THE WATER VAPOR OF THE AIR 225 



Sometimes the beginning of this may be seen first in the few 

 inches of air near the ground, gradually deepening as the air 

 chills higher. Night fogs usually form first on low ground, 

 partly because of air drainage from the slopes (Art. 238), and 

 possibly, in part, because the low ground is usually more 

 moist and may give off more water vapor to the air resting 

 upon it. This raises the humidity . over low ground so that 

 the night cooling reaches the dew-point earlier there, and so 

 begins sooner to form fog over the damp lowlands. 



Deep, widespread fogs are formed in a different way; they 

 are often formed by the mixing of masses of warm air with 

 masses of cold air; sometimes, especially in winter, by warm 

 damp winds blowing over a colder region. 



255. Vapor Condensation ; Clouds. The lowest clouds and 

 all the denser clouds resemble fog. Clouds are formed by the 

 condensation of water vapor in the free air. They constitute 

 an almost endless panorama, full of interest and often of beauty. 

 Clouds are the messengers of fhe air; often they are harbingers 

 of sunny days; frequently they are outriders of a storm; 

 sometimes they are mere idlers in the vault of blue. They are 

 useful indicators of local weather changes to come. What the 

 clouds tell depends on what one knows of them, on the atten- 

 tion given to them, and also to some extent on the section of 

 country. Clouds do not give the knowledge of general storms 

 and their movements that weather maps give us; but they 

 often give additional information of considerable use in one's 

 own locality, and clouds may be valuable to those who do not 

 promptly receive the map forecasts. To make clouds useful, 

 the reader must become acquainted with them. The following 

 paragraphs offer a beginning and suggest how to continue the 

 study. 



Clouds are grouped into several classes. Figure 155 shows 

 the principal types in the order of their elevation. Some of 

 these forms are further illustrated in Figs. 156 to 164. The 

 following table gives the chief divisions and a number of their 

 combinations. 



15 



