THE MOISTURE OF THE AIR: FOG AND CLOUD
Fog
In his "Floating Matter of the Air" Tyndall demonstrated that, when the air pressure under the receiver of an air pump was reduced, the cooling of the air by expansion produced a perceptible fog. He demonstrated also that, if the air admitted to the receiver were filtered, a second exhaustion would produce a fog only to an extent scarcely observable, or not at all. In other words, the dust motes and molecules of hygroscopic gases are necessary for condensation. When there were no longer any dust motes, there was no condensation.
Fog and cloud are the most striking examples of condensation on a large scale; in weather science it is commonly called volume condensation. One cannot readily make a distinction between fog and cloud; in general, fog is cloud on the ground, while cloud is fog high in the air. When the blue sky becomes white, the change in color is due to condensation—perhaps water dust, perhaps ice dust. If the condensation thickens, distant objects become blurred by the accumulated condensation; a moisture-haze, quite distinct in color from the blue dust-haze, occurs. Perhaps the white sky might not be called cloud; but if the condensation increases until the color becomes a dark gray, by common consent it is "cloud" in the air, or "fog" if it extends to the ground. The distinction is merely one of degree. Fog and cloud are examples of condensation; but until the droplets coalesce into drops that fall to the ground they are not precipitation.
It is likely that fog and cloud droplets vary much in size; but definite knowledge of the extent of this variation is wanting. Wells found that fog droplets were approximately 0.0002 inch (0.005 mm) in dimension, and that the fog droplets in a cubic yard were not far from 7 grains in weight.
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