What causes thunder and lightning?
Inside a storm cloud, colliding bits of ice and slushy hail build up electric charge, and lightning is the giant spark that releases it. That spark heats the air around it to roughly 50,000°F in a split second; the air expands so violently that it makes a shock wave you hear as thunder.
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How a cloud gets charged
Thunderstorms have strong updrafts that carry water high above the freezing level. Up there, small ice crystals rising in the updraft collide with heavier, slushy ice pellets called graupel. Each collision moves a little electric charge, and the updraft sorts them: the top of the cloud tends to end up positive and the middle negative. When the difference grows large enough to break down the air, it discharges as lightning, within the cloud, between clouds, or to the ground.
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Why thunder rumbles
Light reaches you almost instantly; sound travels about a mile every five seconds. So count the seconds from the flash to the thunder and divide by five to get the distance in miles. A lightning channel is miles long and different parts of it are different distances from you, so the sound arrives spread out as a rumble instead of a single crack.
Staying safe
Lightning can strike 10 miles or more from the rain, so if you can hear thunder you're close enough to be struck. Go inside a building or a hard-topped car and wait about 30 minutes after the last thunder before going back out.
Questions people ask
- Can it thunder without lightning?
- No. Thunder is always made by lightning, but the flash can be hidden inside clouds or below the horizon, so you may hear thunder without seeing it.
- What is heat lightning?
- It's ordinary lightning from a storm too far away for its thunder to reach you, often seen on summer nights.