Popular Science Monthly/Volume 31/May 1887/Influence of Snow-Masses on Climate
|INFLUENCE OF SNOW-MASSES ON CLIMATE.|
THE masses of snow and ice known as glaciers, which are found upon high mountains, have been the object of many studies; and it is a matter to be wondered at that the same has not been the case with the immense beds of snow that every winter cover parts of Europe, Asia, and America, to disappear in the following spring. It has perhaps been thought that the latter have less influence upon climate in general than upon other more special phenomena. But the observations that follow will tend to show that this influence exists and the subject is one well worthy to be studied.
A bed of snow covering the ground acts as a bad conductor, and renders the exchange of temperature between the surface of the ground and the lower stratum of the atmosphere slower than it would be if the snow were absent. This is a result of the porous structure of snow, the interspaces of which are filled with air. In this matter the condition of the snow is of considerable importance; the minute crystals formed by cold are poorer conductors than the larger flakes; but if the snow by alternately thawing and freezing assumes the form called névé, it becomes a much better conductor of heat. Farmers in countries that enjoy cold winters are well aware of the protective nature of the snow-covering, and do not fear for their grain when it is thick.
The presence of snow thus assures a higher temperature to the upper layer of the soil than it would otherwise have, and its thickness is an equally important factor with its structure. But its effect on the air is different, for it separates the air from the warmth which without its presence would escape from the ground. Snow also exercises an important influence through its power of radiation, which is dependent on its whiteness and the extent of its surface, but in which thickness is not a factor.
When air is rarefied, it contains, besides the vapor of water, only a few scattering particles of dust or smoke. Even in the tropics, snow on high mountains does not melt. Although a considerable amount of solar heat is received by the upper surface of the snow, it is all sent back into space, and the air, being very diathermanous, only retains a very small part of it. In the plain countries of high latitudes the air is not rarefied, but, when there is a large extent of snow, the other conditions are similar to those which prevail at great elevations under the tropics, particularly the absence of dust and the small quantity of aqueous vapor. The latter condition is the result of cold, while the former may arise from the fact that there is no dry bare ground near from which dust can be swept up, or from the more or less complete absence of organic life. In this case as in that of high mountains, radiation into space goes on freely, and the solar rays are without power to melt the snow so long as it preserves its light structure, and its surface does not present any dark object to absorb the rays of the sun and cause the snow around it to melt. Thus, we observe that it melts on roofs and immediately around trees, while the clear snow in the vicinity shows no signs of thawing. Snow will melt rapidly under a piece of brown paper lying in a garden; and a thin sprinkling of dust on top of snow will produce a similar effect.
We conclude, from these observations, that if a large extent of continent is covered with snow, that snow will not melt under the rays of the sun. Yet we know that the winter snows in Northern Europe, Asia, and America melt every spring and summer. How does this come about? Observations made in high latitudes show that the temperature generally remains below the point of congelation till the middle of June—that is, till a time when these latitudes receive daily a much more considerable quantity of solar heat than is received at the equator; and that frosts last there for a month or six weeks during which the sun never sets. Besides this, the solar rays are seldom intercepted by clouds, for the month of May is usually very clear in the northern regions. At Polaris Bay, latitude 81° 86', in Northern Greenland, the sun does not set after the 11th of April, and yet, in 1872, on the 1st of June, the temperature of the air had not risen beyond the freezing-point except for ten hours on the 21st of May, while after the 2d of June the temperature was constantly above 32°; and the days during April and May were generally clear. The United States Expedition under Captain Hall passed the winter at Polaris House, latitude 78° 23', where the sun did not set after the 20th of April. Here, again, there was no general thaw before the 31st of May, but only partial thaws on the 16th, 22d, and 27th, although the days were quite clear. On the 8th of May the sky was wholly clear for several hours, before and after noon, and the temperature was -14·4° C. at noon, and -15·1 at three o'clock in the afternoon. It was also perfectly clear on the 31st, at six o'clock in the morning and six o'clock in the evening, and the temperature did not rise above -8·8°. The observations of the Vega at Pitlekaj, near Behring Strait, in a much lower latitude, gave analogous results. We therefore see that in high northern latitudes the heat of the rays of the sun at the end of spring and the beginning of summer can not raise the temperature above the freezing-point. To what, then, shall we attribute the thaw? Probably to the winds that have passed over warmer countries, over continents, or open seas. According to the observations of the Vega, the winds came from the north till the 12th of June, and then, on the 13th, passed around to the south-southeast. These warm winds melt the upper layer of the snow; when it freezes again, it changes into névé, or takes on a condition less diathermanous to solar heat, in which it less readily sends back the warmth which it receives.
