Meteorology Part 5

Accordingly, I concluded that there must be clouds to the west, though I could not see them, which reflected rays of light and faintly illuminated the shadowed part of the moon. It had become much colder, and I concluded that during the night the cloud-particles, if driven near by the wind, would condense into rain. And, a.s.suredly, next morning I was gratified to find that rain had fallen in large quant.i.ties, substantiating the theory.

There is much pleasure in verifying such an interesting problem. The dark body of the moon being more than usually visible is one of our well-known and oldest indications of coming bad weather. And at once came to my memory the lines of Sir Patrick Spens, as he foreboded rain for his crossing the North Sea:--

"I saw the new moon late yestreen Wi' the auld moon in her arm; And if we gang to sea, master, I fear we'll come to harm."

This lunar indication, then, has a sound physical basis, showing that near the observer there are vast areas of clouds, which are reflecting light upon the moon at the time, before they condense into rain by the chilling of the air. According to the old Greek poet, Aratus: "If the new moon is ruddy, and you can trace the shadow of the complete circle, a storm is approaching."

CHAPTER XVIII

AN AUTUMN AFTERGLOW

A brilliant afterglow is welcomed for its surpa.s.sing beauty and a precursor of fine fixed weather.

A glorious sunset has always had a charm for the lover of nature's beauties. The zenith spreads its canopy of sapphire, and not a breath creeps through the rosy air. A magnificent array of clouds of numberless shapes come smartly into view. Some, far off, are voyaging their sun-bright paths in silvery folds; others float in golden groups. Some ma.s.ses are embroidered with burning crimson; others are like "islands all lovely in an emerald sea." Over the glowing sky are splendid colourings.

The flood of rosy light looks as if a great conflagration were below the horizon.

I remember witnessing an especially brilliant sunset last autumn on the high-road between Kirriemuir and Blairgowrie. The setting sun shone upon the back of certain long trailing clouds which were much nearer me than a range behind. The fringes of the front range were brilliantly golden, while the face of those behind was sparklingly bright. Then the sun disappeared over the western hills, and his place was full of spokes of living light.

Looking eastward, I observed on the horizon the base of the northern line of a beautiful rainbow--"the shepherd's delight" for fine weather.

Soon in the west the light faded; but there came out of the east a lovely flush, and the general sky was presently flamboyant with afterglow. The front set of clouds was darker except on the edges, the red being on the clouds behind; and the horizon in the east was particularly rich with dark red hues.

Gradually the eastern glow rose and reddened all the clouds, but the front clouds were still grey. The effect was very fine in contrast. The fleecy clouds overhead became transparently light red, as they stretched over to reach the silver-streaked west. The new moon was just appearing upright against a slightly less bright opening in the sky, betokening the firm hardness of autumn.

Soon the colouring melted away, and the peaceful reign of the later twilight possessed the land.

Now why that brilliancy of the east, when the west was colourless? Most of all you note the immense variety and wealth of reds. These are due to dust in the atmosphere. We are the more convinced of this by the very remarkable and beautiful sunsets which occurred after the tremendous eruption at Krakatoa, in the Straits of Sunda, thirty years ago. There was then ejected an enormous quant.i.ty of fine dust, which spread over the whole world's atmosphere. So long as that vast amount of dust remained in the air did the sunsets and afterglows display an exceptional wealth of colouring. All observers were struck with the vividly brilliant red colours in all shades and tints.

The minute particles of dust in the atmosphere arrest the sun's rays and scatter them in all directions; they are so small, however, that they cannot reflect and scatter all; their power is limited to the scattering of the rays at the blue end of the spectrum, while the red rays pa.s.s on unarrested. The display of the colours of the blue end are found in numberless shades, from the full deep blue in the zenith to the greenish-blue near the horizon.

If there were no fine dust-particles in the upper strata, the sunset effect would be whiter; if there were no large dust-particles, there would be no colouring at all. If there were no dust-particles in the air at all, the light would simply pa.s.s through into s.p.a.ce without revealing itself, and the moment the sun disappeared there would be total darkness. The very existence of our twilight depends on the dust in the air; and its length depends on the amount and extension upwards of the dust-particles.

