AN AUTUMN AFTERGLOW
A brilliant afterglow is welcomed for its surpassing 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
masses 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 quantity 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 pass 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 pass through into space 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
passed 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 assured that Waller Paton really
imitated nature in the characteristic bronze tints of his richly painted
landscapes.