"Frost crack" sounds may come from sky, not trees | Hacker News Reader

"Frost crack" sounds may come from sky, not trees

nautil.us

298 pointsjnord82 comments

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.

A small nitpick, the main character is only 13 years old. Great book!

If he read it at the same age range I did, then a 13 year old certainly seemed like a man by comparison.

I loved this book and have read it a few times now. I was probably 13 too! The movie was okay. It just left a lot of important scenes out. (Like all book->movie things)

A good book, I also read it as a child.

That’s the alternate story line as told in Brian’s Winter. He did not ordeal the winter in The Hatchet.

One of my favorite books as a kid, just recently read Hatchet to my boy

I just read the synopsis, I’m curious what age your son is or how old you were when you read it? I’m not sure my son is old enough for it.

Not OP but Hatchet was an assigned book when I was in 5th grade (American Midwest school), it’s pretty common for early school age kids to read. It’s got some slightly intense scenes in it, but nothing that 5th grade me was too scarred by (the scene where he has to go into the lake to get something from the plane and sees the corpse of the dead pilot was pretty scary to 10-year-old me but that’s about as bad as it got.)

Yeah I think it was 6th grade for me, which I think is about right.

Hatchet was one of my favorite books growing up. I believe the first time I read it I was in 4th grade if that helps you gage (it was pretty common in 4th and 5th grades if I recall correctly). That said, my mother was a librarian and didn't care what I read as long as I was reading (including Faulkner and East of Eden when I was in 6th grade which was way too mature for that age :-)) so depending on your son maturity your milage may vary. That said, Hatchet is a great a book and I give it some credit to my life long love of the outdoors and adventure.

I think it's appropriate for as soon as your son would like to read it. I read it in third grade, even though it was introduced in fifth.

Speaking from memory, I really appreciated being able to read books with relatively "mature" topics like that (isolation, survival, etc.)

I found Hatchet when I was in grade 4, and damn near read it straight from the library checkout counter until I was done a few hours past midnight. I'm pretty sure that my parents made me take a break for dinner.

>I just read the synopsis, I’m curious what age your son is or how old you were when you read it? I’m not sure my son is old enough for it.

If he can read chapter books at all, he's probably old enough.

He’s 7.5yo. Don’t think he really understood The Secret Brian’s mom had, and probably still hard for him grasp severity of the situation, but he was very engaged and enjoyed the book. I don’t recall how old I was when i first read it… but I do recall accidentally finding a Gary Paulson book in the adult literature section of the library and quickly realizing it was not for a young juvenile :D

Don’t let your kids read Roald Dahls adult literature either, Switch Bitch is no Giant Peach

That reminds me, there’s a really great survival game called The Long Dark in which you survive a plane crash in something like the arctic circle and must survive. If anyone is into survival games, definitely check this one out!

It's an extremely captivating game with an unparalleled atmosphere.

Before I started playing the game, I saw in passing an in-game video of a well-stocked gas station. Lights were lit and shelves were stocked. It was made like an in-game advertisement of some sort.

Later, when I started to play the game, I recalled the video, and decided to reach the gas station and set up my base there. Imagine the food! The warm indoor temperatures! Brand new clothes!

After a perilous journey I reached the gas station. For some reason I was expecting the lights to be on and warmth, but of course I was greeted by a half-broken gas station, no lights of course, shelves were almost empty and cold indoors.

But it was a good base, lots of loot.

Was that the Quonset hut surrounded by wolves? I think on the broken highway map?

Yes, Quonset Garage on the Coastal Highway.

It was maybe this video, or a clip of it: https://www.youtube.com/watch?v=gs2IYnNBVhQ

There was another novel along the same lines I remembering liking as a kid called “my side of the mountain”

A story about a kid basically muddling through living ‘off-grid” before it was cool.

My Side of the Mountain, The Hatchet, and Peak were like the trilogy of adventure books for our cohort I think.

I blame my side of the mountain for me, sitting right now, in Breckenridge about to head out into the woods for the day.

Absolutely captivating book to read as a kid.

"Hatchet" by Gary Paulsen

https://en.wikipedia.org/wiki/Hatchet_(novel)

I got to meet Gary Paulsen at a conference once. He's a legit mountain man, and one of the absolute nicest people on the face of the earth. He stayed at a book signing for like 6 hours after to talk to fans. Super cool guy.

Hatchet is part of a series. They’re all pretty good.

The one thing I remember is how he found a rifle and ammo but eventually went back to the bow because it’s quieter and ammo is reusable. What a gamer.

Think I used Hatchet for a book report every year from like 3rd to 7th grade.

> 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.

> or coming from 250' in the air above you when you're on a frozen lake.

If you're on a frozen lake, you much prefer the crack coming from above than below.

And then of course, there are the "ice making" sounds that do come from below. And different lakes and ponds even seem to have somewhat different voices in that way.

And something equally chilling -- the sound of a glacier cracking (more of a deep thud sound).

I think that framing comes from the article rather than the Aurora researchers (I skimmed some of their papers & didn't see it mentioned), but the article claims:

> 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.

Though if you told me you'd heard a tree make a groan or a crack, I'd be inclined to believe it, it doesn't strike me as outlandish.

That quote seems odd to me. Specifically,

1. The sound is attributed to trees cracking

2. The cracking is caused by sap freezing and expanding

3. Sap freezing produces sound in the ultrasonic range

4. So it's not the trees

Point 4 is not a valid conclusion from 1-3, because it was never stated that the sap freezing is what is being heard. Rather, it's the trees cracking, which is _caused_ by the sap freezing... but its own thing with its own sound.

