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Ice formed from frozen seawater in polar regions. Most sea ice occurs as pack ice, which drifts across the ocean surface; other types of sea ice include fast ice, which is attached to coasts and sometimes the seafloor or between grounded icebergs, and marine ice, which forms at the bottom of ice shelves in Antarctica. The sea ice of the Northern Hemisphere covers about 3 million sq mi (8 million sq km) in September and about 6 million sq mi (15 million sq km) in March. Sea ice in the Southern Hemisphere ranges from about 1.5 million sq mi (4 million sq km) in February to about 8 million sq mi (20 million sq km) in September.
This entry comes from Encyclopædia Britannica Concise. For the full entry on sea ice, visit Britannica.com.
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