Daily Read / Weather & Waves
Weather & Waves · Use cause-and-effect and timing evidence to compare how light and sound reach an observer
Why Do We See Lightning Before We Hear Thunder?
Explain why a lightning flash reaches your eyes before thunder reaches your ears, how thunder forms, why it can rumble, and how the flash-to-thunder delay gives only an approximate distance.
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Lightning and thunder come from the same event
Lightning is a powerful electrical discharge. As it passes through air, it heats the air extremely quickly. The heated air expands rapidly and creates a shock wave that we hear as thunder.
Environment and Climate Change Canada describes thunder as the sound of the shock wave produced when lightning heats the surrounding air. The flash and the thunder therefore belong to the same lightning event.[1]
Light wins the race to you
The flash and the sound begin at nearly the same time, but they do not travel at the same speed. Light moves enormously faster than sound through the atmosphere.
Canada's lightning science page gives a useful comparison: light travels about 300,000 kilometres per second, while sound is roughly 0.3 kilometre per second. At ordinary storm distances, the light-travel time is too small for us to notice, but the sound delay is easy to hear.[1]
The delay can give an estimate
Weather agencies use a simple classroom rule: thunder takes about 3 seconds to travel 1 kilometre, or about 5 seconds per mile. If 6 seconds pass between the flash and thunder, the lightning was roughly 2 kilometres away.
This is an estimate because sound speed changes with air temperature and atmospheric conditions. Counting seconds by hand also adds measurement error.[2]
Why thunder can rumble instead of making one click
A lightning channel can stretch for kilometres through the sky. Different parts of that channel are different distances from you, so their sound arrives at different times.
The National Weather Service explains that the nearest part can produce a sharper crack, while sound from more distant parts arrives later and helps create a longer rumble. The atmosphere can also change the sound as it travels.[2]
What the timing does not tell you
A flash-to-thunder count is not a precise map of the entire storm. It gives an approximate distance to the lightning that produced the sound you heard.
Cloud shape, multiple lightning channels, wind, temperature and the way you count can all affect what you hear. The safe conclusion is an estimate, not an exact measurement.[2]
Thunder is also a safety signal
The National Weather Service warns that if you can hear thunder, you are close enough to a storm for lightning to be a danger. Counting is useful for learning about travel time, but it should not delay moving to a safe shelter.
The science lesson and the safety rule fit together: the delay tells us sound took time to arrive, not that the storm is safely far away.[2]
Fast lesson
Use these checks before you answer the practice questions.
Avoid saying thunder happens several seconds after lightning begins. The sound is produced by the lightning event; the delay is mainly travel time.
The 3 seconds per kilometre rule is a practical approximation. Sound speed varies with conditions, and hand counting adds uncertainty.
If thunder is audible, students should already be in safe shelter. Any timing activity should be done from indoors rather than by staying outside to count.
Common mistakes
Why this fails: Thunder is sound produced by the rapid heating and expansion of air around the lightning channel.
Why this fails: It is a useful approximation; actual sound speed depends on atmospheric conditions.
Why this fails: Weather agencies advise seeking shelter whenever thunder is audible.
Word Lab
Use the meaning, hear the word, then try it in your own sentence.
lightning
nouna powerful electrical discharge in the atmosphere
A bright lightning flash lit the sky.
thunder
nounthe sound produced by rapidly heated and expanding air around lightning
Thunder followed the flash several seconds later.
shock wave
nouna strong pressure wave caused by a sudden disturbance
Rapidly expanding air creates a shock wave.
channel
nounthe path through which a lightning discharge travels
The lightning channel stretched across the cloud.
delay
nouna period of waiting between two events
There was a six-second delay before the thunder.
estimate
nouna reasonable approximate value rather than an exact measurement
Two kilometres was only an estimate.
atmosphere
nounthe layer of gases surrounding Earth
Sound travels through the atmosphere to the listener.
kilometre
nouna metric unit equal to 1,000 metres
The storm was estimated to be about two kilometres away.
Check your understanding
Choose an answer, explain your choice, and then check. Hints and retries are welcome.
Try the questions here. Public practice does not save a learning record.
Level 1 · Trace the cause
Level 1 · Trace the cause practice
Connect lightning, heated air, sound and arrival order.
