When we look up into the sky on a cloudless day we behold a continuous canopy, the prevailing color of which is blue. This canopy is a veil that hides the starry hosts beyond, and its presence seems, at first sight, incompatible with the fact that the air is a transparent medium. We see the stars by night through the same intervening atmosphere. Why are they cut off from our sight by day? The answer to this question can, perhaps, best be made plain by a simple experiment. Place a lighted candle behind a sheet hung across a room not otherwise illuminated. The flame of the candle will be distinctly visible through the sheet. Next, let the room be brightly lighted, say with electric light or daylight. The candle can now no longer be seen through the sheet, owing to the bright illumination of the latter as compared with the feeble light of the candle.
In the atmosphere the counterpart of our sheet is a layer, several miles in depth, of minute particles, which by day are lighted up by the sun. Some of the particles are tiny dust motes, others are fine droplets of water or bits of ice, and the rest are the molecules of the atmospheric gases themselves. It is the light that comes to us from these particles that makes our eyes insensitive to the fainter light of the stars, and makes the sky itself a visible luminous vault.
Next, why is the clear sky generally blue, rather than some other color? To answer this question, we must recall the fact that sunlight is made up of ether waves of many different sizes. In combination, these waves produce upon our eyes the sensation of white light. When they are separated, as by passing through a prism, the smallest waves—or, in more technical language, the vibrations of shortest wave length—register the sensation of violet, and the largest or longest waves that of red. The whole sequence of colors runs in the order violet, indigo, blue, green, yellow, orange and red (easily fixed in the memory by means of the word VIBGYOR, formed from the initial letters of these words).
Now the passage of sunlight through the atmosphere is obstructed to a certain extent, not only by suspended dust particles, but also by the molecules of the air. Let us consider, first, the effect of air molecules and of the finest dust particles, not much above molecular size. These tiny objects have different effects on light waves of different lengths. The longest waves are little disturbed by them, just as ordinary waves in water are little affected by a floating cork, for instance. The shortest waves are so small in proportion to the size of the obstacles that they are diffused or scattered by them, as a tiny ripple in water might be broken up by a floating cork. It is this diffuse light, of short wave length, that gives the sky its color. A large part of the violet and indigo light is lost by further scattering before it reaches the earth, leaving a preponderance of blue in the sky as we see it. When the air contains a considerable amount of suspended particles larger than those above considered—whether in the form of solid dust or crystals of ice or tiny droplets of water—light of all wave lengths is reflected by them, and the sky looks white or grayish.
On account of the action of atmospheric particles in filtering out the shorter light waves, as just described, sunlight becomes relatively rich in red and orange in passing through the air. When the sun is high, the path of the sunbeams to the earth is short, and the color of their light is but little affected. Near the time of sunrise and sunset, however, sunlight comes to us through a much greater extent of air, and the filtering process is much more effective. Hence the sunshine is both enfeebled and reddened when the sun is near the horizon. The diffuse light of the sky around the sun is filtered in the same manner, and therefore is commonly red when the sun is low.
A gray sunset sky after a clear day is due to the presence of water drops in the air, and indicates conditions favorable for rain, since, unless the air were saturated to a considerable altitude, the comparatively warm sunshine of the afternoon would favor evaporation rather than condensation of moisture. A gray sunrise sky has, as a general rule, just the opposite meaning. It often indicates the presence in the air of water drops formed on dust particles during the night, after the manner of dew, because the upper air has been dry enough to permit rapid radiation from the dust. These drops will be speedily evaporated by the rising sun, and the general dryness of the atmosphere will not favor further condensation. Several familiar weather proverbs are thus justified, e. g.:
Evening red and morning gray
Help the traveler on his way;
Evening gray and morning red
Bring down rain upon his head.
There are many other interesting optical phenomena connected with sunrise and sunset, including, first of all, the morning and evening twilight. When the sun, or any other heavenly body, is only a little below a clear horizon, it is still visible, on account of the bending of its rays by the atmosphere. This lifting effect, known as astronomical refraction, amounts to about half a degree (at the horizon), which is about equivalent to the apparent diameter of the sun or moon. As the sun sinks farther below the horizon, in the evening, the only daylight that comes to us is that reflected from the upper levels of the atmosphere, which are still illuminated. This is called twilight, and it lasts until the sun is about 18 degrees below the horizon, when total darkness sets in. The period as a whole is sometimes called astronomical twilight, in distinction from the briefer period known as civil twilight, during which there is light enough for outdoor occupations; the latter lasts from sunset until the sun is about 6 degrees below the horizon. Morning twilight is more commonly called “dawn.”
An interesting succession of light and color effects is observed before and after sunset and, in inverse position and order, about sunrise. Considering sunset only: After the sun has sunk out of sight, a broad band of golden light, called the bright segment, is seen along the western horizon. Above this, in the western sky, appears a more or less circular expanse of rosy glow, known as the purple light. In the eastern heavens, after sunset, there rises steadily from the horizon the so-called dark segment, which is the blue or ashy shadow of the earth on the sky. This is bordered above by the pink or purplish antitwilight arch. As time goes on, the purple light in the west, after increasing in brightness for a while, finally sinks behind the bright segment; while in the east the rising dark segment encroaches upon and finally obliterates the antitwilight arch. Sometimes, in clear weather, there is a fainter repetition of these lights and colors (second purple light, etc.).
Among the Alps and other snow-capped mountains, these sunset and sunrise phenomena assume a particularly beautiful form, known as the Alpenglow. In fine weather, just before sunset, the peaks to the eastward begin to show a reddish or golden hue. This fades gradually, but in a few minutes, when the sun is a little below the horizon of the observer, but the peaks themselves are still bathed in direct sunlight, an intense red glow, beginning down the slopes, moves upward to the summits. This is identical with the antitwilight arch described above. Presently this glow is succeeded by an ashy tint, as the peaks are immersed in the rising shadow of the earth (the dark segment). Their rocks and snows assume a livid appearance, aptly described by the inhabitants of Chamonix, whence the phenomena in question are well seen on the summit of Mont Blanc, as the teinte cadavéreuse. In ordinary weather darkness succeeds without any further notable phenomena, but occasionally there occurs a remarkable renewal of rosy light upon the peaks, known as the recoloration or afterglow. At Chamonix this is termed the “resurrection of Mont Blanc.” The afterglow has been variously explained, but it is probably due, mainly at least, to the reflection of the purple light in the western sky. Sometimes it lasts until an hour after sunset, and it passes away from below upward. On very rare occasions there is a second afterglow, presumably the reflection of the second purple light mentioned above. Similar phenomena are often seen in reverse order at sunrise.