32 POLAR PROBLEMS 



of -4° C. and hence be much warmer than the surface air. There- 

 fore it cannot descend within the polar zone, but if drawn off into 

 lower latitudes it can descend to the surface and become a cold wave. 

 In the clear dry air of the cold wave the surface radiation is intensified, 

 so that surface radiation becomes a factor in intensifying the cold. 

 The rate of radiation decreases rapidly with decreasing temperature; 

 hence the polar air, if undisturbed, would probably not cool much 

 below —70° C, when absorption and radiation would balance. The 

 air between i and 6 kilometers is prevented from reaching these 

 temperatures because it flows equatorward to form the "cold waves" 

 and "polar fronts" of the modern meteorologist® and is replaced 

 by warmer air coming from lower latitudes. 



The details of the processes and methods of formation of these 

 "cold waves" and "polar fronts" which play so important a role 

 in the weather of lower latitudes are not yet determined, and this 

 forms one of the unsolved problems of polar research. For this 

 purpose a network of stations in the polar regions is needed, supplied 

 with kites, sounding balloons, or airplanes for the purpose of observ- 

 ing conditions in the upper air. Professor W. H. Hobbs has recently 

 initiated a praiseworthy attempt to study these processes around and 

 over Greenland, the reconnaissance field work having been done 

 during the summer of 1926.^ He has secured a corps of able assistants, 

 two of whom have had much experience in the exploration of the free 

 air, Mr. S. P. Fergusson, of the U. S. Weather Bureau, having been 

 active in the study of the upper air at Blue Hill, Mass., and at St. 

 Louis, Mo., and Professor J. E. Church, Jr., of the University of 

 Nevada, at Mt. Rose, Nevada. 



My own researches^ indicate clearly that the impulses which 

 set these cold waves in motion are in some way intimately associated 

 with variations in the intensity of solar radiation as measured by the 

 Smithsonian Astrophysical Observatory. 



The physical processes involved are not yet well understood, but 

 my conception of them is that without solar variation the atmospheric 

 circulation set up by the sun would be a balanced circulation disturbed 

 only by the daily and annual chartges. The solar variations give rise 

 to the pulsations in the atmospheric circulation which cause the 

 changes we call weather. 



With an increased solar radiation the pressure falls in the equato- 

 rial belt, presumably because of the absorption of heat by the upper 



^ See v. Bjerknes: The Meteorology of the Temperate Zone and the General Atmospheric Circu- 

 lation, Nature, Vol. 105, 1920, pp. 522-524; J. Bjerknes and H. Solberg: Life Cycles of Cyclones and 

 the Polar Front Theory of Atmospheric Circulation, Geofys. Publikationer , Vol. 3, No. i, Christiania, 

 1922. See also, on atmospheric radiation, W. J. Humphreys: Physics of the Air, Philadelphia, 1920, 

 p. 44. 



' W. H. Hobbs: The University of Michigan Greenland Expedition of 1926-1927, Geogr. Rev., 

 Vol. 16, 1926, pp. 256-263; idem: The First Greenland Expedition of the University of Michigan, 

 ibid.. Vol. 17, 1927, pp. 1-35 (meteorological results, pp. 20-32). 



5 op. cit., p. 13. 



