"Frost Crack" Sounds May Come from Sky, Not Trees

In grade school I read a book called ‘The Hatchet’. It was a story about a man who survived a plane crash near the arctic circle and had survived many days on his own with not much more than a hatchet. He experienced these sounds after a number days in the harsh wilderness and thought it was gun shots and that he was going to be saved. The book then went on to explain that it was the trees cracking from the extreme cold. I was mesmerized by this as a kid. Knowing now the true origin of the ‘frost crack’, I’m twice as captivated.

> It’s not to say that trees don’t crack—but rather that spooky noises long attributed to trees may emerge from the night sky itself.

Well, it seems like he demonstrated that the night sky itself can make sounds under certain conditions, not that these sounds are always the night sky.

By the way, I don't recall ever hearing the supposed tree cracking sound in an area where there were no trees. If it's always just the sky, you'd expect to hear it at least occasionally on the plains, or coming from 250' in the air above you when you're on a frozen lake.

>> Indeed, these loud cracking sounds are often attributed to large pressure splits in tree trunks, caused by sap freezing and expanding inside the tree’s interior. But while freezing sap in trees has been found to produce sounds at ultrasonic frequencies, outside of the range of human hearing, scientists have found no evidence this phenomenon might make sounds that are audible to the human ear.

Personally, i have not just heard them but have seen it happen. At -40 and below, in certain evergreen forrests not used to such temperatures, a tree can randomly "explode". An internal crack shakes the tree, throwing snow everywhere. It lookes and sounds like an explosion. You hear gunshot and then see the tree shake off all its snow. The tree stands out as the one dark with branches no longer held down by snow. It is like an angry ent waking up about to eat a passing human.

https://youtube.com/shorts/oG-N2LCYEc4

Does anyone really believe that a crack like that wouldnt make a gunshot sound?

Here is the sound, after about 0:30. Not much snow to shake off but you can see them moving.

https://youtu.be/Rz3TqqNkEBU?feature=shared

Really fascinating. After reading a little more, I learned that while Laine proposed the inversion layer hypothesis in 2016, the Auroral Acoustics group he headed was informally started in 2000. The reason the linked article is coming out now is due to Laine’s latest paper that details the triangulation of the sounds[1][2].

Would love to try and record this myself. I’ve been recording some VLF “sferics” for some time now for an art project[3]; it seems the auroral sound recordings often peak in the same frequency range (and perhaps there is overlap without me realizing it).

[1] http://research.spa.aalto.fi/projects/aurora/index.html

[2] https://www.researchgate.net/profile/Unto-Laine/publication/...

[3] https://www.mattdesl.com/sferics

This reminds me of "aircraft wake snapping" or "vortex snapping", which is a very audible sound one can sometimes clearly hear shortly after a plane passes over you if it's low enough, such as on final landing approach. I seriously thought I was imagining it the first few times I experienced it - so weird to hear sound coming from apparently empty air.

edit to add an example: https://www.youtube.com/watch?v=UA-NONhZIN8

Fascinating. Wonder if this is related to these "sky trumpets" reported around the world usually near forests and mountains

https://www.youtube.com/watch?v=081FuoN8K40

https://www.youtube.com/watch?v=gtLNmdZTf_g

https://www.youtube.com/watch?v=PJpucJkRa6c

https://www.youtube.com/watch?v=_P4XOiMKLhk

https://www.youtube.com/watch?v=gHeoPMg3_zM

https://www.youtube.com/watch?v=stISzPngwh4

In February 2021, in the Willamette valley of Oregon, we had a weather event unprecedented in my lifetime. Winters here are usually overcast and rainy, with little to no snow and a handful of mild freezes. In 2021 we had a significant rainfall followed immediately by a deep freeze due to a polar vortex.

That morning was like nothing I've ever experienced. About once per minute there would be a loud crack like a gunshot, coming from all directions.

After several days, power was restored, the roads were cleared, and it was obvious what happened. Countless deciduous trees had split from what I assume was the accumulated water from the preceding rain storm. There were so many downed and permanently damaged trees that it took around a year for property owners and the city to finish cleanup.

Usually, when we get freezing temperatures, it's because there's no cloud cover. It's extremely unusual to swing from heavy rainfall to a deep freeze like that.

Anyway, I don't know if this article is talking about something different, but the cracking I heard was definitely deciduous trees cracking due to expanding, freezing water. Few conifers were damaged.

Is it possible that the inversion layer creates a structure for sound to reflect/refract back down towards the sensor, when it in fact the original source was on the ground? You might not detect it laterally if there were a bunch of trees in the way.

Wouldn’t the tree explanation be refuted by simply recording the sounds in an open field, far away from trees?

At first I thought this was going to be about frost quakes[0], which are mini quakes we get in the Midwest when there's a deep cold.

[0] https://en.wikipedia.org/wiki/Cryoseism

First I heard prof Laine talk about recording auroras in the early 00s I and many of my student friends thought he was an old eccentric (in some less polite words too).

Seeing him come through with such a solid long term effort, rigorously done and communicated with clarity is amazing, with a pinch of healthy embarrasment.

(I studied in the same academic cluster of music/audio/acoustic labs he made his career at)

For someone living in equator the article will be perfect if it has the sound mentioned in the article.

Would be cool if there was a recording, I don't know what this article is talking about.

This reminds me of a time I was stargazing, when suddenly a meteor streaked overhead making a distinct sizzling/hissing sound that tracked with its movement…which seemed improbable since light obviously travels faster than sound. I later read the theory as to how this phenomenon occurs is that the sound is created by low frequency radio waves.

https://ethw.org/Electrophonic_Meteors

>As this warm air collides with cooler air from above, it forms an “inversion” layer of warmer air layered over cold air, which traps the ions.

I think this is reversed, inversions are usually cold air sitting atop warmer air. (Warmer air is lighter, defining the typical sequence with which an inversion is relative to.)

It was so weird reading a science article that uses feet, and even more so since the research was done in Finland.

Fascinating read! I'm surprised this wasn't already commonly known.

I'm amazed that this phenomena is hitherto unexplained when it's entirely common knowledge in rural scandinavia, barely worth talking about.

My first thought was "bullshit, these are _obviously_ trees cracking." Well, using triangulation, it is obviously coming from 250+ ft in the air. Good to test assumptions!

This is so neat. I love science.

Nonsense.

If you hear a loud crack and are near trees, look up and get out if the way.

This idiot is going to get people killed.

> Laine was able to triangulate the origins of the sounds from calculations based on the distance between the microphones and the speed of sound. The triangulation data revealed the origin of the sounds was indeed the sky.

Triangulate doesn't work, it's in the sky remember?

You need 4 for 3D space theoretically. But in practice it's more like 6-7. Any wind or temperature difference adds dimensions which you have to computer away.

The paper seems to confirm it's literal. 3 mics. Which is fine to find stuff but why not do it to spec in the real paper, do the results disappear?

They talk about "virtual microphones", not convinced.

Interesting, but kinda hard to believe that the sounds we hear in the forest could be coming from that high up I'm no expert, but if that's true, that's pretty mind-blowing