CHAPTER VI 

 ICE ACCRETION 



Section 82. The Centers of Freezing 



All observers are amazed by the speed with which the initial forms of ice appear on the sur- 

 face of the sea after the water reaches the freezing point. At several degrees below freezing, only 

 a few hours are required for the ice sludge to extend as far as the eye can see; after several more 

 hours, the sludge turns into nilas, or pancake ice. 



However, the surface layers of the sea never cool to the freezing point simultaneously over 

 the entire sea. For one reason or another, such cooling ends in certain areas earlier than in 

 others, and then from these areas, as from centers of freezing, it expands in all directions but 

 not at the same rate . 



When making theoretical estimates of the position of these centers of freezing, one should 

 consider first the distribution, at the given moment, of the freezing indexes in the sector of the 

 sea under investigation, and secondly, the rate of cooling of the sea. 



As we have seen, by freezing index, we mean the amount of heat released by 1 square cm of 

 sea surface on cooling to the freezing point. In the given instance, let us use the term "rate of 

 cooling" to indicate the decrease of the freezing index per unit time, as a function of meteorological 

 conditions during the pre- winter period. 



It is clear that the time interval prior to the appearance of the initial forms of ice will be 



where q is the freezing index and dq /dT is the rate of cooling. 



If we were to compute the intervals of time prior to freezing for some moment at points 

 equally distributed over the area of the sea, and then draw isolines of time (isochrones), the 

 centers of freezing would be located at the points where the time interval before freezing is 

 minimum . 



From the formula, it follows that the periods of time before freezing, are the smaller, the 

 lower the freezing index and the higher the rate of cooling, but, of course, even high freezing 

 indexes can be quickly destroyed in the presence of a high rate of cooling. 



As we have seen, other conditions being equal, the freezing index is the smaller, the shal- 

 lower the location, the lower the temperature of the sea, and the greater the vertical salinity 

 gradient which limits the distribution of vertical winter circulation. Thus, it can be considered 

 that, generally, the freezing index is smaller in shallow water, especially offshore (slight depth), 

 at high latitudes and near ice massifs (low water temperature), near the estuaries of rivers and 

 near ice massifs (high vertical salinity gradients). 



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