The eruption of the Tonga Volcano at 4:10 GMT (5:10 Italian time) was certainly one of the strongest natural explosions ever recorded. The incredible thing is how the pressure wave (the displacement of air) that was generated started from the volcano and spread in all directions, being recorded by barometers all over the world.
In Italy we have taken as reference two extreme points monitored with our control units. The first image (A) shows the data from the INFN (National Institute for Nuclear Physics, which we thank for sharing) located in Valle d’Aosta, at 3480m.
The averaged data per hour show the peak of the first wave and the subsequent peak of the second. It is actually the same phenomenon, but the double recording refers to the two different directions taken by the wave from the place of the explosion, which reached the station from two opposite directions, covering different distances.
The distance between Tonga and this station is approximately 17,300km in one direction and 22,800km in the opposite. The eruption occurred at 5:10 am Italian time and created a wave that, traveling at 1092 km/h (or 300 m/s, speed of sound), took almost 16 hours to arrive from one side and 21 from the other.
In the graph, the first peak shows the wave that arrived from the shortest distance, shortly after 9pm, Italian time, and the next with a 5 hour delay due to the greater distance. The disturbance in terms of atmospheric pressure however persisted for several hours.
The second image (B) refers to one of our stations in Marina di Ragusa in Sicily. In this case the barometric measurement is per minute, therefore the graph is much more defined and the presence of the two perturbations and their duration are better understood.