Why is there fog in severe frosts? How does fog form? Where is the fog.
Finding yourself in the middle of a solid white cloud, so dense that at arm's length it is almost impossible to discern anything, you often ask yourself the question: why such a thick fog has formed, why is it white and you start to wonder how long this phenomenon usually lasts, and also why any fog dissipates.
Fogs form when droplets or ice crystals accumulate in the air in the lower layers of the atmosphere, due to which a cloud-like veil forms along the earth's surface, limiting visibility so much that space is not visible beyond one kilometer, and in some cases objects become difficult to distinguish even at a distance several meters.
If the ambient temperature exceeds -10 ° C, the vapor layer consists only of drops. If the temperature indicators fluctuate from -10 to -15 ° C - from water droplets and ice crystals, and when it's -15 ° C outside - the fog consists of small ice crystals, shimmering in the light of night lights.
Why this phenomenon is formed is easy to answer: it owes its appearance 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 terrestrial clouds can often be observed in the evening or in the morning after the temperature of the soil and vegetation (grass) has dropped, 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 shroud above it, spreading over the water surface. This happens because the temperature of the water during frosts is warmer than the surrounding ice and the air in contact with it (because of this, the air above the water is always warmer than the rest and the fog over the river in the area of the ice hole is almost always there).
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 the Canadian island of Newfoundland located in the Atlantic Ocean. Due to the fact that here two currents collide with each other - the warm Gulf Stream and the cold Labrador one, local residents are forced to spend about one hundred and twenty foggy days a year among the haze.
Ground cloud formation
When water-saturated air cools or mixes with colder air currents, droplets begin to form in the atmosphere. After that, if there is over ground surface the smallest dust particles, they begin to adhere to them, layering on top of each other and forming droplets of more large sizes(the more dust in the air, the faster the cloud forms, therefore big cities almost always shrouded in a faint, almost imperceptible veil).
In the warm season, the size of such a drop ranges from 5 to 15 microns, during frosts - from 2 to 5 microns, so the winter cold fog is not as thick as the summer one. As soon as the droplets reach the required volumes, the objects are vague and difficult to distinguish: the air becomes whitish in strong fog and bluish in weak fog.
The answer to the question of why this phenomenon occurs in different colors is simple: smaller drops scatter short blue rays better, while in thick ground clouds, larger drops 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 the vegetation of the arid regions of the planet). How impenetrable the veil turns out to be depends largely on the humidity of the air, which during the appearance of ground clouds usually ranges from 85 to 100%:
- if the visibility does not exceed 50 meters, there is a thick fog, and the number of drops is 1200 per cubic centimeter;
- if the space is viewed at a distance of 50 to 500 meters - moderate (water drops in this case from 100 to 600);
- if the visibility is a kilometer - weak (drops - from 50 to 100).
Fogs are not uncommon during frosts, and the phenomenon can be seen even when the humidity does not exceed fifty percent. They can usually be seen in cities, especially at train and bus stations, where the veil is formed by the steam that appears during the combustion of fuel and is released into the air through chimneys and exhaust pipes.
Views
Terrestrial clouds do not always owe their origin only to nature: big number fogs arise in cities, and therefore they consist not only of drops and dust, but also smoke, soot that are emitted by factory or chimneys, or arise after or during fires, when a forest, peat or steppe is burning. By origin, meteorologists divide fogs into dry (smoke, soot, etc. are to blame for their formation) and wet (only water and dust are involved), while often the second form flows into the first.
In turn, humid fogs, the formation of which was directly influenced by nature - this is evening, night or morning fog (it is this period that is optimal for the appearance of clouds creeping on 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 can be continuous, or it can go in separate shreds, and the 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 the clouds are almost impossible to distinguish. This is mainly evening, night and morning fog, and during cold weather, when the temperature indicators cold fog can be seen during the day.
Why do fogs disappear
The duration of this phenomenon is different and can range from half an hour to several days (especially during cold weather or in the collision of warm and cold air and water flows e.g. fog over a river). The main reason why any fog dissipates is the heating of the air. Since a shroud forms near the surface, after Sun rays it is heated, the air is also heated, as a result of which the drops 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 vapor is converted into water droplets and forms clouds.
Almost any parent is once faced with the need to answer many questions of 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 the child, adults themselves should be well versed in the topic of the issue, only in this case it is possible for the child to become an indisputable authority in everything.
So, what is fog, why does it form and is it not 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 to the eye, droplets of water condensing 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 line of sight between one and ten kilometers is called haze.
Just as steam appears above a saucepan with hot soup - the result of intense evaporation of water and its condensation when it comes into contact with air at room temperature - fog appears when warm layers of air cool sharply with the formation of tiny droplets of moisture.
If the air cools down to a temperature below zero, the moisture droplets immediately freeze, forming equally small ice crystals.
Types of fog
Meteorologists distinguish between several types of fog, depending on the method of formation and geographic conditions terrain. They are classified into two main types: evaporation and cooling mists.
Cooling fogs are as follows:
Radiation fog 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. The lower layers of the air cool down at night faster than water, which evaporates and condenses again in cold air, forming 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 landline. The width of the coastal line, where active fog formation is observed, can reach several hundred kilometers.
Slope mists are formed on the slopes of mountains due to the rise of warm air mass from the surface of the earth and its adiabatic cooling.
Types of evaporation mists:
Sea fogs most often formed in the cold season due to the evaporation of water from non-freezing areas of the sea. Entering the layers of frosty air, steam condenses to form fog.
Autumn fogs are formed due to the evaporation of water from the surface of a river or lake, when these vapors come into contact with the cold air of the land, since water retains heat longer than land.
