Fog

(fig.) (Dan.,a storm.] (Meteor.) Acloud resting on or near the surface of the earth. Fogs appear whenever the temperature of the air falls markedly below the dew-point; so that, if any circumstance occurs either, (1) to lower the temperature of the air considerably, or (2) to pour more vapor into the air than it can hold in the form of invisible vapor, a mist or fog-the aggregate of the particles of condensed vapor-makes its appearance. Owing to the fact that a F. maybe caused in either of these two ways, fogs result from apparently contradictory causes. Thus, a river flowing from a cold to a warm region will often be covered with F., because it is colder than the surrounding air, which, becoming cooled below the dew-point, discharges its moisture in the form of P.; but, again, a river flowing from a warm to a cold région will also often be covered with F., because it pours more vapor into the air than can be retained in the invisible form. Fogs often appear on mountain slopes. The air which blows up the slopes is gradually lowered in temperature, and at length reaches a level where its temperature is lowered below the dewpoint, when condensation takes place. The fogs which occur in the winter months in large cities built on rivers are due to cold winds which flow in upon an accumulation of warm moisture-laden air. After mild weather, with prevalent southerly wind, a steady easterly current almost invariably causes a dense F. to make its appearance, the air being compelled to resign its moisture as the temperature gradually falls. [s. 372]