Page:Handbook of Meteorology.djvu/73

Rh proportional to the difference in the readings of the dry bulb and the wet bulb of a sling psychrometer. Evaporation increases very rapidly with a dry wind and more slowly as the relative humidity of the air increases. It increases rapidly with rising temperature and decreases with falling temperature. It increases inversely with barometric pressure. The rate of evaporation of sea water is about 95 per cent that of fresh water, all other conditions being the same.

Condensation.—The process whereby water vapor changes to a liquid form is condensation. Condensation may occur as a result of mechanical processes, such as pressure and artificial cooling; in the free air, however, it results from cooling by contact, cooling by mixture, or cooling by expansion—practically adiabatic cooling. More definitely: warm air resting on the ground, or on the sea, may be cooled by contact therewith, until some of its moisture is condensed. An area of warm, moist air may be invaded by a cold wind and the mixing process may cool the vapor to the temperature of condensation. A body of air warmed above the temperature of the surrounding air is pushed upward. Its expansion causes adiabatic cooling and if the temperature falls below that of saturation, condensation of the water vapor occurs. Practically all the cases of condensation with which weather science has to do result from one or another of the causes named. The condensation resulting from contact causes dew and frost; that which results from mixing causes fog and cloud; that resulting from adiabatic cooling—that is, updraught—causes rain and snow. There are occasional exceptions to the fore-going, especially where superficial turbulence of the air is involved; in the main, however, these processes of condensation are fundamental in weather science.

Dust Motes and Condensation.—The invisible, floating dust motes of the air and many of the gaseous products of combustion are important factors in condensation. Each droplet of cloud or fog condenses upon a dust mote or upon a hygroscopic gas product. In general, the dust motes which cool most