How Loud Is Thunder? Why Meteorologists Publish No Figure

Thunder is the acoustic shock from air heated explosively along a lightning channel. That channel can extend for kilometres, so different parts of it are at very different distances from you, and they arrive at different times. The result is that one flash produces a sharp crack for someone beneath it and a long rumble for someone further away.

The peak current, channel geometry, altitude and atmospheric temperature profile all change what reaches a listener. Sound refracts upward in typical daytime conditions, which is one reason thunder becomes inaudible well before the flash becomes invisible.

The National Weather Service's lightning safety material is built around audibility: if you can hear thunder, you are close enough to be struck. It gives an approximate audible range of about ten miles under typical conditions, and it uses the flash-to-bang interval as a distance estimate rather than as a loudness statement.

That framing is deliberate. The operationally useful quantity for safety is whether thunder is audible at all, not how many decibels it registers.

Experimental work on triggered lightning places microphones close to a known discharge and reports acoustic pressure waveforms and radiated energy. These are physics measurements of specific events at specific short distances, often tens of metres, in a highly instrumented setting.

They tell you a great deal about how thunder is generated. They do not give a representative level for a thunderstorm heard from a house, and their authors do not present them that way.

Claims that thunder reaches 120 dB, or that a nearby strike produces 165 to 180 dB at the channel, appear widely. We could not trace either to a measurement with a stated distance, weighting and instrument in a source we can cite.

It is entirely plausible that the acoustic pressure very close to a lightning channel is enormous. Plausibility is not evidence, and a page that prints a number without a source is not more useful than one that explains the gap.

A phone measurement during a storm records something real — the relative difference between rumbles, the level indoors against outdoors — but it is a poor instrument for impulsive peaks. Microphones and their preamplifiers compress and clip sharp transients, so the loudest crack is exactly what a phone measures worst.

The safety advice is simpler than the acoustics: if thunder is audible, lightning is close enough to matter, and indoors is where you should be.