Often, especially in the fall and in the winter, over a surface of the earth, the rivers, seas there are fogs. They can be both absolutely imperceptible, and such dense that through them it is difficult to see something.
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Fog represents the atmospheric phenomenon which is characterized by formation of a layered cloud on a terrestrial surface. It consists of the smallest drops of water or crystals of ice.
Fogs happen several types. It depends on the processes which are the cornerstone of their education and on the district where there is their emergence. Fog can be radiation, advective and frontal.
Radiation fog is not connected with harmful radiation in any way. Its second name "ground". The lower ground layer of air as a result of heat exchange with the earth is quickly cooled. Thus, warmer air rises above. If there is a windless weather, this atmospheric phenomenon or does not arise at all, or is expressed very poorly. At a small wind formation of fog goes more intensively. If flaws strong, it dissipates since layers of air mix up.
Most often radiation fog arises in the fall and in the winter when high relative humidity of air and long nights is observed. It also appears in the centers of areas with an elevated pressure which are usually characterized by a light breeze and lack of rainfall. The arisen in the evening or at night, such fog can keep for all day if air is rather steady.
Advective fog is formed over the district which temperature is lower than air temperature over it. In this case air quickly is cooled, begins fast process of vapor condensation. There is a dense and low fog. In other words, in the lower layer of the atmosphere of couples becomes saturated, and near a terrestrial surface the layered cloud of considerable vertical extent is formed. Advective fog can be formed at any time. Most often it happens on the sea coasts, and also in the districts which are partially covered with snow. In midlatitudes such fog can be formed in conditions when warm southern air masses are transferred to the North. Advective fog — the frequent guest over the high sea. It arises from movements of warm air over colder surface of the sea. Sea fogs can be long. Sometimes they do not dissipate for weeks.
Frontal fogs arise from interaction of two air masses with various properties. The place of their meeting is called frontal zones or fronts. Such areas often are in the atmosphere, however not all of them are followed by fogs. More often frontal fog can be observed before the warm front. Accompanied with rainfall, it can be rather long. Frontal fogs are very dangerous to all means of transport, especially to air traffic.
Fogs in megalopolises, mixing up with a smoke and exhaust gases, represent huge health hazard of people, especially for those who has cardiovascular diseases and diseases of respiratory organs. Besides, fogs in large industrial regions show, air is how polluted: the smog interferes with natural fall of temperature of air at night.