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Duration:02:47
Uploaded:2017-09-16
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You’re in the woods, there’s a full moon, and the wind begins to howl. We can’t take you out of this horror movie scenario, but we can explain why the wind sounds so spooky.

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Sources:

https://method-behind-the-music.com/mechanics/physics/
http://hmf.enseeiht.fr/travaux/CD0102/travaux/optmfn/gpfmho/01-02/grp1/phy_know.htm
https://ocw.mit.edu/courses/mechanical-engineering/2-22-design-principles-for-ocean-vehicles-13-42-spring-2005/readings/lec20_viv1.pdf
http://www.ambius.com/blog/how-plants-reduce-noise/
https://www.physicsforums.com/threads/how-does-wind-produce-sounds.118880/
https://arxiv.org/abs/1202.2245
http://www.slate.com/blogs/bad_astronomy/2013/04/12/atmospheric_vortices_flow_past_an_island_seen_from_space.html
It's a dark and stormy night. There's a strange creaking sound coming from somewhere. The wind is howling through the trees, and you just know the main character in the story is in some dire straits.
But what makes the wind howl in the first place? And would a horror story's classic forest of dead trees really make for louder winds than a forest full of leafy branches?
Let's start with sounds, which are just vibrations in the air around us that get picked up by our ears. Faster vibrations translate to a higher-pitched sound, slower vibrations translate to a low-pitched sound, and everything in between - that's in-between. So, to get a certain sound, all you need is air vibrating at the right rate. These vibrations are usually measured in units of Hertz, or the number of movements back and forth per second. And wind gets air wiggling back and forth when it curves around obstacles, like trees or buildings. When wind goes around a tree, for example, it has to split up as it goes past and then come back together on the other side; but because of natural randomness and things like the air speed and the surface of the tree, one side's wind is gonna be slightly stronger when the winds recombine, so it pushes the other side's wind out of the way. That builds up pressure on the second side, which eventually overpowers the wind from the first side and pushes it out of the way; and that builds pressure back up on the first side, and the cycle starts all over again. As the two trade back and forth, the wind carries a series of high-pressure waves and whirlpools past the tree, which vibrate the air as they interact with each other - they're called von Karman vortices. The rate of switching back and forth depends on the wind speeds and the size of the obstacle, but, since sounds are just vibrations, fast wind going past trees or buildings can lead to vortices that make vibrations at just the right rate for us to hear it as a howling wind (and for us to get kind of creeped out by it).
Different trees or buildings with slightly different shapes will make slightly different sounds in the same winds, creating a sort of a howling chorus that you can hear in the woods. If those trees are leafy, the leaves will absorb some of the vibrations in the air and dull the sound, but without leaves, like when it's in the middle of the winter or the entire forest is dead, the howling will travel a lot farther. 
But winds can also howl without even hitting something. There are always going to be random pockets of air with higher and lower pressure, and wind can bend around or through those patches, just like it would bend around a tree - which is how you can get howling winds even on the flattest planes, and why windy nights in the middle of nowhere can be downright creepy. 
Thanks for asking, and if you're interested in learning more about how wind works, cause, like, why wouldn't you be, you can check out our video where I explain what wind is, what factors influence it, and how fast it can go.