What does lightning sound like? The obvious answer is in the boom of thunder: an explosion of expanding, superheated air. But there are more subtle and less understood noises associated with lightning, known as brontophonic sounds, which are heard far less frequently.

Two features make these sounds distinguishable from thunder. One is that in contrast to the deep reverberation of thunder, brontophonic sounds sound like the hissing of a red-hot iron in water or the tearing of fabric.

The other is that there is no time delay. Thunder travels at the speed of sound and is usually heard several seconds after a lightning flash, but brontophonic sounds are perceived at the same time as the flash.

One theory is that brontophonic sounds come from induced charge. The same potential difference that generates a lightning stroke may create smaller pockets of electric charge in the surrounding area. These may be strong enough, at the instant of the lightning, to make crackling electrical sounds similar to static electricity discharge.

Brontophonic sounds are part of a larger category of electrophonic sounds, anomalous rustles and pops also associated with meteors and the aurora borealis, all of which remain essentially mysterious.