Dry fogs are characterised by a dull opaque appearance of the atmosphere. They are most common in certain parts of North America, though they sometimes occur in Germany and in England. They are generally referred to the electrical state of the atmosphere, but the theory of them is still a matter of doubt.
The term mist is generally applied to vapours that rise over marshy places, or the surfaces of water, and roll or move over the land.
369. What is the difference between a mist and a fog?
Fogs, as they are known to us, generally arise over the land, and are usually mingled with the smoke of large towns. Mists generally arise over water, or wet surfaces.
370. Why do mists and fogs disappear at sunrise?
Because the condensed vapours are again expanded and dispersed by the heat of the sun's rays.
371. Why do fogs frequently rise in the morning and fall again in the evening?
Because, warmed by the sun's rays, they become more rarefied, and fly away at an altitude where they appear to be altogether dispelled; but at night, when the earth cools by radiation, the vapours near the earth again condense, and settle in the form of fog.
372. Why do fogs sometimes rest upon a given locality for several days together, and then disappear?