OPTICAL PHENOMENA IN METEOROLOGY
THE colors of the sky and clouds depend principally on the presence of water vapor, clouds and dust in the air; but there is one important exception that we will consider first--namely, the blue and associated tints of the clearest atmosphere.
The Blue Sky. --The researches of Lord Rayleigh have made it seem almost certain that we should have an azure sky even without the presence of moisture or dust, because the individual molecules of oxygen, nitrogen, and other gases, whose diameters are less than the wave-length of the light falling upon them, diffuse it according to the law that the ratio of the scattered to the incident light is inversely proportional to the fourth power of the wave-length.
Probably in many cases selective absorption, especially of the blue, and selective reflection from the earth and from the clouds, modify the colors of the sky. There is also some evidence of fluorescence in the upper atmosphere.1 Because of the inverse fourth-power law an excess of blue light emanates in all directions from every gaseous particle and fine dust- mote in the atmosphere, and the result is the blue light of the sky.
Always Dark at the Top of the Atmosphere. --When an observer is on the summit of a mountain or when he ascends in a balloon, the bluish tint becomes feebler in proportion as the mass of the air above him diminishes and the blue sky gradually becomes darker or blacker until the brighter stars begin to be visible and the zenithal portion of the sky has the color of late twilight, and, granting its possibility, if one should ascend to the upper limits of the atmosphere, where there is nothing to diffuse the light rays, he would find himself surrounded by total darkness.
Whitish Light Reflected only from Large Particles of Lower Atmosphere. --On the other hand, as the observer occupies a lower position, and es-____________________