But how have the particles been increased in size in the east? Because, as the sun was sinking, but before its rays failed to illumine the heavens, the temperature of the air began to fall. This cooling made the dust-particles seize the water-vapour to form haze-particles of a larger size. The particles in the east first lose the sun's heat, and first become cool; and the rays of light are then best sifted, producing a more distinct and darker red. As the sun dipped lower, the particles overhead became a turn larger, and thereby better reflected the red rays.

Accordingly, the roseate bands in the east spread over to the zenith, and pa.s.sed over to the west, producing in a few minutes a universal transformation glow.

To produce the full effect often witnessed, there must be, besides the ordinary dust-particles, small crystals floating in the air, which increase the reflection from their surfaces and enhance the glow effects.

In autumn, after sunset, the water-covered dust-particles become frozen and the red light streams with rare brilliancy, causing all reddish and coloured objects to glow with a rare brightness. Then the air glows with a strange light as of the northern dawn. From all this it is clear that, though the colouring of sunset is produced by the direct rays of the sun, the afterglow is produced by reflection, or, rather, radiation from the illuminated particles near the horizon.

The effect in autumn is a stream of red light, of varied tones, and rare brilliancy in all quarters, unseen during the warmer summer. We have to witness the sunsets at Ballachulish to be a.s.sured that Waller Paton really imitated nature in the characteristic bronze tints of his richly painted landscapes.

CHAPTER XIX

A WINTER FOREGLOW

Little attention has been paid to foreglows compared with afterglows, either with regard to their natural beauty or their weather forecasting.

But either the ordinary red-cloud surroundings at sunrise, or the western foreglow at rarer intervals, betokens to the weather-prophet wet and gloomy weather. The farmer and the sailor do not like the sight, they depend so much on favourable weather conditions.

Of course, sunrise to the aesthetic observer has always its charms. The powerful king of day rejoices "as a bridegroom coming out of his chamber"

as he steps upon the earth over the dewy mountain tops, bathing all in light, and spreading gladness and deep joy before him. The lessening cloud, the kindling azure, and the mountain's brow illumined with golden streaks, mark his approach; he is encompa.s.sed with bright beams, as he throws his unutterable love upon the clouds, "the beauteous robes of heaven." Aslant the dew-bright earth and coloured air he looks in boundless majesty abroad, touching the green leaves all a-tremble with gold light.

But glorious, and educating, and inspiring as is the sunrise in itself in many cases, there is occasionally something very remarkable that is connected with it. Rare is it, but how charming when witnessed, though till very recently it was all but unexplained. This is the _foreglow_.

It is in no respect so splendid as the afterglow succeeding sunset; but, because of its comparative rarity, its beauty is enhanced. I remember a foreglow most vividly which was seen at my manse, in Strathmore, in January 1893. My bedroom window looked due west; I slept with the blind up. On that morning I was struck, just after the darkness was fading away, with a slight colouring all along the western horizon. The skeleton branches of the trees stood out strongly against it. The colouring gradually increased, and the roseate hue stretched higher. The old well-known faces that I used to conjure up out of the thin blended boughs became more life-like, as the cheeks flushed. There was rare warmth on a winter morning to cheer a half-despairing soul, tired out with the long hours of oil reading, and pierced to the heart by the never-ceasing rimes; yet I could not understand it.

I went to the room opposite to watch the sunrise, for I had observed in the diary that the appearance of the sun would not be for a few minutes.

