ATMOSPHERIC MOISTURE 63 



surface of the rock cairn. Similar crystals 18 inches long have been 

 reported from these summits. 



Frost. Frost results from a condensation of the atmospheric 

 moisture at a temperature below the freezing point, in contact with 

 cold objects. It has the same relation to dew that snow has to rain. 

 The atmospheric moisture freezing in the finer rock crevices becomes 

 a powerful agent in causing rock disruption. The freezing of water 

 in these fissures likewise acts as a powerful wedging agent (see, 

 further, Chapter IV.) 



Clouds and Fogs. Clouds are the condensed atmospheric vapor 

 remaining suspended in the air, and fogs are clouds resting upon 

 the surface of the earth. When condensation takes place above the 

 freezing point, the clouds and fogs will consist of water particles ; 

 if below that point, they will be ice particles, constituting air frost. 

 Clouds form an effective thermal blanket, preventing radiation and 

 the cooling of rock surfaces. In regions of much cloudiness, shat- 

 tering of rocks by insolation is reduced to a minimum. The diame- 

 ter of the droplets of water in clouds and fogs has been estimated 

 at 0.0085 mm., and it is owing to this small size that they remain 

 suspended. The types of forms which clouds assume are: cumulus, 

 thick clouds with horizontal base and more or less dome-shaped up- 

 per surface ; stratus, horizontal sheets of lifted fog ; nunbiis, or rain 

 clouds, of thick layers of dark clouds with ragged edges ; and cirrus, 

 high, thin, feathery, or fibrous clouds, often consisting of particles 

 of snow or ice. Intermediate types are also recognized. 



Rain, Snow, and Hail, The moisture of the atmosphere is 

 precipitated as rain, snow, or hail, when the particles become heavy 

 enough to fall. If they begin as frozen particles of snow in the 

 upper air, they may melt on passing through a stratum of warmer 

 lower air and so change to rain, or, if the air is dry, even evaporate. 

 Thus snow may fall on the mountain top and rain in the valley, 

 or no precipitation occur in the valley, while rain or even snow 

 falls on the summits. When water falling as rain passes through 

 a colder stratum of air, it may freeze, and hail will result. Hail 

 also results from the enlargement of snow crystals which pass 

 through a very moist atmosphere and so surround themselves with 

 layers of ice. 



Amount of Rainfall. Sir John Murray (68) estimates the total 

 annual rainfall of the globe at 29,350 cubic miles. Of this amount 

 2,243 cubic miles of rain falls on the inland drainage areas of the 

 globe, such as the Caspian, the Sahara, and similar districts. Of 

 this the Sahara alone gets 728.9 cubic miles, or an average of 

 31,106,860 cubic feet per square mile of area, while the Kalahari 



