Page:Handbook of Meteorology.djvu/81

Rh Fog Types.—A common illustration of fog formation may be observed when a cake of ice is at the doorstep. Almost immediately it begins to “steam.” The ice chills the air in contact below the dew-point, and condensation is at once apparent in the form of fog. Condensation liberates enough latent heat to give the moisture a certain amount of updraught, and therefore a steaming effect. It is an instructive illustration of contact cooling, and the fog produced is the radiation fog of weather science.

On still nights during spring and fall, fog is frequent over rivers and ponds, especially in relatively low places. If the air is still over such bodies of water during the day, it is apt to be moist. Therefore the normal lowering of temperature soon reaches the dew-point and, as a result, a radiation fog forms. Sometimes its depth is only a few feet; occasionally it overtops buildings and trees.

In various instances fogs hover over manufacturing districts when nearby rural areas are free from them. It is pretty certain that the products of combustion are the “favorable nuclei” in such cases. Dr. Owen of the British Meteorological Office found that many such floating particles were extremely hygroscopic, and that they tended to produce condensation when it did not occur in air free from them. At all events, the city fog has become a factor in meteorology as well as in city traffic.

Advection fog is the name given to fogs that result when warm moist air invades a surface so cold that dew-point temperature is reached. The sea fogs of the North Atlantic are an example. Warm, moist winds of a southerly origin invade the region of cold Arctic currents, and condensation of the moisture brought to the region occurs. “Skin friction” between wind and water causes the eddying movements of the air known as turbulence, and the fog blanket extends higher and higher as