Popular Science Monthly/Volume 38/February 1891/The Storage of Cold
|THE STORAGE OF COLD.|
THERE are two processes constantly active upon the surface of the earth which are of the utmost importance as regards its suitability for human habitation—the storage of heat and the storage of cold. Of these we are here concerned only with the latter. The source and method of the storage of cold (a negative process, which we may here treat as a positive) are much less evident and not so generally known as those of heat-storage, and a review of them may be of interest.
The source of the stored cold is the upper atmosphere, and the principal storing substance snow. Here we are on ground familiar only to scientists. Readers generally are not aware of the vitally important part which snow plays in the economy of nature. The lightly falling snow-flake, with its poetic affiliation and its attractive aspects, has its aspect of terror as well, for the feathery snow has done more to limit man's dominion of the earth than any other of the unfriendly agencies of Nature, even if we count the fiery ravage of the volcano and the ruinous work of the earthquake. While the rains are friends to man, and efficient agents in the progress of civilization, the snows are his enemies, and the most persistently hostile of his foes.
It need scarcely be said that the invigorating beams of the sun visit the earth in very differing measure, varying from tropical profusion to frigid sparseness. This diversity of heat distribution is partly overcome by the agency of the winds and waters, particularly the latter, since the great ocean currents carry vast supplies of heat from the torrid zone toward the poles, and drive far backward the boundaries of the realm of frost. The agency of the air in this heat convection is of less importance. The anti-trade winds move through the upper atmosphere, and lose their heat before descending to the earth; but surface winds from the tropics convey a considerable share of the torrid heats to the colder zones.
Snow is the great opponent to the full effect of this distributed heat. It constitutes an agent of Nature by which the chill of the upper atmosphere is conveyed to the earth's surface, and stored there in a more or less persistent form, which requires much of the solar heat and the warmth of tropic winds and waters to overcome. If it be asked how snow can produce such an effect, we must advert to the heat relations of water. A large supply of insensible heat—latent heat, as it is called—exists in liquid and gaseous matter. In the freezing process this heat becomes sensible, and is absorbed by the surrounding substances. Such, a process takes place on a large scale in the chilled fields of the upper air, the water vapor of the atmosphere being condensed into snow and its latent heat lost to the surrounding frosty air. In a word, snow is water which has lost its latent heat, or—in a negative sense—has absorbed cold from the upper atmosphere. The falling snow conveys this chill to the earth, and thus acts as a great refrigerating agent. To overcome the cold thus conveyed from mid-air to the earth heat is necessary, and large supplies, which might have been usefully employed in the service of man, are lost in the conversion of vapor into snow, and thus indirectly consumed in warming the upper air.
It may be said here that the conversion of vapor into rain is also exhaustive of latent heat. In the evaporation of the oceanic waters a very considerable quantity of heat is absorbed, and conveyed to the upper air as the latent heat of water vapor. Of this heat a part is lost in the formation of rain, and a larger part in the formation of snow. But the rain reaches the earth in a condition suitable for service. It does not, like the snow, need to be changed in its physical state, at a great expense in heat, to render it serviceable. In fact, the chilling influence of rain is inconsiderable, the heat-consuming agency of snow great and important, and the mode in which its work is performed calls for some consideration.
Snow has several curious methods of extending the sphere of its hostile influence. The comparatively light snows which fall in our latitude are of minor importance, since they readily yield to the early spring sunbeams. They are in some degree beneficial to the fertile surface and protective to its more tender annual plants, while their only important adverse effect is the dangerous flooding of the rivers, due to their rapid melting. But the deep and persistent snows of northern regions are far more exhaustive of solar heat, and reduce the agricultural season of those regions to a dangerously short period. In their melting, also, the surface air is chilled, and winds from the north convey this chilled air far to the south, thus spreading widely over the warmer zones the frost-inducing influence of the melting snows.
We have seen how the tropic heats are carried toward the poles by winds and waters. The frigid cold is carried toward the tropics by the same agencies—chilled winds and cold ocean currents. It is carried by another agent of great importance, the direct creep of the snow itself toward the lower latitudes. This agency has once—perhaps many times—produced an extraordinary effect upon the surface of the earth, one far surpassing that of volcanic explosions and lava outflows in its adverse influence. At present this glacial action is greatly reduced, but is still of much importance. Were it not for the snow-fall the problem of climate would be materially modified, and the temperature of the earth's surface much ameliorated. The seasons would gain a regularity which they do not now possess, the agricultural period of the colder zones be much extended, and the domain of agriculture be considerably widened, by the recovery of broad regions which are now covered during much or all of the agricultural season by snow.
In the winter the frost-laden strata of the atmosphere descend to the surface over much of the globe, and produce a direct refrigerating influence upon the surface soil and waters. This winter freezing, however, is of minor importance, as it, except in the polar regions, quickly yields to the spring suns, while its influence upon the summer temperature of lower latitudes is but slight. Only for the snow-fall this would be our sole source of cold. But the vast blanket of snow which descends annually upon the colder zones conveys downward the severe chill of higher layers of the air, borrowing from a mighty storehouse of cold which broadly impends above the earth. This snow blanket must be removed, and its stored cold overcome by solar heat, before agriculture can begin, and in this process weeks or months pass away, the effect being greatly to reduce the area of the earth's surface which is suitable for human habitation.
