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I recently heard that tsunamis (meaning "harbour wave") can travel over 800 kilometres per hour (500 mph), not so far from the speed of sound in the air.

May it happen, in general, that a wave in a liquid exceeds the speed of sound in the above gas medium (assuming it is less dense), and what would be the physical implications? Examples of experiments with not standard liquidss and gases are welcome (mercury in methan).

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  • $\begingroup$ The phase speed of a tsunamis can be higher than the speed of sound in air, but the displacement of the above air will almost never exceed the speed of sound in air. As @Dan pointed out, the tsunami will not create a shock wave in the air. $\endgroup$ – honeste_vivere Apr 4 '16 at 22:27
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    $\begingroup$ "Not so far from the speed of sound in the air" -- 800 km/h at sea level is about Mach 0.65. That is pretty far from Mach 1.0... Good question though. $\endgroup$ – DevSolar Apr 5 '16 at 7:52
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    $\begingroup$ @DevSolar I understand your comment, it is more than halfway though, hence the question about more exotic liquids. $\endgroup$ – Laurent Duval Apr 5 '16 at 7:59
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Your question seems to imply that you think that, over a deep-ocean tsunami where the wave is travelling at speeds in excess of the speed of sound in air, the water in the wave is travelling faster than that speed of sound. This is not the case. The wave is travelling faster than the speed of sound in air, but the wavelength of the wave is so long that the water is only rising a few feet in half an hour. In general, the speed of a wave is not the same as the speed of the medium the wave is travelling in.

Since the water itself is travelling at normal speeds relative to the air, there aren't really any unusual effects happening at the surface.

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    $\begingroup$ It does mean that if the wave makes a noise, it'll get to you before you hear it, of course. (Assuming sound is transmitted mostly through the air and not through the water) $\endgroup$ – immibis Apr 5 '16 at 0:34
  • $\begingroup$ So that would mean, when this wave splashes in to my face... wait, no better not. $\endgroup$ – Zaibis Apr 5 '16 at 8:52
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    $\begingroup$ @Zaibis: Your face would get wet. $\endgroup$ – Dan Apr 5 '16 at 13:36

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