A. INTRODUCTION 



The Arctic pack is a floating aggregation of pieces of sea ice of 

 all sizes, from huge masses many miles in diameter to broken pieces not 

 much larger than snowflakes or sleet particles. Principal tj^pes of ice 

 Twhich are now recognized are young ice, vfinter Icej, and polar ice. 

 Yoiong ice is newly-formed thin ice which from the air appears grey or 

 black in color and has an age measured in days or weeks. Winter ice. 

 develops during the wintei- season and may be several feet thickj much 

 of it melts during the succeeding summer, Paleocryetic or polar ice 

 is ice that has remained unmelted through one or more summers and is 

 much harder and thicker than winter ice. It has a pale blue color, 

 and because it partially melts during the summer, it has subdued relief 

 features, the surface being composed of small ridges and valleys in which 

 a system of fresh-water rivulets flows during the summer,-"- 



With the establishment of regular weather reconnaissance flights 

 over the ice pack, observations of the pack in all stages of freezing 

 and melting have been made. It is now possible to formulate theories 

 to attack some of the unsolved problems concerning the nature of the 

 pack. This paper outlines a theory covering the annual cycle of the 

 pack and the age of the polar ice, together with some characteristics 

 of the pack and a consideration of some of the problems of forecasting 

 the movements of the pack, 



B, A THEORY OF POLAR PACK HISTORY AS APPLIED TO THE ALASKAN AREA 



The annual cycle of the Polar Pack is probably as follows. In the 

 winter the whole Arctic basin is covered by an elastic ice sheet in which 

 many short, narrow leads and cracks are opened and closed by the stress 

 of wind and currents and by expansion and contraction of the ice. At the 

 edge of the ice sheet there is continual breaking away of floes, but in 

 the inner portions the wind force is mainly exerted in the formation of 

 pressure ridges. Sea ice is elastic because of its salt content sind thus 

 is readily forced into ridges by pressure, the ice bending instead of 

 breaking. Because of the salt content the final breakup in summer is 

 caused not primarily by the external wind and current stresses but by 

 melting of the salt water and salt crystals imbedded in the ice, which 

 have a melting point lower than that of the less salty clear ice. Since 

 prevailing winds during winter are northeasterly, a northerly stress is 

 applied to the ice sheet, forming large pressure ridges on the shores of 

 northern Alaska, western Banks Island, and other exposed areas. 



The summer breakup of the ice is as follows. West of Point Barrow 

 the northerly transport of warm water begins in spring, the current open- 

 ing a lead from Bering Strait northward. At the same time the land warms 

 up, rivers begin to flow after their winter freezeup is ended, and the 

 wann river water is carried to the ice to act as a melting agent. When 



-K^For a complete description of ice and for an explanation of the 

 technical terms in this paper, reference should be made to H.G. Pub, No. 

 609, A FunctioP-al Glossary of Ice Terminology, 



