In “My Sister Versus the Thunderstorm”, Meher and her brother count the seconds between each flash and its thunder, and the thunder, when it comes, rolls “over the building like furniture being dragged across the floor upstairs”. That description is better physics than it looks. Because if you think about how thunder is made, it shouldn’t roll at all.
The story explains the bang itself. In a few millionths of a second, the lightning heats a thin column of air to around thirty thousand degrees. The air in that column expands so violently that it slams outwards as a shock wave, which within a few metres slows down into an ordinary, very loud sound wave. All of that happens almost instantly. One event, one moment. So why does the sound you hear last for five, ten, sometimes twenty seconds?
The answer is the shape of the bolt. A lightning channel is not a point. A cloud-to-ground bolt is typically several kilometres long, running from the base of the cloud, which can be a couple of kilometres up, down to the ground, and it is never straight. It zigzags and branches, so its total length is longer still. And the whole channel is heated at nearly the same instant, so every part of it cracks at once.
But every part is a different distance from you. Picture a bolt that strikes the ground three kilometres away, with its upper end inside a cloud six kilometres away. The sound from the nearest point sets off at the same moment as the sound from the furthest point, but it has three kilometres less to travel. It arrives about nine seconds sooner. In between, sound from every point along the channel arrives in turn, nearest first, furthest last. What was made as one crack reaches you as a long, continuous stream. Each kink in the channel that happens to lie at the same distance from you adds a louder burst, which is why the rumble swells and fades and seems to growl. The dragged furniture is the sound of the bolt, laid out in time.
This is also why the story’s activity tells you to time the first sound, not the loudest. The first sound comes from the part of the channel nearest you, usually close to where it hit the ground. The loudest part of the rumble may come from further away, a stretch of channel that happened to point broadside at you.
The rumble is low as well as long, and there’s a reason for that too. Close by, thunder is a sharp, ripping crack, full of high pitches. Further away, it’s a deep, soft rumble. That’s because air doesn’t carry all sounds equally well. As a sound wave travels, some of its energy is absorbed by the air itself, and high-pitched sounds, which make the air vibrate faster, lose energy much more quickly than low ones. Over a few kilometres, the high frequencies fade away, and only the deep notes survive. Echoes off buildings, hills and the ground add their own delays, smearing the sound out still further.
Finally, there’s a limit to how far thunder can be heard at all, usually around fifteen to twenty-five kilometres, which is why distant lightning can flash in total silence. You might expect sound simply to fade with distance, but something stranger happens. Air near the ground is usually warmer than the air above it, and sound travels faster in warmer air. So the lower part of a sound wave moving along the ground travels a little faster than the upper part, and the wave tilts upwards, bending away from the ground like a car turning towards its slower wheels. Far enough away, the thunder’s sound has curved up over your head and never reaches your ears.
So Meher’s furniture was really a map of the storm, written in sound. The first thump was the bottom of the bolt, nearest the building. The long drag was the rest of the channel, climbing away into the cloud, one stretch at a time. And the fact that it rolled for so long told her, if she’d known how to listen, just how tall that bolt of lightning was.