the eye level of an observer standing on the ground, i.e. about 2 m above the earth’s surface) from 1 to 9 km. It can be observed before or after fog, and more often as an independent phenomenon. Often observed during precipitation, especially liquid and mixed (rain, drizzle, rain with snow, etc.) due to humidification of air in the surface layer of the atmosphere due to partial evaporation of precipitation.
Haze should not be confused with deterioration in horizontal visibility due to dust, smoke, etc. In contrast to these phenomena, the relative air humidity during haze exceeds 85-90%.
2) Ground fog - fog that spreads low over the earth's surface (or a 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. It is observed how usually in the evening, night and morning hours.
3) 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 (amount and cloud shape). 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.
4) Fog - 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 (amount and cloud shape). 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.
According to the method of occurrence, cooling fogs are divided into evaporation fogs. The first ones occur when the air is cooled below the dew point temperature, the water vapor contained in it reaches saturation and partially condenses; the second - with additional supply of water vapor from a warmer evaporating surface into cold air, as a result of which saturation is also achieved. Cooling fogs are the most common.

According to the synoptic conditions of formation, they are distinguished:
1) intra-mass fogs, formed in homogeneous air masses,
2) frontal fogs, the appearance of which is associated with atmospheric fronts.
Intramass fogs predominate, in most cases these are cooling fogs. Intramass fogs are divided into radiation and advection.
Radiation ones are formed over land when the temperature decreases due to radiative cooling of the earth's surface, and from it the air. They most often occur on clear nights with light winds, mainly in anticyclones. 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.
Advection fogs are formed due to the cooling of warm, moist air as it moves over a colder surface of land or water. The intensity of advective fogs depends on the temperature difference between the air and the underlying surface and on the moisture content of the air. They can develop both over land and over the sea and cover a huge area, sometimes on the order of several tens or even 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. Some advective fogs are evaporative fogs and occur when cold air is transferred to warm water. Fogs of this type are common, for example, in the Arctic, when air flows from the ice cover to the open surface of the sea.
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. Fall plays some role in increasing fog ahead of fronts atmospheric pressure, which creates a slight adiabatic decrease in air temperature. Fogs occur more often in populated areas than far away from them. This is facilitated by the increased content of hygroscopic condensation nuclei (for example, combustion products) in the air.
Fog significantly affects visibility, which is one of the the most important factors safe navigation for the navigator Visibility is the distance at which during the day the last signs of the observed object disappear (its outlines become indistinguishable), and at night an unfocused light source of a certain intensity becomes indistinguishable. Visibility is assessed in points, visually based on a number of objects located on different distances from the observer, according to the international visibility scale (Table 1):
Table 1. International visibility scale.
Score Visibility range Score Visibility range
0
1
2
3
4 0-50 m
50-200 m
200-500 m
500-1000 m
1-2 km 5
6
7
8
9 2-4 km
4-10 km
10-20 km
20-50 km
50 km

Table 2. Designation of fog when plotting data on weather maps.

Continuing the conversation about the weather. Rubric led by Candidate of Economic Sciences M.SOFER

WHAT WE KNOW ABOUT THE FOG

The period of autumn is approaching when bright and beautiful colors will be replaced by faded and dull ones. Increasingly, the objects around us will seem to dissolve in a weightless and intangible environment - the time of fog will come.

Fog is a phenomenon from which you cannot protect yourself or get rid of it; you have to reckon with it. Compared to other meteorological phenomena, such as a hurricane, thunderstorm, snowfall, fog cannot be called a formidable force of nature. However, it has a noticeable impact on human living conditions, affects production processes, the operation and safety of movement of all types of transport, and even the well-being of people.

When there is fog, visibility sharply deteriorates, so for safety reasons, the movement of ships and aircraft is limited, and work on construction sites is suspended. Transport accidents are also common, for example, in 1989 in Italy, when more than 80 cars collided and caught fire due to fog near Milan.

The number of foggy days in a year determines the suitability or unsuitability of territories for health purposes, tourism and simply for comfortable living conditions. At the same time, fogs can be beneficial for Agriculture– in dry areas they supply the soil with additional moisture, which increases productivity.

Who hasn't heard of the famous London or St. Petersburg fogs?

Their descriptions entered classical literature and became business card these cities. The London fogs from a common phenomenon for coastal cities turned into its symbol, a kind of myth. Oddly enough, foggy London gained particular popularity after it was represented in the paintings of the Impressionists.

And what does fog consist of and how is it formed?

The composition of the fog is obvious - water. Wait until the kettle boils, or exhale strongly in the cold - and you will see the process of fog formation. At different temperatures, air may contain different quantities water vapor. The warmer, the more. The degree of proximity of the air to the state of saturation with moisture determines our feeling of “dryness” or “dampness.” If the temperature drops, then some of the moisture condenses. In everyday life these are “puffs of steam”, in nature these are clouds, rain, dew, fog.

