Fig. 248.—Side view of end of glacier. Southeast side of McCormick Bay, North Greenland. Shows structure of ice as well as position of débris.
The variation of temperature at the surface is due primarily to the varying temperature of the air. During the cold season, a wave of low temperature (the winter wave), starting at the surface, penetrates the ice, and during the warm season a wave of higher temperature (the summer wave) takes the same course. The day and night waves and other minor variables are, for present purposes, negligible.
Fig. 249.—Side view of a North Greenland glacier (East glacier), showing position of débris and structure of the ice.
The winter wave.—There are but few observations on the internal temperatures of glaciers during the winter season, but it seems certain that the winter wave diminishes rapidly downward and dies out below, much as does the winter wave which affects land surfaces not covered with ice. Conduction alone considered, the temperature of the ice where the cold wave dies out, should correspond, approximately, to the mean annual temperature of the region, provided that temperature is below the melting point of ice.
Assuming that in the high altitudes and high latitudes where glaciers abound, the temperature of the surface may average about −12° Fahr. (about −25° C.) for the winter half of the year, which is about the case for north Greenland, Spitzbergen, and Franz Josef Land, and that the conductivity of the ice in the C. G. S.[130] system is .005, the temperature would be lowered appreciably only about 40 feet below the surface at the close of the winter period, conduction only being considered. How far the internal temperature may be influenced by air forced through the ice by winds and by variations of the barometer is not known and cannot well be estimated. The wave of low temperature descending from the surface in winter would probably become inappreciable before reaching a depth of 60 feet. At this depth the temperature should be about 15° Fahr.—the mean annual temperature of the region.
Fig. 250.—Contorted lamination shown at the surface. A small glacier south of Forno hut, Engadine, Switzerland. (Reid.)
The summer wave.—The warm wave follows the analogy of the summer wave of ice-free land surfaces much less closely. This is because of the low melting temperature of ice as compared with other forms of solid earth-matter. On this account the summer wave is bi-fold. The one part travels downward by conduction, the other by the descent of water; the one has to do primarily with the temperature before the melting-point of ice is reached; the other, with the temperature after that point is reached; the first conforms measurably to the warm wave affecting other solid earth-matter, while the second is governed by special laws. After the surface portion of the ice is brought to the melting temperature, the additional heat which it receives melts the ice and is transformed from sensible into potential heat. Ice charged with water is potentially, but not sensibly, warmer than ice which has just reached the melting temperature.