What causes lightning?

Inside a thunderstorm, violent updrafts hurl ice crystals and soft hail past each other, and the collisions strip electrons, charging the cloud like a colossal battery: positive up top, negative at the base. When the electrical difference grows too large for air to insulate, hundreds of millions of volts, the air breaks down and charge tears through in a spark miles long. Most lightning stays inside clouds; the strikes we fear reach the ground, usually meeting a rising positive streamer from a tree, tower, or rod in the final meters.

The bolt heats its channel to roughly 50,000 degrees Fahrenheit, about five times the sun's surface, and the surrounding air explodes outward. That shockwave is thunder; the flash and boom are one event heard late.

Safety math worth keeping: sound travels a mile every five seconds, and if you can hear thunder at all, you are in range. When thunder roars, go indoors, and stay ten minutes past the last rumble.