The melting of the snow may be speeded by the dust which the wind brings from the continental spaces whence it has already disappeared. If the warm winds do not last long enough and are not strong enough, they will not produce durable results; but a new fall of snow will give a new layer, which is only slightly diathermanous, and possesses great radiating power. As a large quantity of caloric is expended in the melting of snow, the warm winds lose much of their heat, and may thereby produce a considerable refrigerant effect. But along the frontiers of the region covered by snow, the surface of the ground, after that is nearly melted, may be warmed by the sun, and thus become a source of heat to countries farther north. The movement begins along seas that never freeze and continental spaces in which the snow never, even in winter, forms a permanent bed; it then advances more and more northward, till all the low lands of our hemisphere are (so far as we know) freed from their covering.
This advance is not continuous, but proceeds, as we might say, by leaps and bounds. Warm winds coming from the south, or from the sea, further it, but cold winds arrest it, and sometimes reduce the temperature, where the thaw has already begun, to below the freezing-point. The disappearance of the snow in all the plains of the northern hemisphere is due to the geographical conditions of the hemisphere, or because all its known parts are reached by warm, melting winds. Moreover, in some countries in high latitudes but little snow falls, so that there is not much to melt. This, however, is not always the case, for snow may be seen to cover the ground, and the temperature of the freezing-point to prevail even during the summer. The fact is not only possible, but is a reality in very high northern latitudes. We learn from observations by Sir James Ross, that on the shores of the Antarctic lands the mean temperature, even in the height of summer, is considerably below the freezing-point, and never rises above it. This is explained by geographical conditions. The shores in question are at least 20° away from all other land, and can be influenced only by the seas north of them, while it has been observed that the temperature of these seas, down to 68° of south latitude remains below the freezing-point all through the summer. The Antarctic lands, therefore, do not receive from any quarter winds which can cause a thawing of the snow; and as this remains during the summer, the rays of the sun, notwithstanding its greater nearness to the earth at that time, can not raise the temperature above this point.
The existence of a very extensive bed of snow produces another important effect which has not received the attention it deserves: it keeps the temperature at but a little distance from the freezing-point, and below it. The mean temperature in February is the same at Bogoslowsk, at the eastern base of the Ural Mountains, and at Barnaul, on the upper Obi, at the foot of the Altai Mountains. But toward the southwest, not far from the Altais, in the Kirghiz steppes, there is usually but little snow. The mean and maximum of February are therefore higher. The same difference is observed between Ustsinolsk, in the government of Vologda, and Irgirs, in the Kirghiz steppes; and analogous differences may be remarked in other places. Wherever the snow-bed is less regular, the mean and extreme maxima of temperature are higher; and the difference goes on augmenting toward the south. It is especially considerable between Mitau, near the Baltic, and Nukurs, on the lower Amou (Oxus). In February the mean temperatures of these places differ by only 0·7° C, but the mean maximum is 10·2° and the absolute maximum ll·7°. The last result is all the more striking, for it is deduced from only six years of observations, while the observations at Mitau include a space of more than forty years. An analogous result is presented in December, when the monthly means and the mean minima differ very little, but the mean maxima of Nukurs are higher by 7° and the absolute maxima by more than 10°. This is because snow rarely falls at Nukurs, and a covering lasting for several days seldom occurs. In the absence of snow there is nothing to interfere with the action of warm winds, and, in that latitude, the sun heats the ground sensibly, even in the middle of winter.
The influence of a bed of snow on the maxima should vary according as the temperature is below or above the freezing-point. In the latter case, the melting of the snow, involving the absorption of heat, would tend to prevent increase of temperature. Not only does it prevent high maxima, but it keeps the temperature near the freezing-point long after it has begun. This is why April is colder than October in Russia, Central Europe, Canada, and the Northern United States. Farther north, for the same cause, the mean temperature of May is below that of September. It can not be doubted that this cause of cold, or rather this conversion of heat into melting action, is proportional, all other things being equal, to the mass of snow that remains on the ground. Hence, in countries having cold winters, the principal obstacle to the rise of temperature in spring is found in the quantity of snow on the ground, and not in the previous cold of the winter. It is only in countries situated near the sea, or in the neighborhood of large frozen lakes, that the mean temperature of the winter has much influence on the depression of temperature in the following spring and summer, because a larger quantity of ice is found in such situations during cold seasons, and a greater sum of heat is necessary to make the melting complete than in average seasons. Snow is the only cause that can produce an analogous result in countries distant from seas and lakes. It may be concluded, from the observations on these subjects, that, at the moment when the mean temperature begins to rise above the freezing-point, everything depends upon the sum of cold existing in the form of snow and ice. The greater it is, the slower and more irregular will be the rise of temperature.
