Page:Popular Science Monthly Volume 88.djvu/270

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��Popular Science Monthly

��the recondite, let us consider briefly the anatomy of a cloud. The highest clouds, cirrus, cirrostratus, and probably also true cirro-cuinulus, with an average al- titude of six or seven miles above the earth, consist of tiny needles of ice. All other clouds are composed of drops of Avater, and do not differ at all in struc- ture from an ordinary fog, which is simply a cloud resting on the earth.

These cloud particles are formed by the condensation of the invisible water- vapor (water in a gaseous state) which is at all times present in the air. Just as water-vapor condenses and becomes visible on the cold surface of an ice- pitcher, so, it is supposed, condensation occurs in the free air on the surface of extremely minute (mostly ultra-micro- scopic) grains of so-called "dust," when cooled to the critical temperature with respect to the amount of water-vapor present (the "dew-point"). The exact nature of this "dust" is not fully under- stood.

You will perhaps wonder how clouds composed of water can exist in cold weather, when our ponds and streams are all frozen to ice ; especially as it is a matter of common knowledge that the temperature of the air diminishes with altitude, so that wintry weather on earth implies wintrier weather in Cloudland. To find the clue to this enigma we con- sult the books on physics, and learn that, with proper precautions, it is possible to cool a liquid far below its ordinary freezing point (32 degrees Fahr. in the case of water). Clouds of "supercooled" water-drops are seen even in the polar regions. A sudden jar turns a super- cooled liquid instantly to a solid ; and thus it happens that, in cold weather, raindrops or fog particles turn to ice on coming in contact with terrestrial ob- jects, such as trees, telegraph wires, and the like, giving us the interesting spec- tacle of the "ice storm."

Clouds are Always Falling

Another paradox is the fact that the bits of ice and drops of water composing the clouds should appear to "float" in the air, though of much greater density than the latter. As a matter of fact they do not. Cloud particles are all the

��time falling relatively to the air around them ; though since this air itself may constitute an ascending current, they are not always falling in an absolute sense. The speed at which a cloud par- ticle falls through the air depends upon its size; the smaller the particle, the more slowly it falls. The smallest have diameters of the order of .0004 inch and fall in still air at the rate of about a tenth of an inch per second. The larg- est range up to more than a fifth of an inch in diameter, and fall at the rate of about twenty-six feet per second. Rain- drops and snowflakes are cloud particles which, in virtue of their size and other favorable conditions, succeed in falling all the way to the earth. Many a shower of rain or snow never reaches the earth, but evaporates in midair.

Reverting to the aspects of clouds as we see them from the earth, there are a few interesting phenomena that require notice. Cirrus and cirro-stratus clouds sometimes occur in long, narrow strips, extending across the sky, and, while really parallel, seem to converge toward two opposite points on the horizon on account of perspective. These strips are called "polar bands," or, popularly, "Noah's Ark." Parallel bands of cloud, whether in continuous strips or in sep- arate cloudlets, reveal the presence of waves in the atmosphere. Where a wave carries a body of water vapor upward the latter cools by expansion and con- denses to visible moisture. Thus the clouds mark the crests of the waves.

The "White Flag of the Chinook"

A kindred phenomenon is that of the "cloud cap" often seen over a mountain. Here the ascent of the air, with its charge of water vapor, is due to the up- ward deflection of the wind by the slope of the mountain. Sometimes the cloud cap, once formed, spreads far away to leeward of the mountain peak, consti- tuting a "cloud banner." .Such is the "white flag of the chinook," seen stretch- ing from the crest of a mountain ridge in our Western states when the chinook wind is blowing over it. The same phe- nomenon constitutes the "foehn wall" attending the foehn wind in the Alps. One of the most famous and striking of

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