Popular Science Monthly/Volume 7/June 1875/The Cyclone in the Universe
|THE CYCLONE IN THE UNIVERSE.|
By JAMES MACKINTOSH, M. A.
THE science of meteorology is but of yesterday, and yet it has already developed results which throw light upon the genesis of the universe. It has revealed to us the true nature of atmospheric disturbances throughout time and space. The winds no longer blow where they list, and we hear the sound of them and can tell whence they come and whither they go. We know their producing causes, and can foretell, with a considerable degree of accuracy, their force, duration, and direction. Accordingly, the great majority of civilized countries, including China, the oldest of them, have already established weather bureaus, whose business it is to forewarn the mariner of coming tempests, and to give us all a timely notice when we shall require an umbrella or a great-coat.
While these practical results are exceedingly worthy of attention, and inspire us with the hope that the time shall come when perfected instruments, improved methods, and increased knowledge, shall enable the meteorologist to predict with the utmost certainty every atmospheric disturbance, they yet fall into the shade when compared with the magnificent and luminous conceptions which meteorology has added to cosmological science. It has opened to us visions of beauty and order reaching through infinity and eternity. It has given us a clearer glimpse into the workshop of the Almighty.
The principal scientific result of meteorology is the theory of the cyclone. This is its central idea, the point of reference from which every thing is explained. So long as meteorologists tried to explain storms by encountering currents of wind, as did Dove and his school, 60 long did the science remain merely a laborious, interminable, and apparently useless collection of tabulated facts. It was then in its empirico-historical stage, and could, at the best, only produce such bare generalizations as isobaric and isothermal lines. Averages and darkness ruled throughout. But, when once the light-giving idea of the cyclone was fully grasped, a heavenly radiance dispelled the uncertain gloom, and the science was at once taken out of the range of the merely empirical, and established securely upon a deductive basis.
What, then, is this light-bringing conception of the cyclone? Briefly, as follows: A cyclone consists essentially of a rapidly-ascending current of air. This involves two other functions: 1. A rushing in of the air at the under part of the ascending current; 2. An outrushing at the upper. Upon the former of these functions, combined with modifying circumstances, depend the peculiar character and career of the under-currents and of the clouds they bear; upon the latter, combined with the same circumstances, the proportions and direction of motion of the upper currents and of the heavy masses of clouds they bear. There appears to be nothing in the nature of the cyclone itself which can determine the motion of either the upper or under current more toward any one point of the compass than toward the others. This direction of motion relative to the ascending column depends upon the direction and velocity of motion of the latter, and of the atmospheric strata in which the influx and efflux take place, modified to some extent by the differing velocities of revolution of the surface of the earth at different parallels of latitude, by the form of the earth's surface, and by the variation in the constitution of the atmosphere. If the cyclone column and the atmospheric strata which it penetrates move in the same direction, and with the same velocity, the influx and efflux will take place in nearly equal quantity, on all sides of the column. If they move with different velocities, the directions of exaggeration and diminution of the influx and efflux can be calculated in the same way as the direction of a vane on a ship's mast, given the directions and velocities of the motion of the wind and of the ship.
This gives the general idea of the cyclone for all space and time, but not the sources of its power. These are to be found in the less specific gravity, potential or actual, of the lower atmospheric strata as compared with that of the higher. Air, and gases generally, expand when heated, and become specifically lighter. It thus tends to rise above the superincumbent colder air. (We see this illustrated in our chimneys every day.) If the air over the surface of a plain becomes heated by contact with it, its specific levity is increased, and it tends to rise. But the density, and therefore weight of air, the temperature and humidity being the same, is inversely as the pressure upon it. Consequently, so long as the diminution of specific gravity caused by increased temperature is balanced in the strata above by the diminution of pressure due to elevation, the heated air cannot ascend very fast. If it had a chimney to rush up, the case would be different; but, not having one, it can only rise slowly by intermingling itself with the superincumbent air. As soon, however, as the diminution of specific gravity due to higher temperature is greater than that due to diminished pressure, the lower stratum will break itself a way through the air above it, and rush up through the opening. This result may be attained, and is generally reached, by favoring circumstances, long before equilibrium is totally destroyed. When once the' heated air has thus at a particular point formed itself a channel of escape, the warm stratum along the surface of the earth rushes toward the opening, and there ascends, while the colder air above descends to take its place. If the ascending column of heated air remained stationary, it is evident that its supply of warm air would soon be exhausted, and the process therefore speedily come to an end. But this is not so. It moves forward to where there is more heated air, just as one might fancy a chimney to travel after a moving fire. The operation can thus go on for a considerable time.
