ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 231 



change is signalized in the neutral band of calm by a dense yellow 

 haze, producing great darkness in winter, the result of a banking up of 

 smoke to some altitude, together with the condensation of vapor by the 

 mixture of currents differing in temperature. The darkness in such a 

 band lasts much longer with lighter winds, and I have known a west 

 wind to prevail at Richmond simultaneously with an east wind in 

 London, both 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 equal. 



FOG, SMOKE, GASEOUS AND SOLID IMPURITIES IN THE AIR. 



FOG. 



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 

 grows 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 those 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. 1 The persistence of the fog 

 depends upon the coldness of the ground, which is shielded from the 

 sun, and upon the very large difference 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 

 uppermost stratum rises, owing to diminished specific gravity, and 



1 These observations were taken during the prevalence of a ground fog, in the 

 country surrounding the Malvern Hills, in February, 1890. 



