The ocean does freeze, just at a colder temperature than fresh water — and when it does, something surprising happens to the salt. At least 15 percent of the ocean is covered by sea ice for some part of the year, and on average sea ice covers about 10 million square miles of the Earth.
The Arctic. At least 15 percent of the ocean is covered by sea ice for some part of the year. Credit: NOAA.
A lower freezing point
Fresh water freezes at 32 degrees Fahrenheit. Seawater, because of the salt in it, freezes at about 28.4 degrees Fahrenheit.
Salt stays behind
When seawater freezes, only the water freezes, so the ice contains very little salt. Melted down, sea ice can be used as drinking water.
Colder and denser, all the way down
Seawater and fresh water also behave differently as they cool:
| Seawater | Fresh water | |
|---|---|---|
| Densest at | its freezing point — it keeps getting denser as it cools | 39.2 degrees Fahrenheit, well above freezing |
| Freezes at | about 28.4 degrees Fahrenheit | 32 degrees Fahrenheit |
The average temperature of all the water in the ocean is about 38.3 degrees Fahrenheit.
Sources
Based on "Can the ocean freeze?" from NOAA's National Ocean Service, a work of the United States government in the public domain.
Licence: CC0 1.0 (public domain) · Adapted from oceanservice.noaa.gov
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