Sea Ice: Some Facts

Take a moment to imagine a beach. What do you see?

Perhaps you think back to a hot summer day, with the sun shining and warm sand underfoot. The beach in your mind is filled with people in bathing suits tossing a ball, building a sandcastle, or riding the waves. Or maybe you think about a tropical vacation, with clear blue waters and palm trees reaching toward the sky.

What if I told you some beaches get so cold that you can ski on them? Or the waves take on the sound of wind chimes, with balls of ice crashing into each other? You might be thinking this only happens in the Arctic. But with the right conditions, ice can form in many oceans. In fact, about 15% of the ocean is covered with sea ice throughout the year!

What is sea ice?

Sea ice is different from big glaciers or icebergs, which can also be found floating in ocean waters. The difference is that glaciers and icebergs originate from tightly packed snow on land. When a chunk of that compacted snow breaks off and drifts into open waters, it is considered an iceberg or glacier.

Sea ice, on the other hand, forms, grows and melts in ocean water. In other words, sea ice is frozen sea water!

How sea ice forms?

Freshwater—like the water found in lakes, ponds, and streams—freezes at 32℉ (0℃). The same is true for the water used to make ice cubes in your freezer. When water freezes, it changes from a liquid you can drink into a solid that is firm and stable in shape (at least until it melts!). This happens because the molecules in the water slow down enough to form an organized crystal—what we call ice.

Just like freshwater, ocean water can freeze, but the temperature has to be even lower. You see, any time a substance is dissolved in water, it changes the boiling and freezing points of the liquid. Since seawater contains salt, the temperature must drop to about 28.8℉ (-1.8℃) to freeze.

Here’s something really interesting: only the water freezes into ice, not the salt. This means sea ice can be melted down for drinking water. We’ll talk more about this in a bit. First, let’s find out where sea ice can form.

Where to find sea ice?

Sea ice can be found anywhere where the temperature drops low enough to freeze salt water. Let’s start with when you can find sea ice in the two polar regions of the world.

The Arctic is a polar region located in the northernmost part of Earth, also known as the North Pole. Here, it is very cold all year long. A large mass of sea ice stays in Arctic waters for 12 months of the year. In the Antarctic, or the polar region known as the South Pole, sea ice is seasonal. This means the ice melts away during the warmest part of the year and refreezes when temperatures drop.

Sea ice isn’t limited to polar regions or the areas around the North and South Poles. In the United States, certain types of sea ice can form on coastlines with the right conditions, such as high altitudes or cold climates. Ice has been found in the waters of Lake Michigan, along the shorelines of New England, and of course, in waters surrounding Alaska!

Stages of sea ice

You already know that for sea ice to form, temperatures must drop 28.8℉ (-1.8℃) and hold steady at a low temperature. Since ocean waves are always in motion, sea ice can take on many different formations. Because it needs just the right conditions, sea ice forms in stages.

The first stage of sea ice forming is called frazil . This happens as the ocean water begins to freeze, causing small, needle-like ice crystals (frazil) to form. These ice crystals are small, just 3-4 millimeters (0.12 to 0.16 inches) across. During this stage, the salt from the water is forced out of the crystal and drops down further into the water. (This is why sea ice is almost pure fresh water, making it suitable for drinking!).

If temperatures remain cold enough, sea ice moves on to the next stage of formation. Because ice is less dense than water (especially salt water), frazil crystals float to the surface of the ocean. Once enough frazil crystals appear on the surface, they stick together and create a thin sheet of ice. From here, the sheet of ice can then turn into two different types of sea ice, depending on the conditions of the ocean: nilas or pancake ice .

If ocean waters are calm enough, the thin sheet of ice remains smooth. It is called grease ice because it looks like an oil slick on the water. As grease ice grows and thickens, it becomes nilas. Nilas is a newly frozen sheet of ice that is less than 10 centimeters (4 inches) thick.

But what about when the ocean waters are rough with waves? In that case, sheets of ice are broken apart, as frazil crystals form into slushy circular disks called pancake ice. As the waves push the pancake ice around, it smooths out the edges, making them round.

If at any point during sea ice formation, the temperatures begin to rise, you know what that means! The ice melts and turns back into ocean water.

Even though it is typical to find sea ice in polar regions , people who live on cold coasts sometimes get a surprise of frozen sea ice. When this happens, you might find people gathering on the beach for the stunning sight—but instead of bathing suits, they’re wearing winter coats and boots.