I think you missed the last bit:

> scientists have found no evidence this phenomenon might make sounds that are audible to the human ear.

Which I take to mean they’ve measured ultrasounds but no audible sounds.

"Found no evidence" and "didn't even try to measure" isn't really the same, is it?

I'm not sure why you say I missed that. I didn't. Can you expand on what you meant by your reply.

To me, your reply actually highlights what I was talking about; because your use of "this phenomenon" is _somewhat_ ambiguous.

1. "this phenomenon" can be the sound of "large pressure splits in tree trunks, caused by sap freezing and expanding" (presumably audible to humans)

2. "this phenomenon" can be the sound of "freezing sap in trees" (presumably not audible to humans)

Seems like they would've recored both in the field, no? If they were recording sap freezing in the field, presumably the mics would pick up on other parts of the tree undergoing stresses and making audible sounds.

For that to have not been the case, either they would've had to freeze sap in the lab, or they would've had to go way out of their way to isolate recordings of just the sap in the field without the rest of the tree (is that even possible with normal recording tech?)

Point 4 seems to have been made by you, not the article.

I guess maybe it depends on how you read it? To me, it read like

> This sound is often attributed to X. But <some component of X> doesn't make sound that humans can here.

The "people say it's this, but actually ..." reads, to me, like they're saying it's _not_ this. Which was point 4.

Depends whether the landscape is prone to inversions. They happen most often in wide and enclosed valleys.

Maybe trees are affecting the rate and intensity of Inversion.

Well thunder and lightning comes from the sky and lightning is caused by static effects of ice crystals in thunder clouds. So maybe it's something similar.

>> 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?

But what if the discharge requires some specific environmental factor, say, something the height of a tree?

Maybe tree covered ground makes a more effective source of the rising-up component of this process. That relatively warm, negatively ionized rising air might be much diminished over a frozen lake or open plain. Otoh, it'd probably be greater over the ocean, I would think.

Do people at Antarctic research stations ever hear this sound?

Probably not as it requires a gradient, which that deserted and static landscape wouldn't provide.

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.

Surely the recordings are available somewhere on the web, but here's a short clip I found on YouTube:

https://youtube.com/watch?v=cm97VQobbh4&t=435

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

Another thing that uses radio waves in the human hearing range is the invisible fences that cause shopping cart wheels to lock (7kHz, visit begaydocrime.com to hear the corresponding sound). Those don't involve sounds in normal operation, so I wonder what about these is different.

I would guess that induced current is making a sound in some nearby infrastructure and not directly in the head of the observer.

>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.)

You aren't quite right here. Temperature inversions are when the atmosphere warms as you go up instead of cools. The atmosphere usually cools at a fairly constant rate as you go up, at least in the troposphere. This layer of warmer air aloft acts as a cap, limiting vertical motion from rising air parcels from below (which are cooler than the air aloft and thus cannot rise through the inversion).

You are correct afaik - warm air would not trap cold air below it as it would move up, only a layer of could air could trap warm air below it.

Some visualizations show air cold/warm/cold, where as others are just warm air under cold air.

Inversions usually happen when the ground is cooling faster than the air above, due to radiative cooling. That results in a layer of warm air sandwiched between cold air above and below.

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.

The conference paper (which is very well written and approachable) suggests that the precision of instruments was insufficient to establish this as of 20-ish years ago. See the section "First Experimentations": https://www.researchgate.net/profile/Unto-Laine/publication/...

Trivia: I tried to copy "First Experimentations" directly from the pdf and this what came out of my clipboard: -*%.$+/01/%*2/#$3$**&#. Thanks, Researchgate

more like research gatekeeping amirite

Several chunks of text in that PDF start with the printable ASCII characters from ! onwards in order until there is a repetition, e.g. Magneto-acoustic triangulation from the headline corresponds to

  !"#$%&'(")'*+&,)-&.,"$#*/"&,'$

which is

  >>> list('!"#$%&\'(")\'*+&,)-&.,"$#*/"&,\'$'.encode('ascii'))
  [33, 34, 35, 36, 37, 38, 39, 40, 34, 41, 39, 42, 43, 38, 44, 41, 45, 38, 46, 44, 34, 36, 35, 42, 47, 34, 38, 44, 39, 36]

So I think this is probably a PDF font-encoding issue that might've already affected the author's original upload.

You're right, I immediately assumed some annoying DRM, but after looking it up, it does seem more like a bug. I couldn't find any clear explanation for why it happens though.

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

What’s common knowledge in Scandinavia? That the sounds come from the air, not the trees?

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.

Three mics and a loop antenna. The diagram in the May 2024 implies the mics provide a trajectory and antenna-mic difference fixes it in 3D.

Disclaimer: I was in his Basics of Speech Processing class in the university. One of the best (and funniest) courses I had.

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

The article explains that this objection was raised by other researchers, but that the sounds were triangulated to a height of about 250 feet (because they are caused by an electrical interaction at the top of an inversion rather than from the aurorae directly).

One thing that I found super interesting when I studied audio engineering is that our ears are very good at determining direction left/right, but absolutely hopeless at working out if a sound is up or down.

This makes their hypothesis a lot more believable to me; I can understand others incredulity.

> absolutely hopeless at working out if a sound is up or down

Easy, just turn your head to the side :P

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