Fig. 728.—Parhelia, or mock suns.
The colour of the atmosphere—the usual blue tint of the sky—arises from the blue rays of the spectrum being reflected more than the rest by the aerial particles, and the less vapour the bluer the sky, because the vapour gives it a whitish or misty tint. At sunset and sunrise the sky is red or yellow, like gold, or of crimson hue. This is because the sun’s rays have so much farther to come to us at sunrise or sunset, as you will readily perceive if you draw a line from the sun to the sides and then to the top of the arc of the heavens. The blue rays are thus lost in space, while red and yellow, which travel so much faster than blue, are transmitted to the eye, not giving the air time to absorb them.
If you go under water and look at the sun it will appear very fiery indeed, and we may likewise imagine that fiery crimson rays, which betoken atmospherical disturbance, very often are due to the moisture through which they are transmitted. Wet and storm frequently succeed a crimson sunset, which betokens much moisture in the air. The sun is similarly seen through the steam issuing from an engine, and the colours vary according to the density of the steam in its stages of condensation.
Fig. 729.—Mirage at sea.
Vapour, we know, is invisible and transparent, but when it has been condensed into rain-drops, and the sun is shining, if we stand with our backs to the sun we see what we call the rainbow, because a ray of light entering the drop is reflected, and as all rays are not of equal refrangibility, the light, which is composed of three simple rays, is divided and reflected into those and the complementary colours. When the sun is at the horizon, the rainbow, to an observer on the earth (but not on a mountain), will appear to be a semi-circle. The higher the sun rises the lower is the centre of the rainbow. So we can never see rainbows at noon in summer because the sun is too high. A second rainbow is not uncommon, the second reflection producing the colours in a different order. The colours in the “original” range from violet to red; in the “copy” they extend from red to violet. “Rainbows” are often visible in the spray of waterfalls and fountains.
Halos are frequently observed surrounding the moon, and then we are apt to prognosticate rain.
“The nearer the wane
The farther the rain,”