upwards, deep ice sticks to the surface ice and forms ice jams, narrowing the active cross section 

 of the river (on the Svir' River, e. g. , sometimes by 80 per cent). 



LITERATURE: 4, 5, 13, 26, 62, 77. 



Section 38. Peculiarities of Ice Formation in the Sea 



Let us assume that at the beginning of freezing, with the sea at rest, we have a series of par- 

 allel layers, uniform in temperature and salinity in which the density of the layers increases with 

 depth. 



After the surface layer cools somewhat below the freezing point, nuclei or cyrstallization 

 begin to form and ice crystals begin to grow in the fallen needles. These needles, if the sea is 

 calm, develop very rapidly at low temperatures and change to plate crystals. The ends of these 

 crystals usually are irregular and slightly rounded. The salt dissolved in the sea simultaneously 

 pass into the interlayers between the crystals where they form "brine. " The latter, due to its re- 

 latively high salinity, in part drains down along the crystals (which causes convective currents in 

 the entire surface layer of the water), and in part, with sufficiently intense cooling, remains 

 frozen in between the crystals of pure ice in the form of brine-filled cells. With sufficiently rapid 

 freezing, the gasses and air bubbles dissolved in the water are concentrated in similar cells. 



During the summer in the polar basins, the interstratification of the surface layers is clearly 

 expressed in the majority of cases, and the salinity increases sharply with depth. The surface 

 layers are diluted by the melting of ice and by the influx of fresh waters, whereas the deep layers 

 are either of oceanic origin (high salinity and high temperature) or they have been made saline by 

 the winter ice formation. Because of this, the upper layer can be very thin here, and at the same 

 time differ very much in salinity from lower-lying layers. 



Thus, the most favorable conditions for the start of ice formation are in calm seas, the pres- 

 ence of a thin and a very fresh surface layer, and a great release of heat to the atmosphere (which 

 is assisted by low air temperature, causing strong convection, or a very dry transparent air, 

 causing strong evaporation and radiation). The fall of even a small amount of precipitation in solid 

 form on the surface of the sea Intensifies the process still more. As many observers have noted, 

 the more favorable the conditions for ice formation, the finer are the developing crystals and the 

 more uniform and stable are the forms created from them. 



The indicated phenomena are also characteristic for the thicker surface layers of the sea. 

 With the beginning of ice formation, convective currents always arise, whose intensity is deter- 

 mined by the intensity of cooling, and up to the time the surface of the sea is covered by a layer of 

 ice (no matter how thin) the nuclei of crystallization can occur, and around them, and throughout 

 the entire mass of the upper convection-mixed layers, new ice formations can develop. 



But in the sea, aside from convective mixing, one observes extremely intense mixing of upper 

 layers of the water by wave motion or currents. Under such conditions, depth ice and even bottom 

 ice, may be formed first. 



In separate areas of the arctic seas, the vertical winter circulation can be so strong, that 

 when the layer involved in a circulation is thin, the release of heat by the water to the atmosphere 

 may be sufficient for forming embryos of deep ice In this layer. 



93 



