During the decompressing northern winds, the more movable floating ice breaks from the 

 pack ice and moves to the south, forming considerable channels and polynyas at the line of 

 cleavage. During a return movement, they press against the pack ice and undergo hummocking. 



LITERATURE: 62, 77, 88. 



Section 49. Pack Ice 



"Pack ice" is the most finished form of old ice. 



They are large ice fields solidly compressed against each other so that the over-all area 

 of water between them, even during the summer, does not exceed 2 per cent. The thickness of 

 pack fields is not less than 3 m. Their upper surface is level and smoothed. There are no hum- 

 mocks with the sharp configurations of floes. Instead of them, there are high (sometimes up to 

 10 m) but rounded hillocks resembling "sheep's forebrows" in form. Only along the edges of pack 

 fields can hummocks of young ice be observed, which form in fissures between the fields during 

 the winter as a result of the migrations of ice fields which still occur. 



Several processes are necessary for the formation of pack ice: 



1. An initial formation and thickening of ice by the natural method of freezing from the 

 bottom. 



2. An increase in the thickness of the ice due to rafting of separate floes and fields and 

 their fragments upon each other. 



3. The fusion of small floes and packs into large fields, which is particularly character- 

 istic of compression during the winter. 



4. Leveling of the upper and lower surfaces of the ice. This leveling occurs due to the 

 following: 



(a) During the same negative temperature accretion to thin ice is greater than to 

 thick ice. Aside from this, during the period of growth, the lower part of the ice 

 fields is brushlike, consisting of initial ice spicules which are connected very 

 loosely to the lower surface of the ice. With the slightest movement of the ice 



in relation to the water, these spicules break loose from the lower surface of the 

 deeper parts of the ice and float up under the less deep ice. Thus, other condi- 

 tions being equal, the thin ice attempts to equalize its thickness to that of the 

 thicker ice. 



(b) Direct solar radiation, which falls at an angle in the arctic, acts most strongly on 

 the hummocks elevated above the ice and destroys them first of all. The surface 

 of the hummocks, which undergoes scattered solar radiation, is great by com- 

 parison with its volume. In this way, both direct and scattered radiation destroy 

 the elevated parts of the hummocks. 



(c) Melted ice water, which forms as a result of melting during the summer of the 

 hummocks and the snow accumulations near the latter, drains into the depres- 

 sions of the ice fields, and freezing here during the winter, increases the thick- 

 ness of the thinner parts of the ice fields. 



118 



