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Comment by RowanH | Hacker News Reader

Heard it go bang, 2600km away, echoing through the hills around our place at the top of the South Island of New Zealand. That's pretty staggering to think about it. Then to see other peoples Home Assistants around the world pick up the change in pressure on opposite sides of the planet. Mental, just mental...

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I live on the beach on the East coast of Australia. We had a marine tsunami warning. We were a few floors up so I wasn't really worried about myself.

About 12ish hours after the initial eruption I thought that anything that was going to happened had all passed by hours ago and I was talking with friends online.

Randomly I started hearing this white noise kind of sound (it sounded like the bubbling water of a kettle without any whistling). I took my headphones off and opened my door trying to find the source of the noise, thinking that the kettle had turned on.

I looked outside and what was small surf conditions only 3 minutes before was extremely turbulent with white frothy waves moving in every direction. It looked like it was reacting to an earthquake but there was no movement in the ground. (I've felt an earthquake there before which is rare here but since our foundations are in the sands I've had the experience and could tell there was no earthquake in the ground on the morning after the eruption)

Since the waves were moving in every direction, some of them were colliding head on. When this happened and the peaks and troughs were interacting in the right way they were shooting out jets of water spray at my eye level about 15m above sea level (I could tell since they were intersecting the horizon from my perspective).

It continued for about a 30 seconds, calming down over another 30 seconds. After the surf had lowered itself to its previous levels, the whole top of the water going back about 50ish meters was covered in thick brown foam. All of the turbulent waves had dredged up the sand beneath it.

The foam slowly disappeared over about an hour with the last bits being the foam that ended up getting washed ashore and out of the water's reach.

Transport this event and yourself back to the bronze age and you could start a new religion with the other people who saw this.

I've heard that the Krakatoa eruption was so loud that the sound wave reverberated around the globe several times. I assume that means if you're on exactly the other end of the world, the sound is coming at you from every direction? Similarly, no matter where you are on earth, the sound is coming at you from different directions at different times? Of course, I'm assuming that the sound wave was in audible frequencies which may or may not be the case...

Even exactly on the other side of the world, the sound will reach you at different times from different directions due to different pressures, temperatures, mountains, etc. on the way.

But there would be a spot where the pressure waves would cross?

> But there would be a spot where the pressure waves would cross [simultaneously]?

There could be, but it's not immediately obvious that there has to be.

It does seem like certain points would need to be on 'cusps' of the wavefront, though, so something like getting the wave from the north and the northeast 'simultaneously'.

Not necessarily. The set of points reached last by the waves could be multiple points, or a line, or multiple lines.

I think so. I think there is necessarily a circle "eversion" (turning it inside-out) with a crease. Whatever is the point of the crease would hear sound from all directions. Even with something like a figure 8 collapsing wavefront, eventually one or more creases should form.

If the crease is a line, would it not sound like comming from two opposite directions simultaneously?

I think it’s highly unlikely that even half the pressure waves would simultaneously reach a single point on earth. The waves get reflected in all kinds of ways by the ground, mountain ranges, difference in air temperature, etc.

If they did, however, I wonder how strong the force would be at its focus (if that’s the proper name)

(If I did my math right, the antipodes of Tonga live in the south of Algeria, near the Niger/Mali/Algeria tripoint. I haven’t heard of weird phenomena happening there, but then, I don’t think that is the most populated and most twitter-addicted part of the world)

Sounds like the hairy ball theorem (https://en.m.wikipedia.org/wiki/Hairy_ball_theorem)

There's an animation of the antipode from this eruption arriving in northern Africa and it's interesting how distorted the incoming wavefront is.

https://twitter.com/weather_models/status/148300643675002880...

The earth is (almost) an oblate ellipsoid so most geodesics on earth aren't closed and even if they all meet (do they? I need to to think about this or play with GeographicLib) at one point they are of different lengths (imagine a point on the equator, the latitudinal path is quite different from the longitudinal path). And then there are different propagation velocities due to pressure, terrain, etc. So probably not.

My back yard weather station picked up the shockwave here in the bay area: https://twitter.com/DavidCWG/status/1482451910154014720

I had no idea that was even possible. Crazy stuff.

Mine picked it up too - in Portugal.

Teeny little dip and crest, and I wasn’t sure until the meteorological station in a city 30km tweeted that they’d seen it too, and the times coincided (well, I was about a minute later) very neatly.

> the bay area

It's always interesting that anyone who lives in a "Bay", refers to their local as THE Bay. For this phenomenon, which is documented as circling the globe, who knows where this could be!

no, there’s multiple areas with bays, there’s only one Bay Area.

> there’s only one Bay Area

I suspected you were just assuming that, so I googled it, and found quite a few.

The "Bay Area" is well known to refer (only) to the San Francisco Bay, while the "Greater Bay Area" is known to only refer to the Pearl River Delta Bay (HK, Macau, Guangdong). Other bays are typically named more specifically (e.g. Tokyo Bay, Monterey Bay, etc.).

You'd be more on point if OP had referred to San Francisco as "The City" as many in the City do; since that's something that's also used in other cities like NYC.

NB: current resident of the Greater Bay Area in China and former resident of the lesser Bay Area in California.

May I kindly ask the make and model of your weather station please?

