MOISTURE IN THE AIR 109 



will of course be rapid and the temperature registered will be 

 much lower than if the air were moist. The second thermom- 

 eter registers the true temperature of the air, and a comparison 

 of the temperatures registered by the two thermometers will 

 indicate the relative humidity. 



96. Forms of Condensation. Rain, snow, hail, fog, clouds, 

 dew, and frost are some of the different forms which result 

 from a lowering of the temperature of the air below the dew 

 point. Fog and clouds consist of tiny particles of liquid water 

 condensed from the water vapor in the air. The only differ- 

 ence between them is that one is near the ground and the other 

 at a considerable altitude above it. On a mountain top a 

 cloud which drifts across it is found to be only a heavy fog. 

 Clouds or fogs always occur when the temperature of the air 

 falls below its saturation point. A certain amount of dust in 

 the air seems necessary for the formation of clouds, each parti- 

 cle of dust forming a nucleus upon which moisture can con- 

 dense. When single clouds are formed in the sky, each is at 

 the top of a column of moist air which has risen high enough 

 to be condensed. Other clouds may be formed by moist winds 

 blowing into colder regions and their moisture being condensed. 

 Rain occurs when the particles in the cloud become so large 

 from further condensation that they no longer float in the air. 

 If the raindrops fall into layers of warmer air, they may be 

 again evaporated without reaching the earth. Over very hot 

 regions, such rain storms far above the earth are occasionally 

 observed. Dew is the moisture in the air condensed on objects 

 on or near the ground. It is incorrect to assume, however, that 

 all the moisture found on grass and other herbage in the early 

 morning is dew. A great deal of it is moisture given off by 

 the plants themselves in the form of liquid water. Rain and 

 dew occur only when the dew-point is above the freezing point. 

 When the dew-point is lower than the freezing point, hail, 

 snow, or frost results when condensation occurs. It may be 



