COOLING IN THE LOWER ATMOSPHERE AND THE STRUCTURE OF POLAR CONTINENTAL AIR

Abstract

It has been known for some time that during a clear, calm night the temperature of a snow surface falls much below the temperature of the air within even a few centimeters of the surface, The snow surface radiates energy practically as a black body; and receives, by conduction from tho snow and air, and also by radiation from the atmosphere, less energy than it loses through its own radiation; and consequently its temperature decreases. However, if the wind movement increases, considerable heat is transported to the surface from the T H E CHANGE FROM POLAR MARITIME TO POLAR CONTINENTAL AIR MASS CHARACTERISTICS No better introduction to the problem can be given than by presenting actual soundings showing different types of polar air. Ccrve (a) in figure 1 shows a portion of a balloon FIGURE 1.-Soundings at (a) i s Norway, February 2 1933. 15.11 h. (b) Fairbanks Alaska, December 28, 1932, 9.3; h; (e) Ellendale, N. dak., Febmar; 8, 1933, 13.28 h: local time. warmer air above, by turbulence, and the siirface temperature rises. When the wind ceases, the surface tem-pera ture again falls. The question arises as to how much the surface temperaturecan fall without alsoa siniultaneoits decrease of th? temperature of large port,ions of the air above it. It wdl be shown, from considerations of radia-tive balance between tbe air and the snow surface, that it is impossible for the surface temperature to fall below a certain value without a simultaneous decrease of the maximum free-air temperature. In other words, it will be shown how, apart from conduction and mechanical turbulence, cooling from below extends upward through the lower troDosDhere. sounding taken through fresh polar maritime air a t As, Norway (59'40' N., 10'46' E.) February 2, 1933. The synoptic situation for this period is discussed by Palmen (l),. and it is seen from his maps that the air moved rapidly from the north over the Atlantic Ocean and appeared a t As with westerly winds. According to Palm&, the air above the surface la er was in almost showers, hailstorms, and thunderstorms along the Nor-wegian Coast. Curve (a) is a sounding made a t Fairbanks, Alaska (64'51' N., 147'52' W.), December 28, 1932, under clear skies and in a calm. An inversion of over 20°