Ordinary fog is a “cocktail” of droplets of different sizes. It can simultaneously contain both very small and very large droplets. Some more, others less, depending on... the temperature. The higher it is, the more large droplets This means that “warm” fogs consist of “thicker” droplets, and “cold” fogs consist of thinner ones. Accordingly, their water content is different. In the first you can get wet, in the second you can get damp.

However, more often we distinguish fogs not by temperature, but by another feature - simple and practically important. We evaluate the most dangerous and unpleasant as “strong”, “thick”, “dense”. But what is behind these assessments? It turned out that in 1 cm3 of weak fog there are 50-100 droplets, and in strong fog - 500-600, i.e. . almost ten times more. To those who have plunged into the dank dampness of the fog, it seems that everything around is as if saturated with water. This belief is based on the traditional idea of ​​100% fog humidity. However, measurements show that the relative humidity can be lower, sometimes 80-90%, and in some cases even lower. About the same humidity in the clouds. So the expression “fog humidity” is not at all a tautology like “wet water”.

After all, it only seems that in heavy fog, clothes quickly get wet. But in fact, if you try to “squeeze out” several cubic meters of fog, then... there won’t be enough moisture even for one sip. Apparently, we most likely overestimate the water content of fogs. 1 m3 of fog contains 0.2-0.5 g of water. This means that by “squeezing” all the moisture out of 1000 m3 of fog, we will barely quench our thirst.

At the same time, a simple calculation shows that 1 km3 of fog can contain up to half a million liters of water. And this amount of moisture is already enough to irrigate tens of hectares of fields. In many places, fog and dew serve as a significant source of moisture for plants.

What types of fogs are there?

At first glance, all fogs are the same. Most often, we are not interested in the causes of fog; we wait for it to clear and forget about it until the next time it appears. However, meteorologists view fog differently. They know that there is no fog “in general”; there are certain processes that each time lead to the formation of one or another type of fog.

It is easy to say that fog is “excess” moisture released in the form of tiny droplets. It is more difficult to explain where these “surpluses” came from. In any case, they come down to the frightening word “heat and moisture exchange,” or, more simply, to two reasons: evaporation from a warm surface into cold air and cooling of a warm air mass.

Almost everyone has witnessed these processes. Some have observed how the ice-free ice hole soars. Others saw how in winter a damp haze approaches land from the sea, and in summer the fog stubbornly stays away from the shore. Still others cursed the road crossing lowlands, ravines, ravines, where cold air flows and where fogs are regularly born. Fourth, they remember the beautiful in form and accurate in observation poems of Russian poets who did not ignore this seemingly inconspicuous weather phenomenon.

Let us recall only some textbook-famous lines. Pushkinsky: “The sky was already breathing in autumn... Fog was falling on the fields...”, or: “The daylight went out, evening fog fell on the blue sea...”. Compare them with Yesenin’s: “The fields are compressed, the groves are bare, the water is foggy and damp...”, or: “A spicy evening. The dawns are going out. Fog is creeping across the grass...”

It’s amazing how dynamic the fogs are in the eyes of poets, they “blow”, “fall”, “lay down”, “rise”, “creep”... Do many people notice this?

Where are fogs registered? It is quite simple to determine this - wherever there is a collision of air masses with different temperatures and humidity. These are the so-called border areas: land - sea, warm - cold current, border sea ​​ice, snow cover boundary. Such conditions arise in the northern hemisphere, so there are areas there that can safely be considered “fog poles.”

Thus, at the junction of the warm Gulf Stream and the cold Labrador Current in the region of South Africa, there are 120 days of fog per year, especially in summer: on average there are 22 foggy days per month. That is why this area is considered one of the most dangerous for sailors. Its notoriety is transmitted (perhaps through fog?) to the fatal “Bermuda Triangle” located nearby.

Fogs often occur in the area of ​​the Kola Peninsula (50-100 foggy days a year), a little less often on the coasts of the Barents, Northern and Baltic seas, in the Sea of ​​Okhotsk and the Sea of ​​Japan, they are typical for Florida and California. The famous red pillars of the San Francisco Bay Bridge are often hidden behind a whitish blanket. New Yorkers are well aware that in winter, warm winds from the ocean almost always bring fog.

The greater the temperature difference, the more intense the fog. Any of us can become a forecaster. If the difference in water and air temperatures reaches 15 C, then the probability of fog formation is 85-90%. Such conditions exist on the shores south seas and in the foothills.

“Urban” fogs have a completely different pedigree. At unfavorable conditions and polluted air, fog in the city can easily arise and persist for several days. A classic example is London and St. Petersburg of past centuries. Remember the feeling of Dostoevsky’s “Poor People”: “There was no rain, but there was fog, no worse than good rain... There is fog under your feet, there is also fog above your head...”

