Isn’t ice WONDERful? On a hot day, nothing goes down quite as well as lemonade poured over a glass full of ice cubes. In fact, ice makes so many things better. For example, we love to use ice to make homemade ice cream!
When Old Man Winter comes calling, falling temperatures can turn creeks, lakes, ponds and even rivers into frozen roads you can skate on. But what about the ocean? If you’ve ever been to the ocean in the winter, you’ve probably noticed that it doesn’t freeze like a small pond might.
So does the ocean ever freeze? If you’ve seen pictures of the North Pole or the South Pole, you know that there are polar ice caps in those places. If the ocean freezes in those areas, why doesn’t the rest of the ocean freeze during the winter?
The freezing point of regular water is 0° Celsius or 32° Fahrenheit. The presence of salt in water, though, reduces the freezing point of water. The more salt in the water, the lower the freezing point will be.
When regular water freezes, hydrogen and oxygen molecules have to bond together. The presence of salt makes it harder for these molecules to bond, because salt doesn’t naturally bond with ice.
When salt particles replace water particles, the freezing rate slows down. This is why salt is often used on icy roads to slow down freezing and make them safer to travel upon.
Normal ocean water usually has about 35 grams of salt for every 1,000 units of water. This lowers the freezing point of ocean water to about -1.8° C or 28.8° F. So ocean water will freeze. It just needs to reach a lower temperature.
Another factor that affects the freezing of ocean water is its movement. Unlike ponds, ocean waves move around constantly. This helps ocean water retain heat. As a result, only really cold areas, such as the North Pole or South Pole, usually get cold enough for ocean water to freeze.
When ocean water freezes, though, only the water part freezes. The salt molecules are pushed below the surface of the ice. As a result, polar ice ends up being freshwater ice that can be melted for drinking water!
About 15% of the ocean contains sea ice for at least part of the year. That might not sound like a lot, but that amounts to about 10 million square miles of sea ice!