Sea ice is simply frozen sea water. Because sea water contains salt, it freezes at a lower temperature than 0° C, the freezing point of freshwater. The freezing point of sea water varies according to salinity, but generally it freezes at - 1.8° C. There are many different ways to classify sea ice, but there are two major types: first year ice and multi-year ice. As their names imply, first year ice forms and melts in a single year while multi-year ice is ice that has survived at least one melt season.
Sea ice grows and recedes in its extent throughout the year, depending on the season and the temperature of the ocean. In the northern hemisphere, the sea ice extent generally increases from October through March, and decreases from April until September.
For detailed information on the various types of sea ice and its distribution, visit the following web sites:
National Ice Center
Canadian Ice Service
National Snow and Ice Data Center