Popular Science Monthly 



243 



Alto-cumulus clouds 



cloud caps is the "table cloth" that 

 spreads itself over Table Mountain, near 

 Cape Town, when a moist wind blows 

 in from the sea. 



Sometimes the lo- 

 cal topography 

 causes the wind that 

 has swept up over 

 the c r e s i of the 

 mountain to form a 

 second "standing" 

 atmospheric wave to 

 leeward of the 

 mountain, and this 

 may also be marked 

 by a cloud, which, 

 like the cloud cap it- 

 self, presents a de- 

 lusive appearance of 

 permanence, while it 

 is really in constant 

 process of formation 

 on the windward 

 side and dissipation 

 on the leeward. The 



pair of clouds thus formed — one over the 

 mountain and the other at some distance 

 from it — is exemplified in the well-known 

 "helm and bar" of Crossfell, in the Eng- 

 lish Lake District. 



Of all clouds the most majestic are 

 the mountainous masses of cumulo-nim- 

 bus that attend our summer thunder- 

 storms. The formation of these clouds 

 can often be watched from its early 

 stages. On a hot, still day the warm air 

 near the earth's surface streams upward 

 by virtue of the same "convective" proc- 

 ess that accounts for the draft of a chim- 

 ney. The diminished pressure prevail- 

 ing at higher levels permits the air to 

 expand, and expansion causes it to cool. 

 When the ascending column reaches a 

 sufficiently low temperature, its water 

 vapor condenses into cloud. The first 

 visible stage is the appearance of a small 

 cumulus, rounded above and flattened on 

 the under surface, constituting the capi- 

 tal of an invisible column of rising air. 

 This occurs at an average altitude of 

 from four thousand to five thousand 

 feet above the earth. In the course of 

 the afternoon one sees these clouds grow 

 and coalesce, until they have towered up 

 to enormous heights ; often ten thousand 

 feet or more. Very often the summits 



become fringed with feathery ice clouds, 

 called "false cirrus," but really identical 

 in structure with true cirrus or cirro- 



Cumulus and alto-cumulus (above) 



stratus. Sooner or later the violent at- 

 mospheric circulation that produces 

 these clouds culminates in disruptive 

 electrical discharges, rain, and hail. 



Similar clouds are not infrequently 

 formed over great fires, and almost al- 

 ways over a volcano in powerful erup- 

 tion. In the latter case an actual thun- 

 derstorm is commonly generated. 



Apart from their shapes, clouds pre- 

 sent interesting phenomena of color and 

 give rise to a great variety of luminous 

 appearances, including rainbow, halos, 

 coronas, and the like. These yield much 

 information concerning the structure of 

 the clouds in which any occur. Thus 

 halos occur only in ice clouds, rainbows 

 only in water clouds. The corona (not- 

 withstanding statements found in many 

 books on meteorology) probably never 

 occurs in ice clouds, though it is some- 

 times due to fine dust in the air. The 

 colors of the rainbow, often described 

 as invariable, really differ considerably 

 from one bow to another, according to 

 the average size of the water drops in 

 which they are generated. 



Beautiful iridescent colors may some- 

 times be detected in clouds, especially 

 along their borders, and not pertaining 

 to a true halo, corona, or rainbow. 



