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Salinity may also affect the rate of freezing through its influence 

 on the density of the water. Fresh water contracts on cooling and 

 thus sinks below the surface until a temperature of 'Sd.'2° F. is reached. 

 On further cooling it expands, so that its density decreases. If the 

 cooling takes place at the surface with no other process of mixing at 

 work, the coldest water stays there in a layer. It is then necessary 

 for only this surface later to be cooled to the freezing point for ice 

 to form. Water with a salinity of 5%o has its greatest density at 

 37.2° F., so the entire body of water must be cooled to that tempera- 

 ture before density currents cease. The temperature of maximum 

 density decreases faster than the freezing point with increasing salin- 

 ity, as shown in figure 1. The two temperatures coincide at a salinity 

 of 24.T%o. This means that with a salinity of 24.7%o or greater, 

 density currents operate until the freezing point is reached, and the- 

 oretically the entire body must be cooled to this temperature before 

 ice can form on the surface. 



In nature, however, rapid cooling of still water often occurs under 

 conditions where heat is removed from the surface layers faster than 



SALINITY PARTS PER THOUSAND 



Figure 1 . — Relationship between freezing point and temperature of maximum density for 



water of varying salinity. 



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