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Go to Brilliant.org/SciShow to learn more. [♩INTRO]. This weird white stuff looks a bit like a fungus, but it's not.

It's ice! It forms because of the quirky physics of how water freezes in wood. It does need the help of fungi, though.

And that's likely part of why it's incredibly rare. Hair ice, ice wool, or frost beard is a strange form of ice comprised of strands that really are similar to human hairs in terms of size and shape. Each is about 20 microns in diameter and can grow up to 20 centimeters long that'd be about shoulder length if it was coming from your scalp!

But what's really weird is that hair ice only forms on dead, rotting branches of hardwood trees where there's no bark… in forests between 45 degrees and 55 degrees north latitude… when the temperature is just below zero degrees Celsius… and the air is pretty humid. Part of that specificity comes from how the strands form a process known as ice segregation. As the temperature drops below freezing, the water inside a rotting branch starts to freeze.

But, thanks to interactions with the molecules of the wood, it doesn't form solid ice. It becomes supercooled instead. Ice does form near the outer edge, though, in small pores in the wood.

Then, at the interface between ice and supercooled water the ‘root' of the hair, if you will more ice grows. Since the crystalline ice takes up more space than liquid water, and the air outside puts up less of a fight than the water in the wood, the forming ice pushes the already-formed ice out of the branch and a thin strand the size of the wood's pore forms. As each bit of water next to the base of the strand becomes frozen, it pulls more supercooled water out of the interior of the wood.

And this process repeats until all the water in the wood is used up. But for any of this to happen, the temperature has to be just right. If it's too cold, the water inside the branch will freeze solid, and if it's too warm, the starter ice won't form on the outer surface.

So a temperature just below zero degrees Celsius is ideal. The air has to be humid too, because if conditions are too dry, the ice will sublimate into water vapor faster than the ice strands can grow. And hardwoods are optimal because they have radial structures inside them called medullary rays.

These act as channels, allowing water to easily travel from the center of a branch to the outside. So if you have all of this in place, you can end up with ice hairs. And these can stick around for hours, or even days.

Which is surprising, too, because normally, such fine ice crystals would melt ever so slightly and then recrystallise into a less attractive shapeless clump. To explain why this doesn't happen, way back in 1918, a German scientist suggested that fungi might be involved. And it turns out he was right.

A 2015 study found that the fungus Exidiopsis effusa is also present in the dead wood. And it secretes proteins called recrystallisation inhibitors. For the fungus, these act much like the antifreeze proteins some animals have to stop the water in their cells from freezing at temperatures below zero.

And similarly, when they end up in the supercooled water that becomes hair ice, they stop the strands from recrystallising into formless lumps. Researchers actually demonstrated that the same branches will produce ice hair over and over again, until you kill the fungi inside of them. It's not clear if there are multiple fungal species that can facilitate hair ice or it's just this particular fungus.

But either way, forests with hardwood trees at latitudes between 45 and 55 degrees north are probably the only places on the planet where physics and biology come together to allow this awesome and unique kind of ice to form. Hair ice is a great example of why understanding the world takes all sorts of science knowledge. But if it's been a little while since your last math or physics class, well, that's something Brilliant.org can help with.

Brilliant has hands-on, interactive courses which cover topics in science, engineering, computer science and math. So if you want to, say, understand how snowflakes form, you can brush up on complex numbers, and then dive deep into group theory. And if the winter weather makes your internet spotty, you can continue the courses offline using Brilliant's iOS and Android apps.

You can learn more at Brilliant.org/SciShow. Plus, if you're one of the first 200 people to sign up at that link, you'll get 20% off the annual Premium subscription! So check it out and see if Brilliant is right for you.

And as always, thanks for watching SciShow! [♩OUTRO].