TEE FORMATION OF CLOUDS. 495 



THE FORMATION AND MOTIONS OF CLOUDS. 



Bv Pkofessok frank H. BIGELOW, 



U. S. WEATHEB BUREAU. 



^T^HE most beautiful objects in the sky are clouds, and their daily pro- 

 -*- cession from west to east in northern latitudes forms a moving 

 tableau of living pictures for those who have eyes to see. The glories 

 of the sunrise and sunset, decking the fading stars of the morning 

 and the waxing lights of the evening with the pure colors of the 

 spectrum, elevate the heart of man to a loftier adoration for the marvels 

 of nature, than any works of art prepared by his own hand. The 

 exquisite tints of the twilight in the northern, and the even richer 

 tones of the tropic zones, are painted on the memories of those who 

 have crossed the equator. I have seen on the Island of Ascension a 

 set of spectrum bows spread over the evening clouds, as if several 

 rainbows had conspired to illuminate the heavens at the same moment. 

 There are possibly two or three other objects in the sky that rival 

 or even excel these cloud pictures in delicacy of light and shading. 

 They are the aurora with its quivering beams dancing in the cool 

 atmosphere of the polar night, and the star clusters of the nebulae in 

 the milky way near the Southern Cross, viewed through the great 

 refractors of the south. 



Such effects are produced by the prismatic action of the small 

 spheres of condensed aqueous vapor that make up the cloud. The rays 

 from the sun, when it is near the horizon, pass through these crystal 

 spherules at such angles that the emergent light is spread out in 

 numerous spectra. The white light coming from space is singly and 

 doubly reflected and refracted within the surface of each aqueous 

 globule, so as to become separated into the bands of color which cor- 

 respond to the wave-lengths of the solar radiations. The rainbow 

 is a typical illustration of this process, but an illuminated cloudlet 

 represents a million minute bows intersecting each other in all possible 

 directions. The colored clouds of the morning and evening are bright 

 because the light from the sun passes at proper angles to the cloud, 

 and from the cloud to the observer, through a given thickness of the 

 vapor, till the refracted rays are brought down to the earth. The mid- 

 day clouds are white and glistening, because the sun's rays pass through 

 them as scattered instead of as refracted light, dancing from drop to 

 drop in zigzag courses till the last reflection brings it down to the eye 

 of the observer. 



