Page:Popular Science Monthly Volume 60.djvu/503

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THE FORMATION OF CLOUDS.

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THE FORMATION AND MOTIONS OF CLOUDS.
By Professor FRANK H. BIGELOW,

U. S. WEATHER BUREAU.

THE most beautiful objects in the sky are clouds, and their daily procession 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 correspond 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 midday 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.