I noticed the same on all in Aqara Zigbee temperature/pressure/humidity sensors. They're all inside, I could still easily see it twice (many hours apart). Though I only noticed after someone in a Home Automation topic mentioned seeing the effect ;)

Nothing fancy was needed for this, the sensor I'm talking about is battery operated. I have this one: https://www.zigbee2mqtt.io/devices/WSDCGQ11LM.html or this one: https://www.zigbee2mqtt.io/devices/WSDCGQ12LM.html (they look alike too much).

It's a Davis Vantage Vue.

I was outside in my backyard when I heard a mild crashing sound around 7 or 8am in Toronto. I thought the neighbour had dropped a ladder or metal garbage can or something…but after reading the news I realized it could have been the Tonga explosion.

What did the “bang” sound like? Was it the sharp crack of thunder close by, the slower rumbling of distant thunder, the sound of fireworks exploding, or would some other description fit better?

Close up (next island over) it was a low loud fast boom like dynamite going off nearby. Over in an instant. From a Facebook video. Really strange that it would sound that way.

I also heard it in the north of the South Island of NZ. To me it sounded like very distant explosions, like a cannon firing, or avalanche control explosives.

We heard multiple explosions over the span of a minute or two, presumably from the multiple paths the sound took as they all sounded the same.

Here's a video with several pre-thuds then a VERY LOUD bang about 1m50s in ... 'sposed to be from Tonga. https://www.youtube.com/watch?v=KGV5payoRYs

My colleague's backyard weather station picked up the pressure wave in Christchurch several times as it repeatedly circled the globe.

Sea levels in the upper south were a little 'odd', trending higher and more variably than usual, even if not by that much, for a week.

Here in California in the Sierra Nevada, a pressure variation was noticeable around 4am pacific.

Hadn't thought of that. Just checked the data from my weather station, and there's a definite spike, followed by a drop.

30.16 inHg just before, 30.19 spike followed by an immediate drop to 30.14, then basically normalized again.

I find it difficult to believe that even a massive eruption could literally push sea levels up or down in the surrounding area (waves /= sea level). Volcanoes are massive, but the energy to move cubic miles of water, water many hundreds or thousands of miles away, is at different level. Hurricanes can move water like that, but they do so with orders of magnitude greater energy (tens of thousands of nukes).

US tsunami monitoring has a record.

https://www.tsunami.gov/previous.events/?p=03-19-09-Tonga

A tsunami is a wave. Not sea level. Waves literally come and go over a period of seconds to minutes. They wobble back and forth without actually moving much water. Sea levels move on the order of hours and days and involve the physical movement of literally hundreds of cubic-miles of water. Raising or lowering a sea level measurement (not a temporary wave) requires far more energy than a volcano.

From the link: "Listed wave heights are maximum amplitude in cm (above sea level)." In other words, even tsunami waves heights are not changes to sea level.

I don't think anyone - except perhaps you - is suggesting that the eruption caused an actual rise in sea levels.

They were initially responding to a comment that said sea levels were slightly elevated for a week after the eruption. I have to agree that's pretty weird even for a tsunami.

> A tsunami is a wave. Not sea level. Waves literally come and go over a period of seconds to minutes. They wobble back and forth without actually moving much water. Sea levels move on the order of hours and days and involve the physical movement of literally hundreds of cubic-miles of water.

You should be aware that tsunamis behave in the way you're calling "sea level", and not in the way you're calling "wave". It's true that a tsunami is a wave, but it's not true that they come and go over a period of minutes, or that they fail to move much water. They're very large.

Some of them, specifically those involving undersea plates moving upwards, but even those do settle out quickly. That is why the term literally means "port waves" because they bypass normal wave protections and move into ports, similar to how tidal bores move up rivers. But the comment above spoke of sea levels being impacted for many days, something beyond even thrust tunamis and more akin to hurricanes.

Think of it more like a very long period wave, much like a tide, but on a much shorter time period than the tide. Tides, waves (periods on the order of seconds), and tidal waves (or tsunamis) are all fluctuations in sea level. Water can be moved when the waves encounter enclosed bays or harbors. Think of the water level dropping 3 ft over 20 minutes in front of a harbor entrance and imagine the currents that can result.

Here's an example of observations from coastal California (red line shows sea level measured every 6 minutes: https://tidesandcurrents.noaa.gov/waterlevels.html?id=941211...

It is truly amazing - I'm not surprised that it's hard to believe.

If it has a period not tied to the rotation of the earth (ie tides) then it is not a change in sea level but a wave. And Tsunami does not translate as "tidal" wave. It means "port wave" and has nothing to do with tides whatsoever.

A tsunami is an entirely different kind of wave than a normal wave.

A normal wave is transverse, and a tsunami is a pressure wave. The first bobs the water up and down and the wave moves forward, the second shoves water ahead of it, displacing it upward into the wave.

The tsunami won't break when the column gets short, but rather transfer all of the energy of moving a deep water column to a shallow one. That's what makes them dangerous.

That's from 2009. I can't figure out how to navigate the site so as to find the 2022 event.

Talk about long distance effects....

"Tonga volcanic eruption shockwave reached the Netherlands"

https://nltimes.nl/2022/01/16/tonga-volcanic-eruption-shockw...

There was a report from a climate org in Miami that they recognized the pressure pulse 9 times, once each ~35 hours since the eruption.

We also heard it while tramping on the thousand acre plateau just north of Murchison. It was very quiet, so we were lucky to be where we were with no noise pollution to drown it out.

Though at the time we had no clue what it was, we joked about being in the Tomorrow series timeline.

Takaka or Motueka? I used to live in that area. Golden Bay has a pretty unique geographical layout!

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