Mists of mixing- as the name implies, the reason for their formation is the mixing of air currents with different humidity and temperature. Mists of mixing are most common in areas where warm and cold sea currents.
There is another variety - city fogs, the cause of which may be any of the above reasons, enhanced big amount solid microparticles of dust contained in the city air, combustion products and other industrial emissions.
These particles serve as nuclei of moisture condensation, due to which fog in large cities not only forms more often than in the countryside, but also has a number of negative qualities. Such fog in Britain is called smog.
How does fog affect human health?
The usual fog that forms in clean air, 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 industrial enterprises, CHP and other pollution. 
It certainly harms the respiratory and cardiovascular systems the human body, and also negatively affects the entire environment- plants, animals and even buildings and structures in the city.
Mist is formed due to the condensation of water vapor in the air. In winter, this phenomenon can be observed when there is a change atmospheric front... When temperatures rise or fall at night, condensation forms in the morning.
In general, fog is a natural phenomenon, which tends to appear due to the temperature difference between the words of the air: the lower and upper layers. Fog can be present in winter, this phenomenon is especially typical with sharp temperature changes from high to low. There is a process of moisture evaporation (snow also evaporates, oddly enough) and a combination of heat, which dates this moisture with cold air. This is where the fog comes from.
Fog arises from differences temperature regime earth and sky, in winter it does not happen often, or rather not as often as in autumn or spring, but sometimes you can see fog in winter especially in the early morning when the night temperature is still low, but it is already starting to warm because a new day begins.
Usually, fogs are formed at a sufficiently high air humidity, due to the condensation of water vapor. However, it often happens that in severe frost, with an anticyclone and low air humidity, dense fogs can form. As a rule, this phenomenon is typical for big cities, especially industrial centers. In severe frost, moisture from industrial emissions (from pipes) and automobile exhaust begins to condense. Stove heating also contributes - people heat their homes more in the private sector in very coldy... And in ordinary stove smoke there is a lot of water vapor.
A difficult question for an ordinary person to understand
I will try to explain in a simpler way:
It is cold in winter, but the earth maintains a normal temperature to some extent.
Normal temperature emits heat.
When it's warmest and coldest winter air connect - fog is formed.
Severe frosts always mean the corresponding temperature anomaly. In the southern regions, the temperature can be called severe frost 10 degrees. In the more northerly 30 degrees and below. But in any case, this is once quickly supercooled air. However, fog during such frosts is generated not by air as such, but by water and moisture. During fogs of this kind, it does not descend from the sky, but rises from the ground (including - it breaks down from the surface of water bodies, comes out of thousands of hot pipes). Natural (natural) fog becomes mixed with smog. Frost, as it were, transforms e (moisture) in its own way. More precisely, this can always happen, but during drops, this phenomenon becomes most evident. During calm hours, the formed surface cloud, which we call frosty fog, becomes very noticeable. Very often, such fog descends on tree branches and any other surfaces in the form of frost.
Look at your eyelashes during a severe frost. They often become the model for what I described above. 🙂
Fog always appears due to the difference in temperature between the top, that is, the air that descends from the heavens, and the bottom, that is, the earth. So, at the difference in these temperatures, the cold side turns from the influence of warm into droplets of steam that create these low clouds.
Fog is only the result of moisture evaporating. In severe frost, such evaporation quickly cools and turns into fog. This is how humid heat and cold meet. The cold air simply touched the warmth of the still uncooled earth and turned into fog. The structure of the fog is different and depends on the temperature. The lower the temperature, the more ice particles. At not very low temperatures, the foggy cloud consists of water droplets.
In Irkutsk, in severe frosts, fog happens due to the fact that the surface of the non-freezing river (after the hydroelectric power station is heated and flows for several kilometers without freezing) soars. Probably you also have non-freezing reservoirs.
Even in winter, there is moisture in the air, way and in less quantity. And when it is severe frosts, especially after the relative warmth, this moisture turns into ice floes, and we see exactly ice fog. Snow also adds moisture, which evaporates, giving off its warmth. And with strong changes towards the minus, this process is more intense, which adds moisture to the air and thickens fog. The fact that snow and ice also evaporate is proved by the washed linen hung out in the cold. Although the temperature outside is freezing, the linen dries out all the same, albeit not completely.
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 sinks to the ground or water, which still retains heat, condensation occurs, and many water droplets hang in the air. It turns out that a huge cloud hangs directly above the ground or a body of water. In the place where the fog has formed, the air humidity is 100%. Fogs are different in structure. If the air temperature is not very cold, above 10 degrees of frost, then the foggy cloud also 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 an ice fog is formed, when the entire cloud consists of ice crystals. In cities and settlements fogs are denser because condensate mixes with exhaust gases and dust.
What fogs there are.
Fogs are different. It depends on how good the visibility is in the foggy location.
Haze is the weakest type of fog.
Ground fog - fog spreading over the ground or a body of water in a thin layer. This fog has no great influence for visibility.
Translucent fog with visibility ranging from several tens to several hundred meters. Through such a fog, the sun and clouds are visible.
Solid fog, when a whitish cloud envelops the earth, through which it is impossible to see literally nothing at a distance of several meters, and sometimes even over an outstretched arm. With such fog, traffic becomes impossible. If the driver is caught in a cloud of solid 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 industrial activities person. Artificial fog consists of dust, smoke, exhaust gases, chemical substances, other combustion products. Otherwise, it is called smog. Smog-one of critical issues modern cities, as it causes irreparable harm to human health and pollutes the environment.
