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Speaker 1: Welcome to brain stuff from how stuff works, Hey, brain stuff,

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Speaker 1: Lauren vocal bam here. When temperatures dip below freezing, it

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Speaker 1: adds a number of extra hazards to driving, but perhaps

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Speaker 1: none so tricksy as black ice. These are slick patches

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Speaker 1: of ice that blend in with the black pavement beneath

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Speaker 1: them because they're so transparent. When drivers blind to the

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Speaker 1: danger steer their cars over the ice, they can lose

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Speaker 1: traction and wind up in an accident. Other kinds of

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Speaker 1: ice are easier to see. You've no doubt noticed that

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Speaker 1: homemade ice cubes usually look cloudy in the little opaque

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Speaker 1: in the middle, like whitish blocks of cotton candy. That

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Speaker 1: stands in contrast to the frightening clarity of black ice.

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Speaker 1: So how come black ice is see through but the

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Speaker 1: cubes in your typical ice trays are not. Liquid water

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Speaker 1: appears to be clear. That's because the material neither absorbs

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Speaker 1: nor reflects most of the spectrum of visible light. Most

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Speaker 1: wavelengths in the spectrum pass right through it. And that's right,

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Speaker 1: I said. Most technical water is not colorless, although it

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Speaker 1: often looks that way. Believe it or not, The liquid

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Speaker 1: has a natural bluish tint, owing to the fact that

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Speaker 1: it absorbs red, yellow, and orange light more easily than

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Speaker 1: blue light. But our human eyes can only observe this

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Speaker 1: azure quality in deep bodies of water. This helps explain

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Speaker 1: why ocean water and glacial ice look blue to us,

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Speaker 1: while glassfuls of drinking water look transparent. Yet there is

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Speaker 1: a deceptive quality to water. No matter how fresh and

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Speaker 1: clear it may seem, it's never one pure. A sample

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Speaker 1: of H two O might contain floating bits of organic

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Speaker 1: matter like algae or plant remains, along with suspended sediments

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Speaker 1: a dust particles, or flexi minerals like calcium and lime.

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Speaker 1: It may also harbor lots of dissolved gases such as oxygen.

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Speaker 1: With outsid oxygen, fish wouldn't be able to breathe. Gases

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Speaker 1: and physical impurities are the key to understanding why those

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Speaker 1: ice cubes in your tray are so darn cloudy. When

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Speaker 1: water freezes, internal debris and air bubbles can become concentrated.

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Speaker 1: The class stirs impede light, causing all the differently colored

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Speaker 1: frequencies on the visible light spectrum to scatter. When this happens,

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Speaker 1: it makes the ice look white and clouded. But it

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Speaker 1: doesn't always happen. Let's take for a case study, a

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Speaker 1: YouTuber who once appeared to walk on water. Thomas Nunnock

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Speaker 1: filmed himself in a travel companion walking across vel k

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Speaker 1: Hankovo place So, a deep freshwater lake in the Slovakian Mountains.

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Speaker 1: His video went viral because it shows the two men

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Speaker 1: standing on some unbelievably clear ice. The lake bottom rocks

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Speaker 1: below them are pristinely visible through the frozen water. The

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Speaker 1: ice and Nunus video was so transparent that some viewers

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Speaker 1: assumed the footage had been faked, but the phenomenon is

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Speaker 1: very real, and when the lake freezes over, as it

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Speaker 1: often does, the surface ice can be crystal clear. An

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Speaker 1: ice sheet needs to be almost entirely free of both

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Speaker 1: air bubbles and debris in order to have this degree

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Speaker 1: of clarity. There are other requirements as well. Ice is

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Speaker 1: made up of crystals, and when water eases rapidly, those

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Speaker 1: crystals tend to be small and numerous. That makes the

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Speaker 1: ice look whider because each of those crystals have reflective surfaces,

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Speaker 1: so if there are more crystals, there will be more

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Speaker 1: reflection going on, which will help scatter incoming light on

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Speaker 1: the other hand, ice that's been slowly frozen is built

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Speaker 1: out of fewer and larger crystals, and those promote transparency.

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Speaker 1: So ice will only be see through and crystal clear

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Speaker 1: if it freezes slowly and doesn't have too many impurities

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Speaker 1: or crystals. Thomas knocks Now famous YouTube video is what

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Speaker 1: it looks like when all these conditions are met. On

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Speaker 1: the day of his trek, the lake was topped with

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Speaker 1: an icy blanket that was around two centimeters thick. That's

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Speaker 1: about three quarters of an inch. The water must have

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Speaker 1: frozen gradually during a tranquil period with no strong winds

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Speaker 1: to deposit debris in the lake or to churn up

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Speaker 1: the water. We see the same kind of thing happening

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Speaker 1: in roadway black ice, although not all of this stuff

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Speaker 1: comes from the same source. It can start out as fog, mist, drizzle,

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Speaker 1: or melted snow. It always freezes slowly during low wind periods.

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Speaker 1: It's consistently then, and it contains very few impurities. That's

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Speaker 1: why black ice is so transparent and frustrating to motorists,

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Speaker 1: but highway annoyances nonwithstanding, transparent ice isn't as common in

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Speaker 1: nature as the white clouded variety. There are a lot

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Speaker 1: of reasons why given ice chunk you find out in

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Speaker 1: nature might not be clear. Some ice is made up

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Speaker 1: of fallen snowflakes whose angular shapes and regular crystals scattered

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Speaker 1: light and sometimes temperatures drop rapidly, resulting in ice with

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Speaker 1: those smaller crystals. Artificial ice is subject to these same

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Speaker 1: physical laws. Most household ice trays are designed to freeze

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Speaker 1: little blocks of water from the outside. In doing so

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Speaker 1: has the side effect of driving impurities within the water

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Speaker 1: towards the center of each cube. The finished products therefore

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Speaker 1: look white and hazy in the middle. To get clearer,

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Speaker 1: more attractive cubes, some restaurants use expensive machines that freeze

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Speaker 1: water very gradually and exclusively from one side that lets

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Speaker 1: the impurities escape outward. Though if you want to make

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Speaker 1: clearer cubes at home, you can do it on cheap.

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Speaker 1: Try boiling the water first, thus ridding it of dissolved

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Speaker 1: gases before the freezing process starts, and just blood it

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Speaker 1: cool of it before you add it to your ice tray.

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Speaker 1: You wouldn't want to melt the plastic. Today's episode was

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Speaker 1: written by Mark Mancini and produced by Tyler Clay. Shop

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Speaker 1: for your favorite brainy companion in our online shop to

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Speaker 1: public dot com slash brain stuff, and of course, for

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Speaker 1: more on this and lots of other completely clear topics,

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Speaker 1: visit our home planet, how stuff Works dot com