Why. Children's questions. Conditions for fog formation

Fog, an accumulation of small water droplets or ice crystals, or both, in ground layer of the atmosphere. (sometimes to a height of several hundred m), reducing horizontal visibility to 1 km or less.

Fog is formed as a result condensation or sublimation of water vapor on aerosol (liquid or solid) particles contained in the air (the so-called condensation nuclei). Fog of water droplets is observed mainly at air temperatures above -20 °C, but can also occur at temperatures below -40 °C. At temperatures below -20 °C, icy fogs prevail. Visibility in Fog depends on particle size, forming 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 µm at positive air temperatures and 2-5 µm at negative temperature. 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. The number of drops per 1 cm3 ranges from 50-100 in weak Fogs to 500-600 in dense Fogs. In very dense fogs, visibility can be reduced to a few meters.

The most general division of Mists - according to their physical genesis - into Cooling Mists and Mists of evaporation. 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. Fog cooling is most common.

According to the synoptic conditions of formation, a distinction is made between intramass fogs, which form in homogeneous air masses, and frontal fogs, the appearance of which is associated with atmospheric fronts. Intramass Fogs predominate; in most cases these are cooling fogs, and they are divided into radiative and advective. Radiation Mists are formed over land with a decrease in temperature due to radiative cooling 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. Advective 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. Advective Fogs are more persistent than radiation Fogs and often do not dissipate during the day. Some advective fogs are classified as 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. A certain role in the intensification of Fog ahead of fronts is played by the drop in atmospheric pressure observed here, which creates a slight adiabatic decrease in air temperature. Fog in populated areas are more often than away from them. This is facilitated by the increased content of hygroscopic condensation nuclei (for example, combustion products) in urban air. Fogs prevent the normal operation of all types of transport, so the Fog forecast is of great economic importance. The artificial creation of Mists is used in scientific research, in the chemical industry, heat engineering, etc.

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 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 moisture condensation nuclei, causing the fog to big cities not only is it formed more often than in suburban areas, but it 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 the person is dressed appropriately for the weather.

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



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

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.

Let's start with what it is a natural phenomenon is formed 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. At the place where this natural phenomenon occurs, the atmospheric humidity tends to 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.

Everyone on the shores of Canadian Newfoundland is familiar with this natural phenomenon. local residents. 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, Chile hasn’t even seen rain in the last few hundred (or even thousands) of years, so this natural phenomenon definitely has nowhere to come from.

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

Let's figure it out why does fog occur? There is always water vapor in the air around us. If the vapor cools, clouds form from it. When clouds descend to the surface of the earth, there are more and more water droplets in them. They approach the ground and form what we call fog.

Fog is formed as a result of the collision of air flows with different temperatures. It is created when heated air is under cold water or meets a mass of more humid and cold air, this is the main cause of fog formation.

Of course, we are talking about dense fog when visibility is less than 200 meters (660 feet). Fog may be even more dense when visibility is less than 60 meters (198 feet).

Exist Various types fog or types of fog , whoever likes it more. They depend on his weight. If the fog extends only two meters (seven feet) above the ground, it is called ground fog.

Another kind of fog usually collected at night, when the ground gives off a large amount of heat. If there is no wind and stars, the rise of air upward may be completely unnoticeable.

Fog can also form as a result of industrial smoke emissions or the accumulation of a large number of cars.

Such fog, in which water vapor and smoke are mixed, is called “smog” (from the combination of the English words fog (fog) and smoke (smoke) (site note). It represents the present disaster and may not dissipate for many days. The resulting fog interferes with normal life, because moving in it is very dangerous. That is why in big cities they carefully monitor the cleanliness of the air.

At one time, London fogs were very famous due to industrial emissions.

What's happening?

Residents of Tashkent and a number of regions of Uzbekistan in last days Fog is regularly observed, which is especially noticeable at night and in the morning. This natural phenomenon causes not only a noticeable deterioration in visibility, but also a lot of unfounded doubts and fears.

What is fog?

Fog is tiny droplets of water that condense from water vapor in the cold air. Simply put: fog is a cloud formed near the ground or water surface. At low ambient temperatures, this cloud can “freeze.” So at temperatures from -10 to -15 degrees Celsius, fog can consist of a mixture of microscopic drops of water and ice crystals, and below -15 degrees - entirely of small ice crystals. Moreover, the thickness of such a “cloud” can reach from one to tens, and sometimes even hundreds of meters.

Where does water come from in the air?

