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Grades K–2 · Earth & Space

Why do we see lightning before we hear its thunder?

A lightning flash can be seen before its thunder is heard. Both come from the same lightning event. The lightning gives off light and heats nearby air very quickly.

E30 visual explanationOne lightning event sends out light and sound. Light arrives first. The middle path changes how long the sound takes to arrive. The picture shows the signals separately so their travel is easy to follow.One flash sends light and sound at different speedsMiddle pathLightObserverSoundEarSoonerLaterLightSoundSame event: light arrives before sound.Paths and arrival timing are enlarged symbols.

In the diagramOne flash sends light and sound at different speedsMiddle pathLightObserverSoundEarSoonerLaterSame event: light arrives before sound.Paths and arrival timing are enlarged symbols.

One lightning event sends out light and sound. Light reaches the observer first; the thunder arrives later because sound travels much more slowly.

The hot air expands quickly. That sudden push makes a sound wave, which travels through the air as thunder. The light also travels away from the lightning.

Light moves much faster than sound in air. The light reaches an observer first, while the sound is still traveling. Over a longer distance, the sound takes longer to arrive. The drawing separates the two signals to show this difference.

About this explanation

Starting knowledge: Recognizes seeing, hearing, a flash of light, and events that arrive sooner or later.

Sources

  • National Weather Service — Understanding Lightning Science
  • National Weather Service — Dr. Lightning’s Guide to Lightning Science
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