However, real observations do not always correspond to the prevailing fiction ideas. Urban fogs depend on dozens of factors (winds, terrain, proximity to water bodies, type of industry, type of transport, etc.), which can be combined in the most bizarre ways. Therefore, do not be surprised that, according to climatologists, one of the most foggy cities in the world is... Rio de Janeiro. There, in the shadow of the year, there are an average of 164 days with fog. The second place is firmly occupied by the capital of Ecuador Quito - 92 foggy days a year. This is followed by a wide margin by Helsinki - 60 days, Bucharest - 55 days and London - 46 days.

Many northern European cities are not as “foggy” as is sometimes imagined. In Stockholm, for example, fog can be observed only 13 days a year, and in Dublin and Reykjavik only 5-7 days. Paradoxically, fog is almost as rare in another climatic zone- in Mongolia.

“A Brief Climatic Guide to the Countries of the World” (1984) dispassionately testifies that in St. Petersburg and Moscow the number of foggy days per year is almost the same: about 30. This is 7 days more than in Rome, but 14 days less than in Berlin.

There are cities on the planet whose inhabitants cannot even remember when they saw fog in last time. For example, Beirut has one foggy day a year. For a resident of Khartoum and Bombay, this weather phenomenon seems to not exist at all.

Visibility in fog.

None of the characteristics of fog are as important to us as visibility in it. It’s not without reason that when they want to emphasize the density of the fog, they say: nothing is visible at arm’s length. Or: you can’t see it two steps away! Messages about possible fog are always included in weather reports. And information about a sharp decrease in visibility is distributed as an emergency message or storm warning.

It is obvious to everyone that the strength, density, thickness of the fog are manifested through the range of visibility in it. If objects distant at 500-1000 m are visible, the fog is considered weak, if visibility is within 50-500 m - moderate, less than 50 m - strong. It is the limited visibility that makes heavy fog most dangerous weather phenomenon. Dense fog can completely paralyze life large city, limit aviation operations, block roads.

But how is density determined? Just the number and size of moisture droplets per unit volume, i.e. water content of the fog, which in turn depends on temperature. Therefore, “warm” fog is almost always thick and dense, while “cold” fog is more liquid and transparent. If you notice how the fog “whirls”, i.e. air masses alternate in it, which means you observe temperature pulsations, and with them the dynamics of water content and droplet sizes.

Be careful! You may also notice a change in the color of the fog. The smallest drops scatter short wavelengths of light (blue rays) best, which is why weak fogs and haze have a bluish tint. In denser fogs, the droplets are larger and scatter light waves of all wavelengths almost equally. Therefore, the color of such fogs is closer to white. However, poets see a much more diverse palette. Thus, Dostoevsky’s fogs are “milky”, Blok’s are “gray-haired”, Bunin’s are “silver”, Gumilyov’s are “fair-haired”, Tsvetaeva’s are “blond”. What are yours?

Audibility in fog

Have you tried talking in the fog? An observant person will confirm: even in the densest fog, sounds can be heard perfectly. But how far do car or ship signals travel in thick fog? Don't their bass sounds fade away as if in a layer of cotton wool?

Knowledge of this issue was vitally important for sailors, since in heavy fog light signals do not pass through and the risk of ship collision increases sharply. According to British statistics hundred years ago Over the course of ten years, 273 shipwrecks were reported off the coast of England due to thick fog. The problem reached the state level, and long-term research began. They were accompanied by great noise - shots from cannons, whistles and powerful sirens echoed the coast day and night, in any weather, in fog, rain, snow. The noise was not in vain...

It was found that sounds travel well over long distances only in cases where the air environment is continuous and homogeneous. If the atmosphere consists of layers of air of different densities and temperatures, then at the junction of each layer sound energy is lost and the sound “does not come” - it quickly goes out. At the same time, cloudy weather is transparent to sounds, and in the thickest fog sounds can travel a distance almost twice as far as in clear weather. How then can we explain the “dips” of sound, the impression of its “viscous” passage through shaggy, moving clots of water vapor? The answer lies precisely in the structure of the fog itself. If the fog swirls and pulsates, it means it is not homogeneous; the droplets differ not only in temperature and size, but also in sound permeability. This often happens in the mountains.

Ordinary lowland fog consists of a huge number of extremely small droplets of water - less than a thousandth of a millimeter in diameter. They create a relatively dense but homogeneous environment, so there is almost no scattering of sound waves. That's why ships, trains and cars caught in fog sound their horns. Horns and sirens are a reliable source of information in the thickest fog.

But it’s one thing to warn neighbors about your location, and another thing to create conditions for safe movement. Most radical way– elimination of fogs. Humanity has come very close to the practical possibility of combating fogs. This problem is not so much technological as economic and environmental. What is the cost of weather control and what are its consequences? While this question remains open...