Fog, or MIST. Water vapour is always present in the atmosphere, and it condenses either as rain, mist, or fog when the state of saturation is reached. In some cases dust-particles condense moisture from non-saturated air, and this produces what is known as a dry fog. Condensation of moisture takes place on a drop more readily the larger it is (see CAPILLARITY), and experimental proof has recently been given by Aitken in support of the theory that no condensation can occur without the presence of a nucleus. Such a nucleus is furnished by the visible or invisible dust-particles in the atmosphere. The amount of vapour present and the number and size of the dust-particles in part determine whether fog, mist, or rain will be formed, under given conditions of temperature and pressure; but the gathering of moisture into drops sufficiently large to fall as rain seems not to depend merely on the number and size of the particles on which condensation occurs. When a stratum of warm, dusty air gets cooled, a fog may be produced. The great amount of smoke-particles and dust-particles present in the air of large towns furnishes the conditions, in certain states of weather, that give rise to the intense fogs often prevalent in large cities. A morning fog disappears as the temperature rises, because of evaporation of the moisture from the nuclei. A fog is often produced in the evening over the surface of water or moist ground, because the air is sufficiently cold for condensation of vapour to occur. The fogs on the coasts of Nova Scotia and other places are caused by currents of warm air moving over cold water; so also the fogs caused on the coasts of Oregon and California by west and north-west winds. On the outskirts of an anti-cyclone fogs of immense breadth sometimes stretch for hundreds of miles lengthwise. Aitken has pointed out that dust-particles are probably efficient in the production of fog or rain in another way besides acting as nuclei. He believes that dust is a good absorber, and therefore a good radiator, of heat; for he has shown that, on equally clear days, the sun's heat is strong if the number of particles per unit volume of the atmosphere is small, but is weak if the number is large. Hence, when the sun's rays are withdrawn, the air surrounding the particles is rapidly cooled, and its moisture condenses.
Clouds, whether of fog, mist, or rain, though apparently suspended in the air, are in reality falling with extreme slowness. The force which causes drops to fall is their own weight, which is proportional to the cube of their diameter. The force acting upwards is the resistance of the air, which is proportional to the diameter. Hence, if the diameter of a drop becomes of its original value, the resistance is reduced to only of its value, while the weight becomes of what it was before, and so the drop falls far more slowly.
One of the worst fogs on record, alike for its density and protractedness, occurred in London from the beginning of November 1879 to the following February. The deaths for the six weeks ending February 21st were 1730, 1900, 2200, 3376, 2495, and 2016, the deaths in the fourth week being thus nearly double those of the first. Of all diseases the deaths from asthma were most directly influenced in fatality by the fog. In the first three weeks of 1880, when London was largely cleared of fog, the deaths fell 30 per cent. below the average; but in the end of January, when the fog again became severe, the deaths rose to 43 per cent. above the average. Bronchitis, pneumonia, pleurisy, other lung diseases, and whooping-cough, though not showing so strict an obedience to the varying density and persistence of the fog, rose to a much greater fatality, the death-rate from bronchitis rising to 331 per cent. and whooping-cough to 231 per cent. respectively above the averages of these diseases. Fogs are worst in the low-lying districts which are on the lee-side of the city, with respect to the direction of the light drift of the wind at the time, and least felt in the higher districts on the windward-side, the amount of suffering and number of deaths being proportioned to the density and persistence of the fog.