1.2 Shockwave propagation
Explosions that occur during volcanic eruptions can create atmospheric shockwaves that propagate at supersonic speed over long distances. In case of very strong eruptions, such as the Hunga-Tonga eruption in 2022, these shockwaves show up on pressure sensors at ground level (Figure 6) and are even visible from space (Figure 7).
Figure 6: Mean sea level pressure readings of the first shock wave at six MeteoSwiss stations, all south of the Alps. Note the slight time shift between the orange (northernmost) and blue (southernmost) traces. © MeteoSwiss
The impact of atmospheric pressure changes on mid-level water vapor resulting from shock-wave propagation is very small but noticeable when artificially amplified by increasing the water vapor image contrast (Figure 7). The water vapor absorption band at 6.9 µm reflects the mid-tropospheric water vapor content. The propagating shockwave slightly lifts the low-level moisture into higher levels, which results in a higher absorption of radiation emitted from the ground and which can be seen in this absorption band.
Figure 7: GOES-17 ABI mid-level water vapor (6.9 µm) time difference image of the shockwave. ©NOAA/NESDIS/ASPB
The above loop shows several shockwaves propagating from the volcano but only the initial wave has enough energy to travel around the globe.