Marvels of the Universe 



437 



instance, the swiftly changing and passing shower-clouds of springtime are quite different from the 

 beautiful wa\y bands which we call a " mackerel sky." The natural association of certain cloud- 

 forms \\-ith certain kinds of weather demanded some means of distinguishing between these different 

 forms ; and, though space does not permit of our here entering into the history of cloud nomen- 

 clature, it will be sufficient to say that in 1891 an International Meteorological Conference 

 authorized the publication of an atlas and classification of cloud-types for the guidance of observers. 

 This classification has been generally accepted, and the names here mentioned are in accordance 

 therewith. 



Clouds owe their origin to the sun. The sun's raj's heat the ground, and the air lying imme- 

 diateh" above the ground thus becomes warm and charged with e\-aporated moisture, and this 



LENTICUL.AR' FORM OF CIRRO-CUMULUS. 



It is to the Lenticular form of the Cumulus clouds that we owe many of our most gorgeous sunset effects. 



warm, moist air, being lighter than the colder air above it, naturally rises upward. When this 

 ascending air-column reaches a height where the temperature is sufficiently low, the water-vapour 

 which it contains condenses into very minute drops of water, and these then become visible as a 

 cloud. A simple illustration of this process is to be seen in the familiar puffs of steam from the 

 exhaust of a locomotive. This condensation is repeated at successively higher levels, and conse- 

 quently we meet with clouds at greatly differing heights. 



Highest of all float those delicate filaments resembling feathers or wisps of hair, popularly known 

 as " mares' tails," but in cloud classifications called Cirrus. Twenty-nine thousand feet is the average 

 height of such clouds, practically the same as that of Everest's snowy peak. At such a great 

 altitude the temperature is far below freezing-point, and these clouds are therefore composed of 

 ice-crystals. Sometimes the Cirrus becomes sufficiently extensive to cover the whole sky with a 

 pearly or silvery .sheet, and is then known as Cirro-stratus. When this occurs, we frequently see 



