Counting Lightning: How Far Away Is It?


Hello! Well just recently we’ve been having some thunderstoms. (whispering): One, two, three, four, five, six, seven, [THUNDER] Eight seconds. So that must mean the lightning is 8 miles away. Is that right?>>RICHARD 2: No, sorry. That’s completely wrong.>>RICHARD 1: But surely the method’s right?>>RICHARD 2: Well the method is actually ok. You can actually count and find out how far away the lightning is. But it’s still not 8 miles away.>>RICHARD 1: OK, so how far away is the lightning?>>RICHARD 2: Well, I’m afraid you’re just going to have to work that out for yourself.>>RICHARD 1: Right. So, to work it out we actually need to think about what’s actually happening. So anyone who’s been close enough to a lightning strike will know that the thunder and lightning actually happen at the same time. So we see the flash of lightning first because the speed of light is much much quicker than the speed of sound. The speed of light is roughly 300 million metres per second which is approximately 700 million miles per hour. For the distances we can see lightning it arrives almost instantly. The speed of sound on the other hand is very roughly 340 metres per second or about 770 miles per hour. and we can fly planes faster than that. It’s actually quite slow and it can take several seconds to reach us. So if the delay is caused by the speed of sound being quite slow then the amount of time after the flash of lightning that we hear the thunder is dictated to by the speed of sound. Now the speed of sound does vary at little bit depending on things like temperature but it’s very roughly One third of a kilometre per second or around one fifth of a mile per second. It takes three whole seconds to get 1 km and about 5 seconds for one mile. So when my lightning bolt earlier took 8 seconds for the thunder to arrive it really wasn’t 8 miles away. It was less than two! And when you consider that some of that distance will be in the up direction, perhaps the lightning is actually a bit closer than you think? So the new rule is: the counting method really does work. You can count and find out how far away the lightning is but as an approximate rule divide by 3 to find out how many kilometres you are away from it and divide by 5 to find out how many miles away you are from the lightning. So, thanks for watching!

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6 thoughts on “Counting Lightning: How Far Away Is It?”

  1. Eggnogonthebog Productions says:

    Never knew that. Well I knew the thunder always took longer to reach you depending on distance but I never knew to divide the count by 5 for miles.
    Thanks….I learnt something knew today

  2. 007vauxhall says:

    Really well made video Richard, and easily explained…I used to count the seconds between lightning and thunder too, but never really knew why. Thanks :-))

  3. Henllan says:

    Nice … but why didn't the man fall over when he got hit on the head by a flying crotchet ?   :o)

  4. Daniel Schurmann says:

    Used for homeschooling – thank you

  5. Harley King says:

    I heard some that was 2 seconds it was. WELL LOUD

  6. DB-47 says:

    2:34 actually it is almost accurate as sonic speed is ~ 340 m/s and it takes slightly less than 3 seconds to travel 1 km, so for intermediate accuracy 3 sec / 1 km this is good estimate method

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