The snow of the frigid zones does not wait for the sun to reach it. It travels toward the tropics to meet the sun. This creep of the snow, as we may call it, takes the forms of the glacier and the iceberg. It also acts in another curious method, not generally known, but which is described by Nordenskiöld, in his Voyage of the Vega. Speaking of the natural conditions at a winter station near Bering Strait, he says: "The fall of snow was not great, but, as there was in the course of the winter no thaw of such continuance that the snow was at any time covered with a coherent melted crust, a considerable portion of the snow that fell remained so loose that with the least puff of wind it was whirled backward and forward. . . . Even when the wind was slight and the sky clear, there ran a stream of snow some centimetres in height along the ground in the direction of the wind, and thus principally from northwest to southeast. . . . The quantity of water which in a frozen form is thus removed in this certainly not deep but uninterrupted and rapid current, over the north coast of Siberia to more southerly regions, must be equal to the mass of water in the giant rivers of our globe, and plays a sufficiently great rôle among others as a carrier of cold to the most northerly forest regions to receive the attention of meteorologists." It may be that a similar condition prevails over northern America, though concerning this we have no evidence at hand. The wind thus seems to play a double role in conveying cold southward—one through the direct carriage of the snow, the other through the aërial chill caused by the melting snow.
The leading agent in the southward creep of the snow, however, is the glacier, and its offspring, the iceberg. The glacier is due to an important relation of the snows to the solar rays; namely, to that in which the stored cold is too great in quantity for the whole year's supply of heat to overcome, so that a part of each year's snow-fall is carried over to the next. There can be no glacier where the whole of the snow-fall is melted, even if the heat of the whole season is occupied in melting it; but, wherever a portion of the snow-supply is carried over from winter to win-, ter, glacial action is inevitable. In every such case the snows must steadily accumulate, their thickness increasing year by year. The growing pressure converts the under portions of this snow mass into ice, and this, through its normal plasticity, is forced by the weight upon it toward lower levels or more southerly regions, until it reaches its limit at that point in which the melting power of the sun balances the growth of the glacier.
The localities of glacial action are, therefore, the peaks and valleys of lofty mountains and the elevated regions of the frigid zones, or the lower regions of the latter in localities of abundant snow-fall. In all such places the heat derived from the sun is insufficient to melt the snow, which, therefore, necessarily creeps to warmer regions in the form of glacial ice. The principal seats of glacier formation in the north frigid zone are Greenland and Alaska. The remaining surface of northern America and that of Siberia are too low in elevation, and perhaps too light in snowfall, to permit any important glacial effect. Of the northern glacier-forming localities, Greenland is much the most important, and its refrigerating influence upon the coast lands of Europe and America is considerable. The mountains of snow which are heaped upon its elevated regions send down huge glaciers to the coast, which not only aid to chill the waters of the southward flowing currents, but send south an annual fleet of icebergs, borne upon these cold currents, and making their way far into the Gulf Stream domain of the Atlantic. No small quantity of the heat supply of this warm current is exhausted in melting the floating mountains of ice. This heat is lost to the northern continents, and their temperature reduced in consequence, possibly much more than we imagine. There is thus an annual battle between the earth's stores of heat and cold. The former, in the condition of warm ocean currents, makes its way far north. The latter, brought down from mid-air by the snows, and locked up in the glaciers, and their offspring—the—icebergs makes its way far south. They meet in mid-ocean, where an active conflict takes place. The heat conquers, but at a great loss of its valuable supplies, and a consequent refrigeration of the adjacent waters, air, and land.
In the southern seas this effect of the snow-fall is much more considerable. A belt of glacier-forming lands surrounds the south pole, and the annual iceberg fleet is much larger than that of the north. The air indraught to the north polar region is estimated to extend over a disk of fifty-five hundred miles diameter; that to the south polar region over a disk of seven thousand miles diameter. The former is largely composed of land surface; the latter is nearly all water, and its air is therefore much more charged with moisture. In consequence, the moist air which reaches the south frigid zone is greatly in excess of that which reaches the northern zone of cold, and the snow-fall there must be very much more considerable. It is estimated that the south polar ice-cap can not be less than three miles and may be twelve miles in height. The thrust of this vast ice mountain upon the viscid material beneath it is necessarily enormous, and a lofty ice-cliff is pushed off the land at a rate of not less than a quarter of a mile annually, and this around a circle of great extent. Fortunately, the immense fleet of huge icebergs, thus annually launched, has no continental land to act upon, its refrigerating influence being mainly exercised upon stretches of ocean out of the ordinary channels of navigation, and far removed from the important seats of human habitation.
