Here’s Why Ice Usually Freezes Cloudy, Not Clear

Things have gotten chillier up in the Northern Hemisphere. With chilly seasons come their challenges. For example, if there’s one term guaranteed to make car-owners lose their cool, it’s “black ice.” The National Weather Service defines this as “patchy ice on roadways or other transportation surfaces that cannot easily be seen.”

To the human eye, it’s all but invisible. Black ice isn’t really black; it’s transparent. The stuff only looks black when it’s covering a layer of jet-black pavement. Accidents happen once drivers, blind to the danger, steer their cars over the ice and lose traction.

Other kinds of ice are easier to see. You’ve no doubt noticed that homemade ice cubes usually look cloudy and opaque in the middle, like whitish blocks of cotton candy. That stands in contrast to the frightening clarity of black ice. How come black ice is see-through, but the cubes in your typical ice trays aren’t?

Feeling Transparent

Liquid water appears to be clear. That’s because the material does not absorb most of the visible light that passes through it. Yet there’s a deceptive quality to water: No matter how fresh it may seem, it’s never 100 percent pure. A sample of H2O might contain floating bits of organic matter — algae, plant remains, etc. — along with suspended sediments, dust particles or flecks of minerals like calcium and lime. It may also harbor lots of dissolved gases, such as oxygen. (Without said oxygen, fish wouldn’t be able to breathe.) Gases and physical impurities are the key to understanding why those ice cubes in your lemonade pitcher are so darn cloudy.



Walking on Water


An ice sheet needs to be almost entirely free of both air bubbles and debris in order to have this degree of clearness. There are other requirements as well. Ice is made up of crystals — and when H2O freezes rapidly, those crystals tend to be small and numerous. That makes the ice look whiter because crystals have reflective surfaces. So if there are more crystals, there’ll be more reflection going on, which will help scatter incoming light. On the other hand, ice that’s been slowly frozen is built out of fewer and larger crystals. And those promote transparency.

To recap, ice will only become see-through and crystal clear if it freezes slowly and doesn’t have too many impurities or crystals. Tomas Nunuk’s now-famous YouTube video is a case study of what it looks like when all these conditions are met. On the day of his Slovakian trek, Velke Hincovo Pleso was topped with an icy blanket that was around 2 centimeters (or 0.78 inches) thick. The water must have frozen gradually during a tranquil period with no strong winds that might’ve blown debris into the lake or churned up the water.

Black Ice and Tray Trouble

We see the same kind of thing happening in roadway black ice. Although not all of this stuff comes from the same source (it can start out as fog, mist, drizzle, or melted snow), it always freezes slowly during low-wind periods, it’s consistently thin, and it contains very few impurities. That’s why black ice is so transparent — and frustrating to motorists.




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