My Sister Versus the Thunderstorm

The power went at eight, right in the middle of dinner, and Meher was under the blanket before the fan had even stopped turning. She was six and she had decided, sometime during that monsoon, that thunder was a thing that came for you personally.

“It’s far away,” I told her, which was a lie, because I had no idea where it was. The window lit up white and the whole room jumped into view for a second: the steel plates on the table, my mother’s dupatta on the chair, Meher’s feet sticking out of the blanket. Then dark. Then, a while later, the sound, rolling over the building like furniture being dragged across the floor upstairs.

I remembered something from a science class I had mostly not listened to. “Meher,” I said. “Want to know how far away it is? Actually know?”

A small face appeared. “How?”

“Next time it flashes, we count. Slowly. One, two, three. We stop when we hear the thunder.”

The window flashed. We counted together, her voice wobbling, and got to eleven before the rumble came. “Eleven,” I said. “Now divide by three. That’s about four. The storm is about four kilometres away. That’s further than your school.”

She thought about this. “Why three?”

Here is what I had to work out, sitting on the floor in the dark with my sister waiting. The flash and the thunder are made at the same instant, by the same bolt. In a few millionths of a second the lightning heats a thin column of air to something like thirty thousand degrees, and air that hot has nowhere to go but outwards. It slams into the air around it, which slams into the air around that, and the shove spreads out as a wave of squashed-together air. That travelling squash is sound. It has to be handed along from one molecule to the next, like a push passed down a queue, and the hand-offs take time: about 343 metres every second on a warm night. Three seconds, roughly a kilometre.

Light doesn’t wait for anything to pass it along. It is a ripple in electric and magnetic fields, and it crosses four kilometres in about thirteen millionths of a second. As far as your eyes can tell, the flash arrives the moment it happens. So the gap you count is almost entirely the sound’s journey. Count the seconds, divide by three, and you have the distance in kilometres.

“So the counting is how long the sound took to walk here,” Meher said.

“Basically, yes. And it walks faster when it’s hot.” That was the bit that surprised me. In warm air the molecules are already moving faster, so they pass the push on more quickly; sound speeds up by more than half a metre per second for every degree. On a cold winter night the same storm would sound a little further away than it really is. The divide-by-three rule is a good guess for weather like ours, not a law of nature.

The next flash, we got to fourteen. The one after that, seventeen. “It’s going away,” she said, sitting up properly now, the blanket around her shoulders like a cape. “It’s going away and it’s getting bored of us.”

When the power came back an hour later she was at the window with my phone’s stopwatch open, refusing to go to bed until the storm was “at least twenty”. The storm never got to twenty. She fell asleep at nineteen, against the window, still counting under her breath. I’ve never been prouder of anything I didn’t really know until that night.