CHAPTER IV 

 ICE FORMATION AND ICE TYPES IN THE SEA 



Section 36. The Concept of Ice Formation 



In regard to what we have learned about crystallization, ice formation does not begin imme- 

 diately throughout the entire liquid after cooling to the melting point, but at individual, chaotically, 

 but equally distributed points in it, where nuclei of crystallization are already present or are being 

 formed. The process of generation of nuclei of crystallization is unknovra. It is considered that 

 nuclei of crystallization occur around the smallest suspended particles of organic and inorganic ori- 

 gin, which always exist in natural water and which are formations of the most varied size, form, 

 and structure. In particular, Wegener points out that the dust carried by air consists primarily of 

 quartz grains which serve as excellent nuclei of crystallization. Under natural conditions, besides 

 this, the tiniest of ice crystal particles already existing in a given volume of water, are snow crys- 

 tal particles which fall on the surface, these often serve as nuclei of crystallization. Ice and snow 

 crystal particles, as Altberg points out, play a dual role: on one hand, they play the role of nuclei 

 of crystallization around which further ice accretion occurs, on the other hand, they are special 

 seeds, viz. , accelerators of the growth of nuclei of crystallization and their conversion into ele- 

 mentary ice particles. 



The effect of ice crystal particles as seed crystals is easily demonstrated by the experiment 

 described by Shenrok. 



A grain of salt with snow flakes on it was thrown into slightly supercooled water. This grain 

 slowly sank to the bottom of the vessel and during this the effect of "a crawling meteor with a tail 

 behind it" was created. The crystal particles moving through the water in turn continually caused 

 the formation of other crystal particles, so that the water was gradually enriched by them. The 

 process occurred much faster when the water was agitated. 



The initial formation of nuclei of crystallization and elementary ice particles always requires 

 a certain supercooling of the liquid. The purer the liquid and the calmer its state, the greater 

 must the original supercooling be. It is known that under laboratory conditions, pure water can be 

 cooled to -32°C, but it is enough even for a slightly supercooled liquid, to introduce a small piece 

 of ice to start rapid ice formation immediately. As Altberg shows, a single grain of ice thrown 

 into a tank of water supercooled to -0. 1° creates 2 to 3 kg of ice in 30 seconds. 



In natural water there are always some types of impurities on which nuclei of crystallization 

 will form, and furthermore natural waters are always involved in some movement for some reason 

 or another. Because of this, the supercooling necessary for the formation of nuclei of crystalliza- 

 tion in natural water, is always extremely slight; it is less, the more intense is the movement of 

 the water and the more particles are suspended in it. 



Supercooling of water is also necessary for the further accretion of ice on nuclei of crystal- 

 lization. Actually, this process requires a slight temperature difference between water and ice. 



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