What Is Fog?
The fog which we generally think of when we speak this word is the fog at or on the sea or other body of water—the one that makes the ships stand by and blow their fog horns. A fog of this kind is nothing more nor less than a cloud, come right down to earth and spread out a little more. People who have gone up into the air in balloons and other airships through the clouds, say that the clouds are only fogs, and that above them it is as clear as it is on a sunshiny day on the water when there is no fog.
There is another kind of fog which settles down over the land, especially in the cities. It is a damp mist which combines with the smoke and other impurities in the air and forms a black and dirty cloud about everything. This occurs when the upper air prevents the smoke which rises from a city with all its people and fires in the furnaces from passing up and away. The upper air acts like a blanket and keeps the misty, smoky air down, until the wind comes along and blows it away.