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How Come? Salty seas stay sloshy

Why don't the oceans freeze? Zoe Gunderson, a student in Brookville

Even as ponds and lakes obligingly freeze over in winter, the ocean stays stubbornly liquid, waves rolling into shore in January just like in July.

How come? First, a big parcel of water acts as a heat reservoir. Since oceans are so much bigger than lakes, they hold onto much more heat, and for much longer. Second, oceans never stand still; tides and rolling waves make it difficult for ice to form.

Finally, there's the freezing point of water. Which -- surprisingly -- isn't a constant. Under everyday conditions, fresh water (like tap water) begins to freeze at 32 degrees Fahrenheit. At 32 degrees some water molecules are locking into crystal lattices (freezing), even as others are breaking away (melting). But as the temperature drops, freezing outpaces melting. Presto: ice cubes!

But when salt is dissolved in water, freezing is short-circuited. A dissolved salt molecule separates into its atoms, sodium and chlorine. These charged atoms (ions), electrically attracted to water's H2O molecules, cluster around them. Result: Some water molecules are kept from attaching to the forming ice crystals. And freezing slows down, while melting continues as usual.

So salt dissolved in water lowers its freezing point. Which is why Earth's salty ocean water must reach a temperature of about 28.4 degrees. before it starts to freeze. Combine a lowered freezing point with the ocean's retained heat and constant movement, and salty seas stay sloshy.

However, ocean water will freeze if the temperature drops low enough. In the frosty waters around Antarctica, ice forms on the sea's surface in winter, when temperatures dip to as low as -58 degrees. The cap of frozen saltwater will gradually extend down 3 to 6 feet. In the Arctic, where sea ice lingers longer, ice can jut 15 feet under the surface.

Some of Earth's lakes are freeze-resistant, too. There's the Dead Sea in the Middle East, a deep lake with water that's nearly 34 percent salt. There's Djibouti's Lake Assal, a body of water in the Horn of Africa with a salinity level of nearly 35 percent. And there's Don Juan Pond in Antarctica, winner of the sodium sweepstakes with an astonishing 44 percent salt.

In the United States we can visit Utah's Great Salt Lake. Depending on evaporation and water levels, the lake is about three to eight times saltier than Earth's oceans. Wade in, and you'd find yourself in a (pungent) soup, topping out at 27 percent salinity. When the water is at its saltiest, the temperature would have to fall to a frigid 12 degrees. for the Great Salt Lake to freeze over.

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