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原始链接: https://news.ycombinator.com/item?id=41240631

一位用户描述说，在异常寒冷的冬季条件下，他听到了巨大的破裂声，并将其归因于树液冻结造成的树木劈裂。然而科学研究表明，这些声音发生的超声波频率超出了人类听觉范围。该用户声称个人观察与此相矛盾，描述了在零下温度的常绿森林中看到和听到的“爆炸”。用户分享包含支持其主张的视频的 YouTube 链接。用户还讨论了影响可读性的 PDF 文件编码问题。他们描述了 2021 年 2 月经历的一次独特的天气事件，其中包括强降雨，随后因极地涡旋而发生深度冰冻，导致落叶树大面积受损。用户得出的结论是，他们相信听到的声音确实是落叶树由于膨胀的冰冻水而破裂。用户提到该文章可能讨论不同的主题。

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.

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

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.)

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 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.

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.)

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.

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.

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.

> 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.

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

> This sound is often attributed to X. But 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.

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.

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?)

> 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.

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.

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

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.

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.

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.

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)

>> 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

>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.

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

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.

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.

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!

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

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.

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).

> 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.

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