There was a time, far in the past, but within the era of man's occupancy of the earth, when the influence of the snow was enormously greater than at present, and when the atmospheric chill, stored in the falling flakes, rendered a vast region of the northern continents unfit for human habitation, and extended the border of the frozen zone far toward the present limits of tropical heat. Doubtless if at present all the snow which forms in the upper air should reach the earth's surface, a glacial epoch would now exist in the north temperate zone. The experience of balloonists and of mountain-climbers teaches us that snow forms and falls in all seasons of the year. This is melted by the warmed lower strata of air, and the earth thus saved from its chilling influence. The solar heat, which has already done good work for man upon the surface, performs new and useful labor for him in the atmosphere, by melting this falling snow, so that its water reaches the earth only in the form of rain.
At the period mentioned the snow limit in the atmosphere was much lower than at present, and the great bulk of the snowfall reached the surface unmelted. As a result, the region of an annual snow surplus extended much farther south than at present, covering much and perhaps all of British America, and a broad zone of northern Europe. It is not necessary to dwell upon its . They have already been abundantly told. It will suffice to say, briefly, that the glacial ice thus formed, accumulating until it became of mountain height, crept steadily southward, combating with the sun as it went, until the front of the polar line of battle reached a limit extending across central Pennsylvania, and westward to the Rocky Mountain slope. In Europe it covered many of the active seats of modern civilization. Along this extended line conditions existed resembling those now found along the coast-line of Greenland. At this line the arrows of the sun checked the hosts of the snow, the annual heat balancing the yearly supply of cold, while great streams of chilled water poured from the melting ice. The mountain ranges farther south also sent out their glaciers over wide regions, and a vast extent of the now habitable earth was held prisoner by the snow.
To what extent the remaining regions of the continents were chilled by these vast glaciers can not be easily determined. The cold winds blowing south must have interfered seriously with vegetation over a broad zone. And the oceans of those days must have been crowded with icebergs to an extent far surpassing the commercial fleets of modern times. These may have floated to the tropic seas, and gone far toward exhausting the heat of the torrid zone, and chilling at their source the great ocean currents.
A time at length came when victory perched upon the banners of the sun. Step by step the cohorts of the snow retreated. The earth slowly reappeared from under its crushing weight of ice. Northward went the ice front, as the solar power increased, until it reached the arctic seas, and the northern continents were released from the foe which had so long held them in captivity. But the surface of the continents emerged in a greatly changed aspect. Great masses of rock had been torn by the gliding ice from the mountains, carried far southward, and deposited in a mighty breastwork of rounded and polished stones. The mountains themselves had been scratched and polished by rigid tools of stone, frozen into the ice. Large quantities of gravel and fine mud had been formed by the grinding of the rocks, and carried south by the flowing waters, to be deposited as hills of gravel and beds of clay many miles away from the glacial front. Enormous labor had been done in scooping out the earth's surface into hollows and basins, which became filled with water from the melting ice, and formed the host of lakes, large and small, which now exist over much of the formerly ice-covered region.
Such were some of the permanent effects of this long dominion of the snow, in its secondary form of glacial ice. Undoubtedly the growth of human culture was greatly interfered with by the long-continued inhospitable condition thus produced, and it is quite possible that, but for the glacial period, the civilization of mankind would have been much further advanced than at present, and most of the awkward questions which are troubling us now would have been settled ages ago. They might, however, have been succeeded by other questions quite as awkward, for the solving of perplexing problems of social relations seems part of the destiny of man. It has been suggested that the glacial age may have aided human advancement, by forcing primitive man to adopt new methods of shelter, clothing, and food-getting, in self-defense against the cold. Thus, instead of hindering it may have helped to break the reign of savagery.
Here it may be well to advert to another probable refrigerating agency of snow to which no attention has hitherto been paid. Aërial snow—snow that forms in the upper strata and is melted at lower levels of the air—may have always been an important agent in the cooling of the earth, aiding essentially in the upward transport of heat during the ages when the surface was at a high temperature. In those ages the great quantity of water vapor in the air hindered the free radiation of heat, whose conveyance upward was mainly accomplished by warm ascending currents. This may have been greatly aided by the conversion of the vapor of these vertical winds into snow in the upper air, the descent of this snow, and the exhaustion of much of the lower heat in melting it.
Such a state of affairs may have extended much further back in time than would at first thought be deemed possible; perhaps to that period when the earth was still too hot to permit the existence of liquid water, and the substance of the present oceans was held in the air as water vapor. Even then the rarer regions of the atmosphere were probably chilled below the temperature of congelation, and a snow limit existed, though very much higher than at present. The range of vapor must also have extended much higher than at present, possibly far within the region of congelation. Therefore, at the period when the surface heat prevented the existence of liquid water, there may have been a continuous formation and fall of snow in the upper strata of the atmosphere. The melting of this snow at lower levels, and the vaporizing, at still lower levels, of the rain which it yielded, must have been highly important agents in the upward transit of the surface heat.
There is thus much reason to believe that the snow-fall, which within the recent period has played so prominent a part in terrestrial affairs, has been from a very early era an active agent in the cooling of the earth, the snow limit of the atmosphere gradually descending through the ages until, in the glacial era, it nearly approached the surface, and vastly extended the ocean domain by covering a broad region of the land surface with frozen water of almost oceanic depth. With this must have been associated a marked lowering of the level of the oceans, though to what extent it would not be safe to estimate.