# Counting seconds between lightning and thunder

I’ve been told that, during a thunderstorm, if you count the seconds between when you see lightning and hear thunder (or is it the other way around?) you can estimate how far away from you the thunderstorm is. If it’s true, how accurate is it?

The speed of sound in air at normal temperature and pressure is ~1000’/s. So counting the seconds tells you the approximate distance. A five second lag would represent a distance of ~1 mile.

Someone will surely arrive with the correct answer shortly. I recall the answer to be close to 4.5 seconds between flash and sound per mile of distance. How accurate it is would depend on how accurately the time is measured.

The old country boy’s trick is to determine how far the lightening strike is from you.

You see the strike, you start counting (a hundred and one, a hundred and two) until you hear the thunder. Rule of thumb: Every count of 5 = 1 mile.

The reasoning behind this is pretty simple. You see the light from the lightening strike almost immediately (I know there is the speed of light to take into account but at this distance it’s almost negligible).
Thunder moves at the speed of sound which in air roughly takes 5 seconds per mile (the estimate is rough because air temperature and pressure change things…but heck, you’re counting under your breath…we aren’t doing brain surgery here).

Lightening strike, count of 5, thunder…1 mile.

I suppose the truly anal retentive could try to factor the 1/186,000 second per mile for the lightning strike to be seen, but that seems a bit much for me.

There was the one time I was standing outside (at Boy Scout Camp) in a thunderstorm trying to use this method. It turned out to be unnecessary, as the lightning strike that came next was the tree about 50 feet directly ahead of me. I went for cover pretty quickly.

–You’ve been on this board for a long time, waterj2. You understand that we have an anal retentive or two who would, indeed, bring that up (given time).
I mentioned it as “negligible” and pointed out that this isn’t brain surgery for just that reason.
I’m in your camp. There are some questions that really don’t need a slide rule.

I was merely amused by your description of it as “almost negligible”. Correcting for the differences in the speed of sound due to the current air pressure, and the altitude of the clouds from whence the sound comes would be necessary to even approach the point where the margin of error is not several orders of magnitude higher than the difference that could be attributed to the speed of light.

Actually, I think I might just be one of those anal retentive people I mentioned. Sometimes at least (for example, my contributions to this thread). But, hey, it’s the Straight Dope™, we’re nothing if not thorough.

And if you use metric measures you can use 330m/s as a (rough) estimate of speed of sound, and so every 3 secs is approximately 1 km.

Hey, I resemble that!

Actually, the lightspeed errors are tiny compared to the errors caused by air temperature and air pressure. Sound waves propagate at 1129ft/sec at 70 deg F, with approx 0.1% change per degree if the temperature is not at 70F. See:

http://arts.ucsc.edu/EMS/Music/tech_background/TE-01/soundSpeed.html

Um, would it be anal retentive to point out that the logo up there says “Straight Dope®”, not “Straight Dope™”?

OK, who’s the first one telling me it’s actually “THE STRAIGHT DOP®E”?

A true anal-retentive would point out that the speed of light in air is not a constant either, but varies with humidity, pressure and temperature.

While the finesse has been discussed, the fact of the matter is that your instruments are usually a limiting factor. A lag of ~5 seconds between flash and rumble means it hit about a mile away.

So, anal retentives on board, where is the source point for the thunderclap?

what about the distance between the cloud and the ground? some big ass lightnin strikes don’t even hit the ground.

jb

Aren’t all of the above errors negligible compared to the inprecision of the “one-thousand-one, one-thousand-two…” method of keeping time?

/hijack/

Does anyone remember that scene in Poltergeist where the boy is counting the seconds between lightning and thunder, and the tree tries to murder him??? That freaked me out as a child, and still does. That’s why I don’t do it.

/end hijack/

:eek: ,

Lorie