Fog is a cloud near the surface of the earth. There is no difference between fog and cloud in the sky. When a cloud is near the surface of the earth or sea, we call it "fog".
Fog usually forms at night and early in the morning in lowlands and over water bodies. It is associated with a cold flow of air that descends onto the warm surfaces of land or water.
Fogs are more common in autumn, when the air cools faster than the ground or water. In calm weather, with the onset of darkness, thin layers of fog form in low places above the ground. As the ground cools at night, the lower layers of air also become colder. When such cool air comes into contact with warm air, fog is formed.
As a rule, urban fogs are denser than rural ones. City air is full of dust and soot, which, when combined with water particles, form a dense blanket.
The most foggy region of the Earth is the Atlantic coast of Newfoundland (Canada), where fogs are formed when the passage of humid warm air over cold waters that move south from the Arctic Circle. The coldness of the water condenses the moisture in the air into small droplets of water. These droplets are not large enough to form rain. They are in the air in the form of fog.
But fogs in the San Francisco area form completely differently. Here, the cool morning breeze blows towards the warm sand dunes, and if rain the day before has soaked the sand, a dense layer of fog forms from the evaporating moisture.
Often the fog appears denser than the clouds. This is because the mist droplets are smaller in size.
Many tiny droplets absorb more light than the larger (but smaller) droplets that form a cloud. So it seems to us that the fogs are thicker than the clouds.
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Fog is formed as a result of collision of air flows with different temperature. It is created when heated air comes over cold water or meets with a mass of more humid and cold air.Finding yourself in the middle of a continuous white cloud, so dense that it is practically impossible to distinguish anything at arm's length, you often ask yourself the question: why such a thick fog formed, why did it white and you begin to wonder how long this phenomenon usually lasts, and also why any fog dissipates.
Fogs are formed when drops or ice crystals accumulate in the air in the lower layers of the atmosphere, due to which a cloud-like veil is formed along the earth's surface, limiting visibility so much that space beyond one kilometer is not visible, and in some cases objects become difficult to distinguish even at a distance several meters.
If the ambient temperature exceeds -10°C, the vapor blanket consists only of droplets. If the temperature fluctuates from -10 to -15°C, it is made up of water droplets and ice crystals, and when it is -15°C outside, the fog consists of small ice crystals, shimmering in the light of night lamps.
Why this phenomenon is formed is not difficult to answer: its appearance is due either to the evaporation of water from warm surface into cold air, or cooling of moisture-saturated warm air currents. For example, the appearance of ground clouds can often be observed in the evening or in the morning after the temperature of the soil and vegetation (grass) drops; the lower layers of the atmosphere cool so much that they begin to release excess moisture in the form of water droplets.
Another example, this time in winter, is fog over a river, lake or other body of water, on the ice of which an ice hole has formed: in cold weather there is always a veil over it, spreading over the water surface. This happens because the temperature of the water during frost is warmer than the ice surrounding it and the air in contact with it (because of this, the air above the water is always warmer than the rest and there is almost always fog over the river in the area of the ice hole).
After warm air mixes with cold air currents, it begins to cool, releasing steam and forming a cloud at the very surface of the Earth. Therefore, the fog over the river and other bodies of water is usually stable and long-lasting: cold and warm air currents and currents constantly mix here.
A striking example of this phenomenon is considered to be located in Atlantic Ocean Canadian island of Newfoundland. Due to the fact that two currents collide with each other here – the warm Gulf Stream and the cold Labrador Stream, local residents are forced to spend about one hundred and twenty foggy days a year among the haze.
Formation of terrestrial clouds
When air saturated with water vapor cools or mixes with colder air currents, droplets begin to be released into the atmosphere. After this, if there are tiny particles of dust above the earth’s surface, they begin to stick to them, layering on top of each other and forming larger droplets (the more dust in the air, the faster a cloud forms, so big cities almost always shrouded in a weak, almost imperceptible veil).
IN warm time year, the size of such a drop ranges from 5 to 15 microns, during frosts - from 2 to 5 microns, so the winter cold fog not as thick as summer. As soon as the drops reach the required volumes, objects turn out to be blurry and difficult to distinguish: the air becomes whitish in heavy fog and bluish in light fog.
The answer to the question of why this phenomenon comes in different colors is simple: smaller droplets scatter short blue rays better, while in dense ground clouds larger droplets and light waves scatter all rays equally, regardless of their length.
The water content of such clouds usually does not exceed 0.5 g/m3, but sometimes dense fog can contain up to 1.5 g/m3 (this water is enough for plants to receive the necessary moisture, this is especially important for vegetation in the arid regions of the planet). How impenetrable the shroud will be depends largely on the humidity of the air, which is usually between 85 and 100% during the occurrence of ground clouds:
- if visibility does not exceed 50 meters, thick fog is observed, and the number of drops is 1200 per cubic centimeter;
- if the space is visible at a distance of 50 to 500 meters - moderate (water drops in this case from 100 to 600);
- if visibility is a kilometer - weak (drops - from 50 to 100).
Fogs are also common during frosts, and the phenomenon can be seen even when the humidity does not exceed fifty percent. They can usually be observed in cities, especially at railway and bus stations, where the haze is formed by steam that appears during the combustion of fuel and is released into the air through chimneys and exhaust pipes.
Kinds
Terrestrial clouds do not always owe their origin only to nature: big number fogs occur in cities, and therefore they consist not only of drops and dust, but also smoke, soot, which are emitted by factory or chimneys, or occur after or during fires when forests, peat or steppe burn. Based on their origin, meteorologists divide fogs into dry (smoke, soot, etc. are to blame for their formation) and wet (only water and dust are involved), and often the second form flows into the first.
