of observing ice formation on the calm, mirrorlike surface of the sea. This ice was in part needles 

 and in part a thin scale. Before this I had already often observed such a phenomenon on an arctic 

 sea, i. e. , I had seen ice formation with an air temperature above 0°. " 



On 17 May 1923 the same phenomenon was observed by Amold-Aliabev from the icebreaker 

 Lenin in the Gulf of Finland at the southern tip of Hogland Island in open water near continuous 

 floating ice. In calm, clear weather, the ice, on being removed from the water, proved to be sev- 

 eral tens of millimeters thick, and looked like flat cakes, i. e. , plates of scaly and branched struc- 

 ture with irregular edges. Solid transparent ice about 2 mm thick had formed in about 15 minutes. 



These examples show that the formation of surface ice during high water and air tempera- 

 tures, but necessarily during the absence of wind and with a clear sky, is an ordinary phenomenon. 

 Even Scoresby explained it by radiation. The observations on the Perseus are characteristic only 

 in that the temperature of the air and water are usually high in comparison with the temperatures 

 observed earlier. 



LITERATURE: 8, 62, 77, 120. 



Section 40. The Basic Varieties of Ice in the Sea 



Ice structure, based on its origin, is arbitrarily divided into "needle ice" and "sponge ice. " 

 As we shall see below, sea ice, in the course of its birth, life and death, undergoes strong 

 physical-chemical and thermodynamic changes, but its basic properties, remain the same. Needle 

 ice forms slowly, a considerable part of the brine drains down from the interlayers between the 

 crystals, and the air is separated out along the vertically located cells. Because of this, needle 

 ice is freer of impurities, is more transparent and durable than sponge ice. The latter always con- 

 tains more of the various types of impurities, which is particularly noticeable if such ice is formed 

 at or near the bottom of the sea. In the latter case, particles of silt, etc. , can be found in the 

 brine cells. 



The division of the sea into needle ice and sponge ice according to its structure, is more or 

 less arbitrary, as has already been pointed out. Transitional forms are usually observed in nature. 

 Furthermore, we can observe individual layers in the same block of ice. Thus, for example, deep 

 ice of spongy structure, after it has floated to the surface begins to grow from below as needle ice. 

 Needle ice which forms, when broken into separate pieces by the wind, can be overgrown with 

 spongy ice, if the surrounding water is sufficiently cooled and agitated. Mechanical causes have a 

 still greater effect on individual ice beams. During agitation, wind and compressions, ice floes 

 may be pushed up on top of one another, and freeze together. As a result, ice is obtained which 

 consists of several more or less uniform layers separated by interlayers formed, in the majority 

 of cases, from the snow which covers the upper surfaces of the lower ice floes. Finally, when one 

 ice flow slides over another, equilibrium can be destroyed and the ice can turn over or the surface 

 of separation can become tilted. Thus, the stratification of the ice is a sign of the changes which it 

 has undergone in the course of its existence. * 



Ice formed from snow occupies a somewhat special place, due to its structure. Snow which 

 has fallen on the sea surface, whose temperature is near the freezing point, does not melt, but is 

 permeated by sea water, becomes denser, and, as we have seen, aids in the freezing of water. 



*Such dynamic stratification of ice can, and must be, differentiated from its thermal stratifi- 

 cation, which is created as a result of changes in the temperature of the air and water during ice 

 formation. I will deal with the question of thermal stratification in Section 84. 



95 



