Page:Popular Science Monthly Volume 31.djvu/72

62 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