I Second Tonga shockwave - why a drop in the pressure?

[Baluncore]

The initial steam explosion is followed by a long slow depression during which the water condenses. The shock decays as the area of the front increases, to become lower frequency sound waves, being followed by the inaudible condensation depression.
Looking at the pressure plot in post #1, it is easy to wrongly assume that the positive pressure has become inverted, but the positive pressure is in the noise before the pressure drop. It is the tail of the pressure and the start of the depression that produces the biggest step down that your eye and brain fix on.

 

[Jonathan Scott]

Looking at the pressure plot in post #1, it is easy to wrongly assume that the positive pressure has become inverted, but the positive pressure is in the noise before the pressure drop. It is the tail of the pressure and the start of the depression that produces the biggest step down that your eye and brain fix on.

I don't see how that can account for the second pass of the pressure wave being so visibly different from the first. Anything in the shape of the curve that was due to the shape of the pressure created by the initial event would be approximately the same for both observations. The line just before the drop for the second pass of the pressure wave looks like a continuation of the earlier state. Noise plus positive pressure would still show up as positive pressure.

I was sceptical about my "change of circumference" hypothesis until I did the rough calculation in my first post in this thread, but I think the numbers make it plausible. If you think there is something wrong with the idea, I'd like to know more details.

 

[Fisic]

How does constructive and destructive interference come into play as the wave makes multiple passes around the globe?

 

[Frabjous]

I don't see how that can account for the second pass of the pressure wave being so visibly different from the first. Anything in the shape of the curve that was due to the shape of the pressure created by the initial event would be approximately the same for both observations. The line just before the drop for the second pass of the pressure wave looks like a continuation of the earlier state. Noise plus positive pressure would still show up as positive pressure.

I was sceptical about my "change of circumference" hypothesis until I did the rough calculation in my first post in this thread, but I think the numbers make it plausible. If you think there is something wrong with the idea, I'd like to know more details.

I think it is wrong.
1) There is the blast.
2) The wave expands radially from the blast point on the surface of the sphere.
3) The portion of the wave traveling the short distance along a great circle arrives.
4) The portion of the wave traveling the long distance along a great circle arrives.
5) Given that the wave does not know which direction it is traveling along the great circle, there should not be a sign difference.
6) I do not think it is a detector issue because then the distribution should be random.

 

[Baluncore]

If you think there is something wrong with the idea, I'd like to know more details.

Firstly, higher audio frequencies are attenuated more rapidly than lower frequencies. The initial shock has energy concentrated in the high frequency range, so it will be strongly attenuated. Each pass will attenuate it further, so it will never look the same twice. The condensation depression has only very low frequencies, so it will remain identifiable for longer than the pressure wave.

The stimulus and the observer lie on a great circle. There will be a short path and the long path. The wave will appear from those two reciprocal directions at different times. Each will have a different attenuation and wind induced characteristics. We cannot expect to see the same high pressure pulse twice.

Then there may be multiple laps of the great circle in both directions, as seen following the 1883 Krakatoa explosion.

 

[Jonathan Scott]

5) Given that the wave does not know which direction it is traveling along the great circle, there should not be a sign difference.

That is true of the pressure profile due to the wave. You'd expect the same shape, gradually smoothed out. But that is exactly what my explanation covers, in that it says that the observed changes in this case were primarily due to the way in which air had been shifted by the wave going past, not by the wave itself.

 

[Frabjous]

That is true of the pressure profile due to the wave. You'd expect the same shape, gradually smoothed out. But that is exactly what my explanation covers, in that it says that the observed changes in this case were primarily due to the way in which air had been shifted by the wave going past, not by the wave itself.

The negative phase of the wave does what you are proposing. According to your theory, at the antipode you would not get a signal which I really do not believe.

 

[Jonathan Scott]

The negative phase of the wave does what you are proposing.

It would be too much of a coincidence for the timing of the negative phase of the initial wave to match the time at the which the original wave reached the observation point the long way round.

According to your theory, at the antipode you would not get a signal which I really do not believe.

No, the change of circumference effect would be minimal at the half way point, at the maximum radius. The effects near the antipode would be a maximum either way if the wave arrived in synchronization, but it seems more likely to arrive at different times from different directions.

And it's quite likely that near the antipode the intensity of the original pressure wave would build up enough to be visible again, depending on how well synchronized it was on arrival.

 

[Baluncore]

Have there been any reports of the antipodean tea cups rattling in southern Algeria, 165 km SSW of Tamanrasset ?

 

[Orthoceras]

Probably there are no meteorologists at the antinode in the Sahara. However, the other convergence point of the pressure wave, Tonga, has a Meteorological Service. Maybe a barographic recording of the pressure wave after traveling around the world survived.

 

[A.T.]

But why is the second event a drop in the pressure? I expected it to be a spike as well, not a drop.

If you correct for the ambient pressure dropping between 01:00 and 04:00, there seems to be a small spike around 02:00, before the drop around 03:00. Looks similar to what was observed at Perth, below:

 

[Baluncore]

The separation in time of peak positive to peak negative increases with distance.
Norfolk Island ½ hour. Warsaw 1 hour.
It appears that the condensation depression is propagating slower than the pressure peak.

 

[Borek]

If you correct for the ambient pressure dropping between 01:00 and 04:00, there seems to be a small spike around 02:00, before the drop around 03:00. Looks similar to what was observed at Perth, below:

But its arrival is not consistent with the time/speed/distance (unless they can be attributed to the weather changes over the path, and fact that the drop comes to me precisely at the time it is expected from the simple distance/speed estimate, is purely accidental).

 

[A.T.]

But its arrival is not consistent with the time/speed/distance (unless they can be attributed to the weather changes over the path, and fact that the drop comes to me precisely at the time it is expected from the simple distance/speed estimate, is purely accidental).

I see what you mean. It would help to have more data from Europe for comparison, like we have for Australia above.

 

[Jonathan Scott]

I've seen quite a few similar graphs from weather stations around the UK. Here's an example containing two such graphs:

 

[Jonathan Scott]

Four results from Swiss weather stations some way down this page:
https://www.eumetsat.int/hunga-tonga-hunga-haapai_2022
These are interesting. The timing at the different stations clearly shows that the first wave is coming from the north and the second from the south. The second wave is more similar in shape to the first than in the UK and Poland, with a clear upwards start, but still seems to show a more negative bias overall than the first.

Given that Switzerland is about 1000km closer to the antipode, one might expect the original pressure wave to be intensified somewhat by the smaller circumference of the wave.

 

[A.T.]

Four results from Swiss weather stations some way down this page:
https://www.eumetsat.int/hunga-tonga-hunga-haapai_2022
These are interesting. The timing at the different stations clearly shows that the first wave is coming from the north and the second from the south. The second wave is more similar in shape to the first than in the UK and Poland, with a clear upwards start, but still seems to show a more negative bias overall than the first.

Here the image:

The weak spike in Warsaw I noticed before seems to be at the same time as the clear spike in Switzerland (02:00 Warsaw time, 01:00 UTC). So it cannot be the same wave, because the drop has a clear delay between Switzerland and Warsaw. And the UK data shows no spike at all.

 

[Borek]

Bydgoszcz (city in Poland).