How it works
Lightning rapidly heats air, creating an expanding shock wave. A nearby section produces a sharp crack, while distant and branching sections add delayed lower sounds.
What people often miss
Terrain, clouds, and atmospheric layers can reflect or refract the sound. Seeing lightning without hearing thunder usually means it is too far away.
Why this question comes up
Counting the delay after a flash provides a rough estimate of distance, but any thunder means lightning is close enough to be dangerous.
Key points
- Thunder rumbles because sound from different parts of a long lightning channel reaches the listener at different times.
- Lightning rapidly heats air, creating an expanding shock wave. A nearby section produces a sharp crack, while distant and branching sections add delayed lower sounds.
- Terrain, clouds, and atmospheric layers can reflect or refract the sound. Seeing lightning without hearing thunder usually means it is too far away.
Bottom line
Thunder rumbles because sound from different parts of a long lightning channel reaches the listener at different times.
Source and verification
This Answer was checked against NOAA National Weather Service. Follow the source for the full official explanation and any later updates.