242 



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 