The air always contains water vapor invisible to the human eye, and even under the hot sun of the Sahara Desert, the relative humidity is at least 20-30%. In cities, water evaporates into the air all year round from the surface of canals and reservoirs, it is released by plants, and water vapor from the combustion of fuel is produced by cars, home stoves and industrial enterprises.

Why does fog form?

Fog usually occurs when water evaporates from a warm surface into cold air, or when warm air currents saturated with moisture cool. For example, fog can often be observed in the evening or in the morning due to the strong night cooling of the air layer near the ground. The cooled air becomes supersaturated with moisture, causing water vapor to turn into small droplets of water. Fog also occurs when warm, moist air flows onto a cold surface.

During the cold season, another option is more common - the formation of fog over the warm surface of water or land. The fact is that in cold weather the air cools noticeably faster than the ground and water, which is why the air above the surface of the water or the ground is always warmer than the rest. After such warm air mixes with cold air currents, it begins to cool, releasing steam, resulting in a “cloud” forming near the ground. Therefore, in the cold season, rivers and open reservoirs are often covered with a thick foggy veil.

At the same time, in cities, fogs are denser and more persistent than in open spaces: in urban air there is noticeably more dust, soot and other particles that serve as “condensation nuclei” and contribute to the active formation of droplets from water vapor.

Why are these drops not visible to the eye?

It's simple - they are microscopic: as a rule, the size of water droplets in fog ranges from 1 to 60 microns. For comparison: the thickness of a human hair is 80−120 microns (0.08−0.12 mm). Typically, in the warm season, the size of droplets in fog ranges from 5 to 15 microns, and during frosts - from 2 to 5 microns, so winter cold fog is not as thick as summer fog.

Where does the fog go?

Any fog dissipates after the air warms up. When the sun's rays warm the surface of the earth, the surface air also heats up, as a result of which the smallest drops of water in it evaporate and turn back into steam.

The duration of fogs usually ranges from half an hour to several days (especially during cold weather or when warm and cold air and water flows, for example, fog over a river).

Is it true that fog can be lilac?

Is it true. Moreover, the fog can be not only lilac, but also blue and even yellow. The color of the fog depends primarily on the size of the water droplets it contains, which affects the refraction of sunlight.

If the diameter of the droplets noticeably exceeds 1 micron, then they evenly scatter light waves throughout the entire range, which is why such fog appears milky white. And small drops with a diameter of less than 1 micron scatter short light waves more, so the fog appears lilac or blue. IN major cities fog can also acquire a yellow tint: this is due to the presence of dust, soot and other chemical compounds in the air.

Why are fogs dangerous?

Fogs are accompanied by a noticeable deterioration in visibility, resulting in unfavorable conditions for driving road, rail and air transport. And the denser the fog has formed, the more danger it poses.

The most unpleasant feature of fog is that it helps to deceive the eye. In such conditions, the human eye can erroneously determine the speed of movement and the distance to other objects, which significantly increases the risk of accidents and other accidents. Airplane pilots have the hardest time in thick fog: for the safety of passengers when landing, they have to go to alternate airfields.

Why is fog dangerous for human health?

Ordinary fog formed in clean air is completely harmless to health, provided that a person is dressed appropriately for the weather. It is rather unusual to breathe such air, because air humidity during fog is usually close to 100%.

Another thing is the smog that forms in megacities, containing not only water droplets, but also dust particles, car exhaust, emissions from industrial enterprises, thermal power plants and other pollution. This explains the specific smell that fog in a city can smell. It can certainly harm the respiratory and cardiovascular systems of the human body, and also negatively affects the entire environment - plants, animals and even buildings in the city.

How should drivers and pedestrians behave in foggy conditions?

Before driving, drivers must indicate vehicle for other road users: turn on the side lights, low beam headlights, fog lights and tail lights. You should not turn on high beam headlights in thick fog: it will not improve visibility in any way, but will only create a bright spot in front of the car, blinding the driver.

A smooth driving style without sudden acceleration and lane changes is the key to safety on the road in the presence of fog. The driver should also reduce the usual speed limit. There is a “golden” rule - the speed of movement in fog should not exceed half the visibility range of the road. For example, if the visibility of the road is within 100 m, the speed should be no higher than 50 km/h.

Pedestrians should cross the road more carefully and not jump out in front of an approaching car. Despite the presence of headlights, the driver may not notice the pedestrian or simply become confused by surprise, which can easily lead to tragedy.

Prepared from materials: Zverev A.S. Fogs and their prediction, L., 1954; Khrgian A.Kh. Atmospheric Physics, M., 1969; Tarasov L.V. Physics in nature M:. Enlightenment, 1995.