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[♩INTRO]
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This weird white stuff looks a bit like a fungus, but it's not.
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It's ice!
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It forms because of the quirky physics of how water freezes in wood.
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It does need the help of fungi, though.
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And that's likely part of why it's incredibly rare.
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Hair ice, ice wool, or frost beard is a strange form of ice comprised of strands
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that really are similar to human hairs in terms of size and shape.
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Each is about 20 microns in diameter and can grow up to 20 centimeters long
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that'd be about shoulder length if it was coming from your scalp!
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But what's really weird is that hair ice only forms on dead, rotting branches
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of hardwood trees where there's no bark…
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in forests between 45 degrees and 55 degrees north latitude…
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when the temperature is just below zero degrees Celsius…
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and the air is pretty humid.
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Part of that specificity comes from how the strands form
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a process known as ice segregation.
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As the temperature drops below freezing,
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the water inside a rotting branch starts to freeze.
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But, thanks to interactions with the molecules of the wood,
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it doesn't form solid ice.
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It becomes supercooled instead.
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Ice does form near the outer edge, though, in small pores in the wood.
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Then, at the interface between ice and supercooled water
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the 'root' of the hair, if you will
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more ice grows.
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Since the crystalline ice takes up more space than liquid water,
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and the air outside puts up less of a fight than the water in the wood,
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the forming ice pushes the already-formed ice out of the branch
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and a thin strand the size of the wood's pore forms.
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As each bit of water next to the base of the strand becomes frozen,
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it pulls more supercooled water out of the interior of the wood.
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And this process repeats until all the water in the wood is used up.
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But for any of this to happen, the temperature has to be just right.
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If it's too cold, the water inside the branch will freeze solid, and if it's too warm,
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the starter ice won't form on the outer surface.
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So a temperature just below zero degrees Celsius is ideal.
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The air has to be humid too, because if conditions are too dry,
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the ice will sublimate into water vapor faster than the ice strands can grow.
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And hardwoods are optimal because they have radial structures
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inside them called medullary rays.
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These act as channels, allowing water to easily travel from the center
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of a branch to the outside.
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So if you have all of this in place, you can end up with ice hairs.
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And these can stick around for hours, or even days.
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Which is surprising, too, because normally, such fine ice crystals would
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melt ever so slightly and then recrystallise into a less attractive shapeless clump.
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To explain why this doesn't happen, way back in 1918, a German scientist
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suggested that fungi might be involved.
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And it turns out he was right.
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A 2015 study found that the fungus Exidiopsis effusa is also present
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in the dead wood.
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And it secretes proteins called recrystallisation inhibitors.
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For the fungus, these act much like the antifreeze proteins some animals
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have to stop the water in their cells from freezing at temperatures below zero.
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And similarly, when they end up in the supercooled water that becomes hair ice,
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they stop the strands from recrystallising into formless lumps.
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Researchers actually demonstrated that the same branches
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will produce ice hair over and over again, until you kill the fungi inside of them.
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It's not clear if there are multiple fungal species that can facilitate hair ice
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or it's just this particular fungus.
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But either way, forests with hardwood trees
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at latitudes between 45 and 55 degrees north
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are probably the only places on the planet
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where physics and biology come together
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to allow this awesome and unique kind of ice to form.
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Hair ice is a great example of why understanding
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the world takes all sorts of science knowledge.
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But if it's been a little while since your last math or physics class,
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well, that's something Brilliant.org can help with.
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Brilliant has hands-on, interactive courses which cover
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topics in science, engineering, computer science and math.
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So if you want to, say, understand how snowflakes form,
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you can brush up on complex numbers, and then dive deep into group theory.
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And if the winter weather makes your internet spotty,
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you can continue the courses offline using Brilliant's iOS and Android apps.
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You can learn more at Brilliant.org/SciShow.
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you'll get 20% off the annual Premium subscription!
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So check it out and see if Brilliant is right for you.
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And as always, thanks for watching SciShow!
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[♩OUTRO]