The ascension of air with a higher sensible temperature would not alone suffice to supply the tremendous power of the cyclone. The difference of temperature, even in extreme eases between lower and higher strata, is wholly insufficient to account for the enormous energy developed by our cyclones of hundreds of miles in diameter. We therefore require another source of power. Nor have we far to seek it. The sun's rays falling upon dry earth heat it, and thus raise the temperature of the air in contact with it. But if they fall upon our oceans, lakes, and rivers, or upon moist earth, there is another result, of a somewhat different though equally familiar kind. It is this, that some of the water is converted into steam or vapor. Now, every one knows that no amount of heat can raise the temperature of boiling-water if it is unconfined. Where, then, does the heat go to? Plainly it is carried off by the vapor in an insensible or latent condition. It is a demonstrated fact that it requires as much heat to convert a quantity of water into steam as it takes to raise the same quantity 1,000° of temperature. The same amount is required to evaporate water without boiling it. Consequently, when the sun's rays evaporate water, a vast amount of heat becomes insensible to our thermometers. It is not annihilated, however, and all that is required in order to make it manifest is simply to condense the vapor into water again.
When the heated air, as already described, rushes up in a column, it becomes subjected to less and less pressure, because there is less and less air above it. Since air in expanding under pressure produces work, and since heat is an equivalent of work, it expends heat in so doing, and is thus lowered in temperature. Consequently, the ascending air rapidly cools as it rises. Now, this air is carrying large quantities of vapor of water with it, which likewise is cooled by expansion. But you cannot cool vapor at any tension below a certain temperature without condensing it; and so, indeed, it happens. The steam carried up by the cyclone is condensed into rain, snow, or hail, and falls to the earth. In condensing it gives forth the enormous quantity of insensible heat which it received from the sun. This heat is imparted to the ascending current, and thus keeps it warmer and therefore specifically lighter than the strata through which it is rising. The heat of the sun, which had been potential in the vapor, is converted into the energy developed by the cyclone.
We thus see that the cyclone is really a kind of vast steam-engine. On our earth its furnace is the sun, and its boiler the moist lands, the rivers, lakes, and oceans. But it is evident that its mode of operation would not be substantially affected if the heat were supplied not as in our case from an outside source, but from the original internal heat of the sun or planet itself. So, also, the essential nature of the cyclone would not be altered whatever be the kind of vapor condensed, whether it be of water, of iron, of copper, of gold, or of granite. The above derivation of the power of the cyclone is therefore applicable throughout time and space.
For the condensation of any vapor whatever would present very much the same phenomena as those with which we are familiar. Let us suppose the earth of such a temperature as to keep iron in nearly the same condition relatively as water is now; that is, partly vapor floating in the atmosphere, partly fluid gathered in oceans, lakes, and rivers, and partly like solid snow and ice as in the colder seasons and latitudes. Evaporation would go on at the surface of the fluid iron until the atmosphere became nearly saturated. As soon as condensation began an ascending current would be formed. Toward the bottom of this current the winds would rush in spirals just as they do now. As the vapor of iron rose and came to the strata of less and less pressure and temperature, it would expand, cool, and condense, and descend in molten showers of liquid metal. Or, if the temperature were low enough, or the summit of the storm high enough, a shower of iron hail, or snow, would be the result.
Nor need we stay our imagination here. The time was, when our globe had no solid or liquid nucleus, but was wholly gaseous. It was literally an atmosphere, and nothing else. All the matter of the earth then floated, a vast globular ocean of vapor. The power which kept its particles apart was heat. Before these particles could come together and the solid foundations of the world be laid, it was necessary that the heat should be got rid of. The means by which this purpose was accomplished was mainly the cyclone. Around the limits of the vaporous world radiation into empty space could go on rapidly. Not so in the interior. Conduction of heat even along a bar of iron is a very slow process. It is million-fold slower through gas. Hence, the quickest way of carrying the heat from the interior to the summit of the atmosphere, where it might escape, was, to carry up the matter itself which contained a large amount of heat, either actual or potential. This work was accomplished by the cyclone.
