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.
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