TEE FORMATION OF CLOUDS. 



497 



or eight miles above the ground. See Chart 3. The alto-cumulus 

 clouds are smaller, more huddled together like the backs of a flock of 

 sheep, and they are from two to three miles high; the cirro-cumulus, 

 from four to six miles high, appear to be still smaller, and they often 

 arrange themselves in ranks or battalions and form the beautiful 

 mackerel sky. All these clouds are formed by the rising of currents of 

 air in a vertical direction, the flat bases showing the level where the 

 temperature begins to be cool enough to cause the vapor to condense, 

 while the sides and tops outline the relative amounts of pressure, tem- 

 perature and vapor which are just sufficient for the saturation to begin. 

 From a study of these elements we may compute the vertical gradients 

 for each 100 meters, or for each 100 feet, of elevation above the surface 

 of the ground, and these quantities are of great value to meteorologists. 

 The stratus or veil clouds are formed in a very different way. Instead 

 of producing the cooling necessary to condense the moisture by raising 

 it to higher elevations, it may also 

 be caused by the flowing of hori- 

 zontal currents of air in contact 

 with each other, as when a warm 

 current passes over a cold one. 

 When air from the south flows 

 northward and air from the north 

 moves southward into the same 

 region, these currents generally 

 overflow one another in two strata 

 instead of mixing, since masses of 

 air at different temperatures are 

 quite reluctant to lose their in- 

 dividuality. This stratification 

 of the air in horizontal sheets, 

 flowing from the tropics and from 



the polar zones, is always taking chart l. Example of the eastward move- 



,.,■,,■■ 1 mentofthe upper air above Washington, D. C, 



place m tne atmospnere, and in mlles per hour. The average eastward veloc- 



the stratus clouds are generally iiy at thesurlaceisemiles perhour; atSmiles 



-, -, 1 1 ±^ high it is 70 miles per hour in clear weather. 



produced somewhere along these 



surfaces of contact. The cumulus clouds therefore indicate, as shown 

 in Chart 1, that the air is rising vertically in certain layers, while drift- 

 ing eastward, and the stratus that it is moving horizontally with a 

 different velocity in adjacent strata. The lowest stratus clouds are 

 elevated fog; the strato-cumulus is about two miles high; the alto- 

 stratus averages four miles and the cirro-stratus six miles above the 

 earth. 



There is furthermore a stratification of the vapor contents of the 

 atmosphere within every high cumulus cloud, which is interesting. 



VOL. LX. — 32. 