Let us endeavor to form some conception of the cyclone of primeval times. Let us fancy ourselves in the solar system ere yet it became separated into insulated worlds, and just as condensation is going on. Gases of different specific gravities tend to intermingle even though at first arranged in separate layers above one another. Many of the gases would also be of nearly the same specific gravity. Hence, although in general the denser gases would tend together toward the centre, and the lighter toward the summit, there would necessarily be a vast amount of confused intermingling. Hence a cyclone of those times could not be attended by the fall of only one kind of rain, as of molten iron, but by that of many different kinds. Doubtless while some substances, such as granite, fell as snow or hail, others, such as iron, would foil as rain.
Moreover, since the strata would be, in the main, according to their specific gravity, and since some of the gases would evaporate and condense at different temperatures from others, showers of different kinds of metals and stones would tend to form at different altitudes. This would be counteracted, at least in part, by the tendency of the cyclone to reach clear up from the lowest depths to the circumference. That the disturbances in our own atmosphere extend to an immensely greater height than is generally supposed, and probably almost to the extreme limit of the atmosphere, is now certain. In my report on the tornado of May 22, 1873 (Chief Signal-Officer's "Annual Report, 1873"), I showed that in all likelihood it reached, at least, to an altitude of sixteen miles. The cyclones in the sun also appear to extend almost to the summit of his atmosphere, otherwise we could not see them so clearly as we do. Judging also from the nature of the case, we should conclude that the cyclone, amid such a vast assemblage of vapors, arranged in layers, would be likely to extend its dimensions almost from the centre to the circumference; for a disturbance and precipitation in one layer would tend to produce a disturbance and precipitation in the stratum above it, as well as in that beneath it. We have thus presented to our imaginations a Vast cyclonic column thousands of millions of miles in height, up which vapors of great variety, and collected at very various altitudes, are rushing with terrific force, and condensing as they go. Those, like granite, that solidify at a high temperature, would freeze in huge blocks which, generating sufficient centrifugal force by the whirling motion, would fly out from the ascending current and rush downward. Substances congealing at different temperatures would thus be likely to be thrown out at different elevations. Much the larger mass of substances, however, would probably be carried up to where the cyclone spread itself out laterally in a huge nimbus-cloud. From that cloud would rush down a fierce deluge of half the substances of the solar system in solid or liquid form. The violence and confusion of the descending hail and rain would be of surpassing grandeur—far more terrible and sublime than that scene described by Milton, where the Satanic host was hurled from the battlements of heaven "with hideous ruin and combustion down to bottomless perdition." All kinds of igneous rocks, mingled with molten metal, chased each other millions of miles down through the fiery gloom. The temperature increased as they descended. Each substance melted and evaporated as it reached the proper temperature, while the substances more difficult to evaporate continued their downward course. Finally, all would again be reduced to a state of vapor.
What, then, had been accomplished by all this turmoil and activity? Merely this: a large quantity of heat had been conveyed from the interior of the system to the exterior; for this it is which all cyclones accomplish. This it is which lends them their power. On our earth it is the sun's heat mainly which the cyclone carries away to the limits of the atmosphere. In the sun itself it is a portion of the primeval stock of heat which is removed.
The cyclone may therefore be defined as the universal cooler of creation. There is not a sun which lights the midnight sky, or which the telescope has brought within mortal ken, which is not vastly indebted to the cyclone. Though so simple as easily to be understood by a child, it is a powerful means by which the Almighty-works. It is a key to very many of the secrets of the universe. When we watch the snow-storm and the rain, we are really watching the method by which God has proceeded in forming his worlds since ever the cooling process began. Thus have the storms raged and the winds howled throughout the universe for countless ages; and by that rain, and snow, and hail, has all the solid and liquid substance of the worlds been formed. Every particle of it has been rained and snowed again and again. Nor is the process yet completed. The cyclone has by no means done its work yet. Its task will be finished only when the last particle of gas is converted into a liquid or solid. It is going on all around us. If there appears to be a balance at present upon the earth, if the solidifying power of the cyclone appears to be at a stand-still, it is only because its efforts are counteracted by the extraneous heat we receive from the sun.
The cyclone may also have assisted at the birth of the planets. Those stupendous meteors of thousands of millions of miles of elevation must necessarily have caused immense gaseous masses to bulge out from the general level of the surface of the incipient solar system. This might be sufficient under exceptional conditions, and when the balance between the centrifugal and the centripetal forces was nearly equal, to turn it in favor of the former, and thus generate a planet.