the land warms still more in the continuous daylight, a shore lead de- 

 velops between the outer sand spits and the main ice pack. Further 

 warming of the ice by the continuing transport of warm water from the 

 Bering Sea and the rise of air temperatures to about 30-35 degrees F. 

 causes much melting of the inner pack, with floes becoming detached 

 at weak points and the water surface being covered with from 8 to 10 

 tenths floes and blocks. At this stage in the melting, wind stresses 

 can cause rapid changes in the concentration, packing the ice from a 

 loose coverage of 6 tenths to a coverage of 10 tenths in a matter of 

 hours. The wind can thus close or open the shore lead temporarily. 

 In midsummer there is always open water toward the middle of the basin, 

 with about one tenth open water observed at BU"^ 144°W and about 3% 

 open water near the North Pole. The heaviest concentrations occur 

 because of wind- and current-driven packing on windward shores. 



Midsummer conditions in the Arctic basin show the pack at the stage 

 of maximum deterioration. North of Alaska there is open water every- 

 where between the ice floes. The size of the remaining floes ranges 

 from blocks to giant floes, with few fields present. Floes have been 

 measured at from Ig to 2| miles in length, but complete listing of 

 sizes is not possible at the present time. Floes which are made up of 

 old ice remain longer than those composed of winter ice. Ice islands, 

 or land shelf ice, float through the pack as isolated, strong, thick, 

 single floes; they are very rare, with probably not more than a score 

 or so existing in the Arctic basin at the present time. (Hundreds of 

 small ice islands are located within the Canadian Archipelago.) The 

 hard, blue polar ice remains throughout the summer, although it becomes 

 heavily puddled. In the summer months the southern edge of the main 

 pack lies approximately 25 to 50 miles offshore from northern Alaska, 

 thence westward at Vl'N to Wi-angell Island. North of Canada the pack 

 boundary runs northeastward toward the northwest tip of Banks Island,^ 

 The edge of the pack is ordinarily well defined, with few areas of ice 

 separated from the main body. Occasionally a belt of blocks and small 

 floes is detached by winds in the Chukchi Sea and floats northward on 

 the current toward Point Barrow, causing interference with shipping 

 during its passage. In the area east of Point Barrow the pack moves 

 almost as a unit, wind forcing the edge inshore or offshore, the concen- 

 tration at the edge changing with wind direction. 



At the end of the short polar sujimer, freezeup occurs, and the sur- 

 face of the Arctic Ocean is transformed from floes separated by open water 

 to the typical winter condition of solid coverage vdth ice under heavy 

 pressure. The freezeup is the reverse of the summer breakup in many re- 

 spects. The freezing begins in the inner areas of the pack v/hen falling 

 air temperatures, due to radiation, cause the formation of young ice in 

 the open leads and cracks. The relative freshness and low salinity of 

 the open water facilitate this freezing, since the freezing point is 

 near 32°F. The first formation of young ice occurs in raid-August, and 

 is follovred by alternate thaws and freezes until late September or early 

 October, v/hen the coverage becomes and remains essentially ten tenths 

 until late April or May. TfYind stress forms pressure ridges between the 

 strong hard floes of polar ice, and the young ice, which is very elastic. 