1. What happens first in the explanation: lightning heats air or thunder reaches your ears?
2. What is a delay?
3. What makes the shock wave?
4. Which reaches you first at normal storm distances: light or sound?
5. What is the lightning channel?
6. How many metres are in one kilometre?
7. Complete: We see the flash first because ______.
Level 2 · Use timing evidence
Level 2 · Use timing evidence practice
Use the approximate 3-seconds-per-kilometre relationship and label uncertainty.
8. A delay is 3 seconds. About how far away is the lightning?
9. A delay is 6 seconds. About how far away is the lightning?
10. A delay is 12 seconds. About how far away is the lightning?
11. About how many seconds of delay would correspond to 5 km?
12. Why write 'about 4 km' instead of 'exactly 4 km'?
13. Which lightning is probably farther away: a 3-second delay or a 15-second delay?
14. A student divides a 9-second delay by 5 and calls the result kilometres. What is wrong?
Level 3 · Explain and evaluate
Level 3 · Explain and evaluate practice
Explain rumbling, distinguish claim from evidence and avoid overclaiming.
15. Why can thunder last longer than one sharp crack?
16. A student says, 'The thunder was late because the cloud waited to make it.' Correct the claim.
17. What observation supports the idea that sound travels slower than light?
18. Name one reason two people might calculate slightly different distances from the same flash.
19. Why should counting not keep someone outside during a storm?
20. How is this delay different from an echo delay?
21. What can a flash-to-thunder count reasonably estimate?
Practice answer key
- 1. Lightning heats the air.
The sound wave is produced after the air is heated and expands. - 2. The time between two events.
Here it is the interval between seeing the flash and hearing thunder. - 3. Rapid expansion of air heated by lightning.
Sudden heating makes the air expand very quickly. - 4. Light.
Light travels vastly faster than sound. - 5. The path of the electrical discharge.
Thunder can come from different parts of this long path. - 6. 1,000 metres.
A kilometre is a metric distance unit equal to 1,000 m. - 7. light travels much faster than sound
Arrival order is explained by different travel speeds. - 8. About 1 km.
3 ÷ 3 ≈ 1. - 9. About 2 km.
6 ÷ 3 ≈ 2. - 10. About 4 km.
12 ÷ 3 ≈ 4. - 11. About 15 seconds.
5 × 3 ≈ 15. - 12. The rule and timing are approximate.
Sound speed and counting can vary. - 13. The 15-second delay.
More travel time generally means greater sound-travel distance. - 14. The 5-second rule is for miles; about 3 seconds is used per kilometre.
The units must match the chosen approximation. - 15. Sound from different parts of a long lightning channel reaches you at different times.
Different path lengths spread the arrival times. - 16. The sound was produced by the lightning event; it took longer to travel to the listener.
Travel time, not a waiting cloud, explains the delay. - 17. The flash is seen before thunder is heard.
The two signals from the same event arrive at noticeably different times. - 18. They may count differently, or atmospheric conditions may differ.
The method is approximate. - 19. Hearing thunder already means lightning can be dangerously close.
Learning activity must not delay shelter. - 20. Thunder delay mainly compares light and sound travel; an echo delay comes from sound making an out-and-back reflected path.
Both involve travel time, but the paths and signals differ. - 21. Approximate distance to the lightning that produced the heard thunder.
It does not precisely map the whole storm.
Related skills
Put it in your own words
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Check your own explanation
This is self-review, not automatic marking.
Compare with a model response
Lightning rapidly heats the surrounding air, and the expanding air creates the sound wave we call thunder. The flash and the sound come from the same event, but light travels far faster than sound, so the flash reaches us first. If thunder arrives about 9 seconds later, a classroom estimate is roughly 3 kilometres because sound takes about 3 seconds per kilometre. The estimate is not exact because sound speed and counting vary. If thunder is audible, the safe action is to be in shelter rather than staying outside to measure.
Sources and revision notes
Source check: 2026-10-04. Grade guidance is an editorial suggestion, not a standardised reading score.
- Environment and Climate Change Canada — How thunder sounds
- U.S. National Weather Service — Understanding Lightning: Thunder
What changed in this edition?
Initial release. Thunder formation, light-versus-sound speed and the Canadian comparison were checked against Environment and Climate Change Canada; timing, rumble and safety statements were checked against the U.S. National Weather Service on 2026-10-04. Distance calculations are labelled as estimates.