Licence: Public Domain Mark
Credit: The atmosphere in relation to human life and health. Source: Wellcome Collection.
Provider: This material has been provided by the Augustus C. Long Health Sciences Library at Columbia University and Columbia University Libraries/Information Services, through the Medical Heritage Library. The original may be consulted at the the Augustus C. Long Health Sciences Library at Columbia University and Columbia University.
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![change is signalized in the neutral band of calm by a dense yellow haze, i)rodu<;ing great darkness in winter, the result of a bunking up of sniok(> to some altitude, together with the condensation of vapor b}' the mixture of currents differing in temperature. The darkness in such a, band lasts much longer with lighter winds, and I have known a Avest wind to i)revail at lliclimond simultaneously with an east wind in London, botli without fog, while at Wandsworth a calm continued for many minutes with dense, almost nocturnally black smoke fog, the pressure in each direction being apparently e(]ual. FOG, SMOKE, GASEOUS AND SOLID IMPURITIES IN THE AIR. Fog is the result of one or both of two principal causes. The first is active radiation into space from the earth and from the air contiguous to it, and the second is a mixture of winds and currents, or of vapor and air at different temperatures. 1. Radiation fogs occur commonly when the atmosphere above the lowest stratum is cold, dry, and nearly still, and when the lowest stratum is greatly cooled by contact with the cold radiating earth, and therefore precipitates vapor into the form of minute globules of water. These globules themselves have a large radiative capacity, so that they tend further to reduce the temperature of the air in which they float, which has no such capacity. The stratum of fog so formed, not extending very many feet above the ground, fails to reflect much of the heat radi- ated from below, and quickly disperses, by radiation into space, what- ever heat it absorbs. Thus earth and fog continue rapidly to part with their heat through the clear sky into space. The stratum of fog often grow^ in height and density through the night, and continues till about noon of the following day, or disperses in the late hours of the morn- ing. If extended over a plain and watched from a height above the upper level, a fog of this character, in somewhat damp and not typical radiation weather, may be seen gradually to move irregularly upward under the influence of the morning sun, and in various directions to present prominences like thobe of the upper edge of cumulo-stratus. Smoke issuing from a tall factory chimney rises through and above the fog, but in a very short time falls back upon its surface and meanders like a dark river on a white ground.^ The persistence of the fog depends upon the coldness of the ground, which is shielded from the sun, and upon the very large diflerence of temperature, sometimes 10 degrees or more, between the fog and the stratum of air a few feet above it. When, however, the sun's heat absorbed by the water particles exceeds the heat lost by radiation, the fog lifts, that is, its upjiermost stratum rises, owing to diminished specific gravity, and ' These observations were taken during the prevalence of a ground fog, in the country surrounding the Malvern Hills, in February, 1890.](https://iiif.wellcomecollection.org/image/b21208724_0043.jp2/full/800%2C/0/default.jpg)
