The minerals dissolved in sea water, which make it taste salty, lower the temperature at which the water will freeze. But at the temperatures found at the far north and south of the globe, the sea is frozen all the time. Further from the Poles, it may also freeze in winter. In fact, the North Pole is permanently frozen sea — there is no land beneath the ice.
Ocean water freezes just like freshwater, but at lower temperatures. Fresh water freezes at 32 degrees Fahrenheit but seawater freezes at about 28.4 degrees Fahrenheit, because of the salt in it. When seawater freezes, however, the ice contains very little salt because only the water part freezes. It can be melted down to use as drinking water. At least 15 percent of the ocean is covered by sea ice some part of the year. On average, sea ice covers almost about 10 million square miles of the Earth.
Sea water becomes more and more dense as it becomes colder, right down to its freezing point. Fresh water, on the other hand, is most dense while still at 39.2 degrees Fahrenheit, well above the freezing point. The average temperature of all ocean water is about 38.3 degrees Fahrenheit.
If the temperature is cold enough, ocean water does freeze. The polar ice cap at earth’s North Pole is a giant slab of frozen ocean water. At earth’s South Pole, the land mass constituting Antarctica complicates the situation, so most of the ice there is compacted snow. Over cold regions such as Antarctica, Greenland, and Canada, the fresh water in the air freezes to snow and falls onto the land without a melting season to get rid of it. Over time, this snow builds up and compacts into an ice mass known as a glacier. Gravity slowly pulls the glacier downhill until it reaches out onto the ocean, forming an ice shelf. The ocean-bound edge of the ice shelf slowly crumbles into icebergs which float off on their own path. For this reason, glaciers, ice shelves, and icebergs are all thick sheets of frozen fresh water and not frozen ocean water. In contrast, when ocean water freezes, it forms a thin flat layer known as sea ice or pack ice. Sea ice has long been the enemy of ships seeking an open route through cold waters, but modern ice breaker ships have no problem breaking a path through the fields of frozen ocean.
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