In turn, wet fogs, the formation of which is directly influenced by nature - this is evening, night or morning fog (this period is optimal for the formation of clouds creeping along the ground), meteorologists are also divided into groups:
- Underground. Evening or morning fog that spreads low over the earth's surface or body of water (for example, fog over a river). The shroud may be continuous, or it may be in separate wisps, and visibility will not exceed a kilometer.
- Translucent. Despite the fact that visibility along the surface is low and in some cases does not exceed several meters, clouds can be clearly distinguished in the sky. This type includes night, evening, and morning fog.
- Solid. Visibility of dense fog is very limited and often does not exceed fifty meters. The sky is almost invisible, so it is almost impossible to distinguish clouds. This is mainly evening, night and morning fog, and during cold weather when temperatures rise, cold fog can be seen during the day.
Why do fogs disappear?
The duration of this phenomenon varies and can range from half an hour to several days (especially during cold weather or when warm and cold air and water currents collide, for example, fog over a river). The main reason why any fog dissipates is the air warming up. Since a veil forms near the surface, after Sun rays it is heated, the air also heats up, 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 steam transforms into water droplets and forms clouds.
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.
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. room temperature– fog appears when warm layers of air cool sharply, forming 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 of the area. 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 form 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 can be any of the reasons listed above, enhanced by the large amount of solid microparticles of dust, combustion products and other industrial emissions contained in the city air.
These particles serve as moisture condensation nuclei, causing the fog to big cities not only forms 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 from 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 tiny droplets of water or ice crystals that accumulate as clouds above the surface of the earth and reduce visibility. This is common atmospheric phenomenon, which is found in almost everyone and at any time.
Fog is also considered one of the varieties, since it has similar characteristics to them. The formation of fog is influenced by a combination of various factors, such as water bodies, wind, relief, temperature and humidity.
Fog formation
Fog forms when the difference between air temperature and dew point is less than 2.5° C, that is, the temperature below which water droplets begin to condense and form dew. When water vapor condenses, it turns into tiny water droplets floating in the air. Water vapor accumulates in water for a variety of reasons, including water evaporation, transpiration from plant leaves, and movement air masses over warm bodies of water.
When and where does fog appear?
Fog usually forms when air humidity is high. However, it will not form when relative humidity 100% when the air is unable to hold any additional moisture. If the air reaches 100% relative humidity, tiny droplets of water begin to combine to form larger ones. In such cases, rain or drizzle occurs.
Fog varies in thickness and density depending on the height of the inversion boundary, which is the point where air temperatures begin to drop.
Types of fogs
Different types of fog are classified depending on the factors of formation and are divided into cooling and evaporation fogs. In turn, cooling mists include:
- Radiation fog is formed due to the cooling of the Earth's surface after sunset. Therefore, radiation fog occurs at night and dissolves shortly after dawn.
- Advection fog occurs when, during the process of advection, airborne moisture passes over cold surfaces. This type of fog usually occurs over the sea.
Other types of fog include steam, ice, sea, frontal, dry, etc.
Effect of fog
As fog forms in the air, it greatly reduces visibility and can affect certain human activities, especially transportation. Droplets in fog vary in concentration, which can result in partial or no visibility.
The aviation industry is particularly susceptible to fog, as reduced visibility can make flying unsafe, especially during takeoff and landing. Because of heavy fog planes remain at airports, which is not only inconvenient for travelers, but also impedes the timely transportation of cargo.
Numerous car accidents occur every year as a result of unsafe road conditions caused by fog.
Fog is essentially a cloud that exists low above the surface. It appears when warm, moist air inevitably comes into contact with colder air.
The level of water vapor concentration in the air is determined by temperature: the colder the air, the less vapor it can contain. If the vapor exceeds the amount possible at a given temperature (a point called the saturated vapor level), it condenses into fog.
If the temperature is low enough, fog can form even in relatively dry air. Fog is most likely to be present in air containing a large number of dust or other particles to which water droplets may attach. In polar regions, where temperatures can drop below -15°C, frozen fog consisting of ice crystals is sometimes observed.
Condensation is everywhere
The same condensation process that creates fog accompanies some ordinary Everyday life phenomena. For example, cold air outside the window cools the warm air in the room (1, above). As the indoor air cools, the water vapor condenses, forming water particles that cause the window to fog up. When warm breath comes out of your mouth wet air(2), it cools quickly and the water vapor condenses, causing the exhaled air to resemble fog. Cold juice in a glass (3) radiates coldness around itself, causing water vapor in the air to condense and form drops on the glass. The water vapor escaping from the kettle (4) is cooled by the air and condenses into a fog-like cloud.
How is radiant fog formed?
At night, as the soil begins to release the heat it absorbed during the day, the air temperature above it begins to drop. When sufficiently cooled, the water vapor condenses into a radiant mist. This type of fog often appears in low-lying areas on clear and relatively windless nights. (In the picture at the top of the article)
Formation of advective fog
Advection fog, common in areas close to the oceans, occurs when a mass of moist, warm air suddenly rises above a cold surface. The lower layer of air cools, causing condensation and the formation of fog particles.
How is creeping fog formed?
As warm, moist air rises up the mountainside, it expands and cools, causing the water vapor in the air to condense into fog. Climbers often encounter this kind of fog, which can resemble uneven patches. If the air currents continue to rise, the spreading fog eventually turns into clouds.
How is steam mist formed?
Sometimes cold air circulates over places that retain heat at night - such as rivers or ponds. Steam condenses in cold air warm water, forming steam mist. The greater the temperature amplitude between water and air, the thicker the fog will be.