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Meteorology; or Weather Explained

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<h2>CHAPTER XXVI</h2> RADIATION FROM SNOW One night a most interesting paper by Dr. Aitken, on “Radiation from Snow,” was read by Professor Tait to the Fellows of the Royal Society of Edinburgh. I remember that Dr. Alex. Buchan—the greatest meteorologist living—spoke afterwards in the very highest terms of the subject-matter of the paper. This was corroborated by Lord Kelvin, Lord MacLaren, and Professor Chrystal. Dr. Aitken had been testing the radiating powers of different substances. Snow in the shade on a bright day at noon is 7° Fahr. colder than the air that floats upon it, whereas a black surface at the same is only 4° colder. This difference diminishes as the sun gets lower; and at night both radiate almost equally well. I select, among the careful and numerous observations, the notes on January 19, 1886; for I took note of the cold of that day in my diary. It was the coldest day of the whole of that winter. The barometer was 28·8 inches, and the thermometer 4°—that is, 28° of frost. According to Dr. Buchan, that January had only two equal in average cold for fifty years. On January 19, at 10 A.M., when the air was at 20° and the sky clear, a black surface registered 16° and the upper layer of snow 12°, showing a difference of 4° when both surfaces were colder than the superincumbent air. It is curious to note that, on February 5 of the same year, at the same hour, when the sky was overcast, the air was at 23°, the black surface registered 29°, and the snow 25°, showing again the difference of 4°; but, in this case, both surfaces were warmer than the air. In both cases the radiation at night was equal. This small absorbing power of snow for heat reflected and radiated from the sky during the day must have a most important effect on the temperature of the air. The temperature of lands when covered with snow must be much lower than when free from it. And, when once a country has become covered with snow, there will be a tendency towards glacial conditions. But, besides being a bad absorber of heat from the sky, snow is also a very poor conductor of heat. On that very cold night (January 18), when there was a depth of 5½ inches of snow on the ground, and the night clear, with strong radiation, the temperature of the surface of the snow was 3° Fahr., and a minimum thermometer on the snow showed that it had been down to zero some time before. A thermometer, plunged into the snow down to the grass, gave the most remarkable register of 32°. Through the depth of 5½ inches of snow there was a difference of temperature of 29°. This was confirmed by removing the snow, and finding that the grass was unfrozen. As the ground was frozen when the snow fell, it would appear that the earth’s heat slowly thawed it under the protection of the snow. The protection afforded by the bad-conducting power of snow is of great importance in the economy of nature. How vegetation would suffer, were it exposed to a low temperature, unprotected by the snow-mantle! So that, though the continued snow cools the air for animals that can look after their own heating, it keeps warm the soil; and vegetation prospers under the genial covering. The fine rich look of the young wheat-blades, after a continued snow has melted, must strike the most careless observer. Instead of the half-blackened tips and semi-sickly blades, which we see in a field of young wheat after a considerable course of dry frost without snow, we have a rich, healthy green which shows the vital energy at work in the plants. Or even in the town gardens, after a continued snow has been melted away by a soft, western breeze, we are struck with the white, peeping buds of the snowdrop and the finely springing grass in the sward. Yet the snow-covering gives durability to cold weather. This has been demonstrated by Dr. W&oelig;ikof, the distinguished Russian meteorologist. On this account the spring months of Russia and Siberia are intensely cold. The plants, then, which in winter are unable by locomotion to keep themselves in health, are protected by the snow-mantle which chills the air for animals that can keep themselves in heat by exercise. What a grand compensating power is here!     <hr style="width: 50%;" /> <div style="break-after:column;"></div>

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