There were streaks of light in the east above the horizon, but no colour was visible. That hectic flush--slight, yet well marked--which was deepening in the western heavens, had no counterpart in the east, except the colourless light which marked the wintry sun's near approach. As soon as the sun's rays shot up into the eastern clouds, and his...o...b..appeared above the horizon, the western sky paled, the colour left it, as if ashamed of its a.s.sumed glory. A foreglow like that I have very rarely seen, and its existence was a puzzle to me till I studied Dr. Aitken's explanation of the afterglows after sunset. I had never come across any description of a foreglow; and, of course, across no explanation of the curious phenomenon. The western heavens were coloured with fairly bright roseate hues, while the eastern horizon was only silvery bright before the sun rose; whereas, after the sun appeared and coloured the eastern hills and clouds, the western sky resumed its leaden grey and colourless appearance. Why was that? What is the explanation?

I have not s.p.a.ce enough to repeat the explanation given already in the last chapter of the glorious phenomenon of the afterglow. But the explanation is similar. Before sunrise, the rays of the sun are reflected by dust-particles in the zenith to the western clouds. The colouring is intensified by the frozen water-vapour on these particles in the west.

One thing I carefully noted. Ere mid-day, snow began to fall, and for some days a severe snow-storm kept us indoors. Then, at any rate, the foreglow betokened a coming storm. It was, like a rainbow in a summer morning, a decided warning of the approaching wet weather.

CHAPTER XX

THE RAINBOW

The poet Wordsworth rapturously exclaimed--

"My heart leaps up when I behold A rainbow in the sky."

And old and young have always been enchanted with the beautiful phenomenon. How glorious is the parti-coloured girdle which, on an April morning or September evening, is cast o'er mountain, tower, and town, or even mirrored in the ocean's depths! No colours are so vividly bright as when this triumphal arch bespans a dark nimbus: then it unfolds them in due prismatic proportion, "running from the red to where the violet fades into the sky."

A plain description of the formation of the rainbow is not very easily given, but a short sketch may be useful. Beautiful as is the ethereal bow, "born of the shower and colour'd by the sun," yet the marvellous effect is more exquisitely intensified in its gorgeous display when the hand of science points out the path in which the sun's rays, from above the western horizon, fall on the watery cloud, indicating fine weather--"the shepherd's delight."

One law of reflection is that, when a ray of light falls on a plane or spherical surface, it goes off at the same angle to the surface as it fell. One law of refraction is that, when a ray of light pa.s.ses through one medium and enters a denser medium (as from air to water), it is bent back a little. By refraction you see the sun's rays long after the sun has set; when the sun is just below the horizon, an observer, on the surface of the earth, will see it raised by an amount which is generally equal to its apparent diameter.

The rays of different colours are bent back (when pa.s.sing through the water) at different rates, some slightly, others more, from the red to the violet end. The rainbow, then, is produced by refraction and reflection of the several coloured rays of sunlight in the drops of water which make up falling rain.

The sun is behind the observer, and its rays fall in a parallel direction upon the drops of rain before him. In each drop the light is dispersively refracted, and then reflected from the farther face of the drop; it travels back through the drop, and comes out with dispersing colours.

According to the height of the sun, or the slope of its rays, a higher or lower rainbow will be formed. And, strange, no two people can see the very same bow; in fact the rainbow, as seen by the one eye, is not formed by the same water-drops as the rainbow seen by the other eye.

When the primary bow is seen in most vivid colours on a dark cloud, a second arch, larger and fainter, is often seen. But the order of the colours is quite reversed. At a greater elevation, the sun's ray enters the lower side of a drop of rain-water, is refracted, reflected _twice_, and then refracted again before being sent out to the observer's eye. That is why the colours are reversed.

_A one-coloured rainbow_ is a curious and rare phenomenon. It is a strange paradox, for the very idea of a rainbow brings up the seven colours--red, orange, yellow, green, blue, indigo, and violet. Yet Dr. Aitken tells us of a rainbow with one colour which he observed on Christmas Day, in 1888.

He was taking his walk on the high ground south of Falkirk. In the east he observed a strange pillar-like cloud, lit up with the light of the setting sun. Then the red pillar extended, curved over, and formed a perfect arch across the north-eastern sky. When fully developed, this rainbow was the most extraordinary one which he had ever seen. There was no colour in it but red. It consisted simply of a red arch, and even the red had a sameness about it.