The Sound of Air Getting Out of the Way

The rain came down like it had somewhere to be. Maya and Soren pressed their backs against the barn wall, under the part of the roof that stuck out, watching the field turn silver.

"One," said Soren. "Two. Three. Four. Five." The thunder rolled in low and long.

"Five seconds," said Maya. "That's a mile."

"About a mile. Sound goes roughly a fifth of a mile every second." He had the numbers ready. He usually did.

Another flash lit the whole sky white for half a heartbeat. Maya started counting before he did. "One. Two. Three." The thunder cracked hard this time, a snap and then a growl.

"Closer," said Soren.

"Why does it do that," said Maya. Not really a question. More like she was setting it on the table to look at.

"Do what?"

"The far ones rumble. Long and soft. The close one just cracked. Same thunder. Different sound."

Soren opened his mouth to answer and found he didn't have it ready after all. He closed it again. "Huh," he said.

Maya liked that huh. It meant he'd found a hole too.

"It's the same thing making the sound," she said. "The lightning. So why do they sound different."

Soren pulled his notebook out of his jacket, where it stayed dry, and turned to a clean page. He drew a jagged line down the middle. A bolt. "Okay. What is the sound, though. What actually makes it."

"The air," said Maya. "My mom said. The lightning heats the air and the air bangs."

"Heats it how much, though." He tapped the pencil. "That's the part nobody says."

The rain hissed. Lightning again, farther off. They both counted without meaning to. Eight. Nine. Ten. Eleven. The thunder took a long time and arrived soft, like something remembered.

"My uncle told me a number once," said Soren slowly. "He said a bolt is hotter than the sun. I thought he was doing the thing adults do where they make it big so you'll be impressed."

"Was he."

"I don't think so. I think he was under it." Soren wrote hotter than the sun and put a question mark, then crossed the question mark out. "Five times, actually. I looked it up after. The air in the channel goes to about five times as hot as the surface of the sun. For a piece of a second."

Maya turned to look at him. "Five times the sun."

"The surface of it."

"And it's right there." She pointed at the field, at nothing, at the place the last close bolt had been. "That was five times the sun. In our field. And then it was gone and it was just raining again."

"For millionths of a second," said Soren. "Then the air just goes back to being air."

Maya was quiet. The kind of quiet where her hands came up like she was holding something invisible and turning it. "Okay. So the air gets five-times-the-sun hot. What does hot air do."

"Expands. Pushes out."

"How fast, though. That hot, that fast." She was leaning forward now, out past the edge of the roof, and the rain touched her face and she didn't move back. "That's not slow expanding. That's not like a balloon."

Soren felt it arriving before he had the words for it. "It's faster than sound," he said. "It has to be. It gets hot faster than the air can get out of the way politely. So it doesn't get out of the way politely."

"It shoves," said Maya.

"It shoves so hard the shove is the bang." He wrote it. The thunder is the air getting out of the way faster than sound. "The thunder isn't a sound the lightning makes. The thunder is the air. Breaking its own speed limit."

They looked at each other in the gray light.

"Then why do the far ones rumble," said Maya, right back to the hole she'd started with. She wouldn't let it go. She never let it go.

And there it was. Soren saw it whole. "Because the bolt is long," he said. "It's not a dot. It's a crack from the cloud all the way to the ground. Miles, maybe. Every piece of it bangs at the same time. But the pieces are all different distances from us."

"So the bangs arrive spread out," Maya breathed. "The near end of the bolt gets here first. The far end gets here later. Same bang, smeared across time."

"That's the rumble. It's not the storm being gentle. It's the same crack, arriving in pieces, because one end of the lightning is a mile farther from your ear than the other end."

"And the close ones crack because the whole bolt is basically the same distance from us. So all the pieces arrive at once." Maya laughed, delighted, a little wild. "The rumble is a shape. You can hear the shape of the lightning. How long it was. How much closer one end was than the other."

Soren stopped writing. "That's true," he said, testing it, believing it. "That's actually true. You're hearing the geometry."

The rain eased. Somewhere far off, another bolt lit the underside of the clouds, and they counted, and it was long and slow and rolling.

"That one was huge," said Maya, listening to it stretch out. "Miles of it. One end way up there, one end way down."

"A hundred of those a second," said Soren quietly. "Somewhere on Earth. Right now. A hundred times a second the air somewhere is going five times the sun and then getting out of its own way."

"Right now," said Maya. "While we're standing here dry."

They stood at the edge of the roof line with the water running off it in a curtain in front of their faces. Out past the field, the clouds flickered again, and Maya lifted one hand and started counting the seconds on her fingers, listening for the shape of it to arrive.