Those who have followed us through this volume will remember that at the end of Chapter VIII. we remarked upon the spectrum, and made a few observations respecting the heat spectrum, and the velocity of light rays, which became too rapid to be observed, and then they developed heat—invisible heat—produced by non-luminous waves, which proceed from the sun as surely as visible rays or light. Professor Tyndall has written very pleasantly upon this subject, and, with his clear leading, any reader can study for himself.
We have now arrived at the conclusion that there are visible and invisible rays giving us respectively light and heat. These latter are the means whereby the ice is melted, and by which water is evaporated to vapour, and formed into Clouds when it is chilled or condensed. Here is another link in the beautiful chain constructed by Nature. We cannot penetrate far into any portion of the system of the universe without being struck with the wondrous harmony that exists between every portion of it. Thus heat and light, vapour, cloud, rain, dew, and ice are all intimately connected.
A cloud, then, is a visible body of vapour in the atmosphere, which is supported by an invisible body of vapour. It will remain thus invisible so long as the atmosphere is not saturated with moisture. The air can contain a great quantity of moisture without its being rendered visible, and so when the day is hot we see no steam from the locomotive. It is absorbed into the dry atmosphere. But when the day is “damp” we find that the air has nearly as much moisture as it can carry, and the steam is condensed, a portion falling in tiny drops like rain. This is proved every day in cold weather when ice is found in the windows—the cold air has condensed and frozen the water breathed out from our lungs, and snow has been known to fall in a ball-room when a cold current of air was admitted.
People are sometimes apt to think that if the sun were very hot, glaciers, and such icy masses, would diminish; but we think after what has been said respecting the power of the sun’s rays to evaporate water, all will see that the contrary is the fact. Without sun-heat we should have no cloud, and as clouds give us rain and snow and ice and glacier, we must come quickly to the conclusion that glaciers and snow are the direct results of the heat of the sun. The “light” rays of the sun do not penetrate snow, and that is why our eyes are so affected in snowy regions. The poor Jeannette sufferers a short time since were blinded by reflected light, and dark spectacles are worn on all Alpine expeditions. The invisible rays, as we have said, dissolve the ice into rivers.
The atmosphere produces clouds by expansion of vapour, which chills or cools it, and it descends as rain. To prove that expansion cools air is easy by experiment, but if we have no apparatus we must make use of our mouths. In the body the breath is warm, as we can assure ourselves by opening our mouths wide and breathing upon our hands. But close the mouth and blow the same breath outwards through a very small aperture. It is in a slight degree compressed as it issues from the lips, and expanding again in the atmosphere feels colder. Air compressed into a machine and permitted to escape will form ice.
Fig. 717.—Cumulus cloud.
Water is present in clouds which assume very fantastic and beautiful forms. We know nothing more enjoyable than to sit watching the masses of cumuli on a fine afternoon. The grand masses built up like the Alps appear to be actual mountains, and yet we know they are but vapour floating in the air, and presently to meet with clouds of an opposite disposition, and produce a thunderstorm with torrents of rain. Those who will devote a few minutes every day to the steady examination of clouds, will not be disappointed. They give us all the grandeur of terrestrial scenery. Mountains, plains, white “fleecy seas,” upon which tiny cloudlets float, and low upon the imaginary yet apparent horizon, rise other clouds and mimic mountains far and farther away in never-ending distance.
A pretty, light, feathery cloud, with curling tips and fibres, is known as cirrus, and exists at a very great elevation. Gay-Lussac went up in a balloon 23,000 feet, and even at that height the cirri was far above him in space. We can readily understand that at such an extreme elevation they must be very cold, and they are supposed to consist of tiny particles of ice. Such clouds as these are very frequently observed at night, as cirro-cumulus around the moon, and a yellowish halo, apparent to all observers, is thought to be coloured by the icy particles of the lofty cirrus. The beautiful and varied phenomena of perihelia, etc., are due also to the snowy or icy flakes of the cirri and cirri-cumuli, caused by the refraction of light from the frozen particles. These cirri clouds are indicative of changeable weather as “Mares’ tail” skies, and long wisps of cloud, foretelling storm.
The cirro-cumulus is the true “mackerel” sky, and is formed by the cirri falling a little and breaking off into small pieces of cumulus, which is a summer (day) cloud generally, and appears in the beautifully massive and rounded forms so familiar. The stratus is, as its name implies, a cloudy layer formed like strata of rock. It is generally observable at night and in the winter. It often appears suddenly in the sky consequent upon diminished pressure or a rapid fall of temperature. It is low-lying cloud sometimes, and at night forms fogs.