The time of the coming of the snow, its depth, and its extent, have also a very great influence on the beginning and duration of the winter frosts; and this influence is manifested, not only in the particular spot, but in the northern hemisphere far to the south of it. In short, we may say that snow gives duration to the cold, and prevents a rapid rise of temperature. If we knew the exact moment when a bed of snow was formed in the North in fall and winter, and if we could announce it by telegraph, we might predict the time of the freezing of rivers and canals, and thus serve the interests of vast territories as of most of Asiatic and European Russia, Scandinavia, British America, and the United States. The announcement of the closing of rivers by ice, even if it were only four or five days in advance, would prevent the considerable losses caused by premature frosts, and permit the safe continuance of navigation during tardy winters.
In all countries where the snow forms deep masses in winter, the rivers rise at the time of its melting; the quantity of water produced hy the melting of snow and ice is so great, and the evaporation is so little, as to produce much greater floods than ever can arise from rains. This phenomenon is therefore of a character to affect some of the sides of practical life; yet the way in which it operates has never been sufficiently well observed. The results of the thaw depend upon its rapidity as well as upon the quantity of snow that may be upon the ground when the frost breaks up. If the snow melts rapidly, inundations may ensue, while the duration of the high water will be too brief for it to be utilized for navigation; and the contrary will take place if the thaw is slow and gradual.
It is a popular saying in Russia that when there is little snow the waters will be high, and there will be little water when much snow falls. This kind of paradox is justified in the case of the smaller rivers. When little snow falls in winter, the ground freezes to a great depth. The first water that is spread over the surface also freezes, and a crust of ice is formed, over which the water flows as over a rock, without penetrating it. It therefore reaches the rivers quickly and swells their waters. When, on the contrary, the snow is abundant, it protects the ground in such a way that the thaw can begin from below, and the formation of a crust of ice on the surface is not possible. The melted snow penetrates the soil, and does not reach the ravines and rivers till after some time—that is, till after the ground has been saturated. The Russian peasants call this ground-water. Observation teaches that it proceeds from the forests rather than from the fields, because the snow accumulates there to a greater depth, and is less scattered by the winds than in open places.
The melting of snow from below was observed in 1884, at the Agricultural Academy of Petrovsky, near Moscow. Observations were made at the surface and at various depths down to two metres. At seventy-five centimetres the temperature reached the freezing-point on the 5th of March, and it rose to a greater height sooner from this point than at fifty or at twenty-five centimetres. As similar conditions have been observed at various points in the valley of the Volga, the high waters of that river in 1884 did not rise above the mean, notwithstanding the great depth of the snow; but the sources of the stream were so well supplied with water by the gradual melting of the snow that navigation was unimpeded during the whole of the summer and fall. The contrary took place in 1880, when a colder winter with less snow was favorable to a rapid thaw; the freshets were among the highest that had been observed, but the water soon fell off, and from August till October navigation everywhere above the mouth of the Kama was precarious.
Everything relating to the covering of the ground by snow is of such importance to science and practical life that it should be observed and published in detail. The exact moment when it occurs should be ascertained; the structure of the covering in different parts of the cold season; its depth in different places—in forests, parks, fields, and ravines; the time when it begins to melt; the advance of the thaw; the condition of the upper layer of soil under the snow (that is, whether an icy crust is formed and when), and the facts respecting high water in the rivers, should all be looked after. The business of making these observations could be intrusted to students of meteorological and phenological phenomena. The only point to be regarded as difficult is the observation of the depth of the snow under different conditions. This could be facilitated by having stakes fixed in advance, with white and red divisions clearly marked, so as to be visible from a distance. The most exact data on the mean depth of the snow will be furnished by forests, gardens, and parks, where the snow is protected against the wind by trees. In open places, numerous observations will be necessary, in consequence of the variations in the thickness of the snow, caused by the action of the wind. I believe, however, that a good observer would soon become at home, and find great interest in observations of this kind. After they have been continued systematically for a suitable time, and it has become possible to draw a few general deductions from them, telegrams might be dispatched to the central meteorological stations, reporting upon the condition of the snow. From these reports, important practical conclusions could be drawn, as in determining how long the rivers are likely to continue open to navigation, the amount of water that the rivers will have to carry after the thaw, and the probable character of the spring.
The snow-fall in the Himalayas has given rise to predictions respecting the arrival and conditions of spring in the north of India, which have been fully justified; and careful observations of this kind might be made very useful in other countries where the winters are liable to be severe.—Translated for the Popular Science Monthly from Ciel et Terre.