E. Linacre and B. Geerts
The freezing temperature of sea water with the normal 3.5% of salt, approximately, is -1.8°C (1). The rate at which ice thickens depends on two factors, the difference of air temperature below that freezing point (DT), and the duration of such conditions (Dt), i.e. the depth of first-year ice (D) is proportional to a time integral starting at the beginning of the cold season:
D ~ ň DT.dt
For instance, the thickening during four days each with an air temperature averaging - 2.8°C is roughly the same as that during one day at -5.8°C. As long as the maximum temperature remains below -1.8°C, one can use the daily-mean temperature Tm, and ň DT.dt can be simplified to a summation ĺ DTm for all days of the season, and the result is known as the ‘number of freezing degree-days’.
Sea ice contains only about a tenth of the amount of salt that sea water has. The fraction is even less if freezing is slow. The salt migrates out of the ice when it melts, so that the surface ice becomes potable.
Large areas of sea ice exist around Antarctica and the Arctic. The radiation balance and exchanges of heat and moisture over an ice-covered ocean are very different from those over an open ocean. Therefore sea ice may have a significant effect on global climate, and both the extent and the thickness of sea ice need to be carefully monitored.
(1) Budinger, T.F. 1974. Icebergs and pack ice. Encyclopedia Britannica (15th edn) 9, 154-61.