"RUMBLE thy bellyful" King Lear enjoins the storm: "Spit fire! Spout rain!" - and thereby hangs a tale. The most noticeable characteristic of a peal of thunder is its long rumbling roll, and yet the phenomenon originates from an almost instantaneous flash of lightning. Why, one might wonder, it is not just a single, short, sharp clap?
The very sudden rise in temperature accompanying a lightning stroke causes the air in the immediate vicinity of the incandescent channel to expand rapidly. This expansion causes a compression wave that has all the characteristics of a common sound wave, and which travels outwards radically from the flash. We hear it as a peal of thunder, but unlike the light which travels almost instantaneously, the sound wave moves through the atmosphere at a mere 700 mph; it takes five seconds to travel every mile.
Now imagine, if you can, a stroke of lightning a mile or two away, stretching vertically from a high cloud down to the ground below. The explosive expansion of the air resulting in the thunder occurs almost simultaneously at all points along the lightning path. But the resulting sound waves travel, relatively speaking, rather slowly; those from the lowest end of the channel will reach you first, because it is the closest; the waves from the top of the channel, just underneath the cloud, have further to travel, and will take significantly longer.
If the sound from the bottom of a lightning stroke reaches you in, say, 12 seconds, that from the top may not arrive for 15. In between, each segment of the stroke makes its own contribution to the train of sound waves reaching a listener's ears, and the result is a continuous peal some three seconds long. And it may also happen that echoes reflected from the surrounding landscape may extend the rumbling sound still more.
Further acoustic complications are often introduced by confusion as to where the sound is coming from when thunder may seem almost to move in circles around the listener. But Nature does not engage in such gymnastics; thunderstorms generally follow a reasonably straight track, guided by the prevailing winds in the upper levels of the atmosphere. A large thunderstorm, however, may consist of several "thunder cells", each an independent zone of thundery activity within a single storm. It may happen that one cell may travel past an observer, only to be followed shortly afterwards by another heading in the same direction. This orderly procession, with each cell sounding off at different times, produces the illusion of the oscillating or rotating, thunderstorm.