How far away was the lightning?
Count the seconds between a lightning flash and its thunder, and work out how far away the lightning struck. Find out why a short gap means you must go indoors at once.
Core concept
Light arrives almost at once but sound travels only about 343 m a second, so the distance to lightning in km is the seconds from flash to thunder × 0.343
Topic: Science around us
Lightning and thunder happen at the same moment: the flash is the light from a giant spark, and the thunder is the bang of the air it heats. Light travels so fast (300000 km a second) that we see the flash almost at once. Sound is much slower: about 343 m, or 0.343 km, every second in air. So the thunder arrives late, and the longer you wait, the farther away the lightning was. Count the seconds from the flash to the thunder and multiply by 0.343 to get the distance in kilometres (or divide by 3 for a quick guess). Safety experts use the 30-second rule: if the thunder comes less than 30 seconds after the flash, the storm is close enough to strike you, so go indoors.
What you will learn
- Explain why we see lightning before we hear thunder.
- Work out the distance to lightning from the seconds between flash and thunder.
- Use the 30-second rule to decide when to go indoors.
Before you start
- Know that sound needs air to travel and takes time to reach us.
Materials
- A watch or a phone stopwatch (to count during a real storm, from indoors)
Equipment
- None
Procedure
- Record a gap of 10 seconds between the flash and the thunder.
- Make the gap 3 seconds and record.
- Make the gap 30 seconds and record.
How this simulation is simplified
- Sound travels at 343 m/s in air at about 20 °C; light arrives in effect at once.
- The 30-second rule follows common lightning-safety advice.