Fog – the meteorological phenomenon at which in the atmosphere there is a high content of water vapor. At warm air temperature the fog represents a congestion of the smallest drops of water, and at cold ice kristallik which sparkle in the sun are added to them.
Fog is formed over the surface of Earth or water at emergence of the climatic conditions favorable for condensation of water vapor. However fog can be not only the natural phenomenon, but also artificial. Such fogs are called radiation because of cooling of air under the influence of radiation. Natural fogs more dense, than artificial, and their duration is from several hours to several days. In fact, fog is a cloud which is at the Earth's surface or water. Formation of fog happens most often at night and early in the morning in low-lying places and over reservoirs. It is connected with the fact that when cold night or morning air falls by the warm earth or water, moisture is condensed and the set of easy water drops hangs in air. Relative air humidity in the place of emergence of fog is close to 100%. Depending on air temperature the structure of fog has different structure. At a temperature over 10 degrees of a frost this cloud from the smallest drops of water, from-10 to-15 degrees it represents mix from drops of water and small kristallik of ice, at a temperature below-15 degrees the fog completely consists of ice kristallik and is called ice. In settlements the fog is more dense due to condensation of water vapor from exhaust gases. On visibility level the fogs divide into several types: haze, shallow fog translucent and continuous fog. The haze is very weak fog. Shallow fog creeps on the ground or water, as a rule, a continuous thin layer, and not really strongly affects visibility. At the translucent fog the visibility makes from several dozen to several hundred meters, at the same time through it illuminate the sky, clouds, at night – stars and the moon. Continuous fog covers the earth with a whitish cloud through which it is difficult to distinguish objects and buildings at distance of several dozen meters. At this fog in air the dampness is clearly felt, it is impossible to make out the sky, clouds, the sun. Traffic, especially aviation is at a loss. Fog arises not only at contact of cold and warm air, but also at evaporation, for example, over the sea or the damp land area. There are so-called dry fogs which consist not of water, and of smoke, dust and a soot. Sometimes over the cities there is a mix of dry and damp fogs, for example, at emission in damp air of mass of firm particles by smoke or exhaust pipes. Artificial fog is formed as a result of industrial human activity, it is called still a photochemical smog. It arises at emergence in the atmosphere of various pollutants, such as products of burning of fuel, vapors of gasoline, chemical solvents, paints, pesticides, nitrates, etc. The photochemical smog is one of the most important problems of modern megalopolises. High content of harmful chemicals in air leads to deterioration in health and even death. Especially the children and elderly people having weak immunity suffer. Long stay in the field of industrial fog leads to breath difficulty, exacerbation of heart diseases, headaches, cough, poisoning and so forth. However the photochemical smog can arise not only because of the person, but also, for example, at a volcanic eruption when in air there is a high concentration of sulphurous gas.