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Thread: Why is there lightning before thunder?

  1. #1
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    Why is there lightning before thunder?

    The lightning is a warning to drop the cat before the clap of thunder. This time I didn't need bandaids on my stomach, just a small one on my hand.

  2. #2
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    The answer...is distance and the speed of sound. So yea...don't ignore the flash if you have a cat in your arms!
    --

    The most expensive tool is the one you buy "cheaply" and often...

  3. #3
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    Sound is about 5 miles per second. Light is instant for practical purposes on earth. So if lighting hits then 5 seconds later you hear the thunder it was one mile away. Note distance is straight line not just horizontal. Here most thunder is at least one mile high. So five seconds plus some for the horizontal distance as well.
    Bill D

  4. #4
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    Quote Originally Posted by Bill Dufour View Post
    Sound is about 5 miles per second. Light is instant for practical purposes on earth. So if lighting hits then 5 seconds later you hear the thunder it was one mile away. Note distance is straight line not just horizontal. Here most thunder is at least one mile high. So five seconds plus some for the horizontal distance as well.
    Bill D
    I think you mean 5 seconds per mile, Bill.

  5. #5
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    Quote Originally Posted by Steve Demuth View Post
    I think you mean 5 seconds per mile, Bill.
    At the sound of thunder my dog will run to the basement at about 5 miles a second.

    Well, I haven't measured it but haven't dared get in the way.

    -Tom

  6. #6
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    Quote Originally Posted by Steve Demuth View Post
    I think you mean 5 seconds per mile, Bill.
    Yup...about 1100 feet per second which is about 5 seconds to get to a mile.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  7. #7
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    Quote Originally Posted by Jim Becker View Post
    Yup...about 1100 feet per second which is about 5 seconds to get to a mile.
    Or about 1125 fps, depending on tenperature, altittude, your references and perhaps attitude. If you really want to see that subject discussed to death find a rimfire-oriented gun forum and a debate about supersonic vs subsonic ammunition, how you define those and why and whether it matters. In my experience, a squirrel can get to the other side of the tree whether the ammo is supersonic or not.

    That's true about the height of the thunderstorm, but now that we live on the fringe of the lightning capital of the country I've noticed that the sound comes from the whole length of the lightning bolt, including where it hits the ground, if it does. I saw a lightning strike on a neighbor's tree from inside my car about 100 yards away a few years ago and I assure you there was no delay waiting for the sound to travel down from the cloud. Part of me was wishing for instant replay but another part says once was enough.

  8. #8
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    Speed of light 2.998 x 108 m/s which translates to about 5.4 microseconds per mile (In my ultrafast spectroscopy days the rule of thumb was a foot for every nanosecond)
    Speed of sound in dry air is about 343 m/s (varies with temperature and humidity) which translates to about 4.7 seconds a mile
    Higher humidity would increase the speed of sound, since the water molecule is lighter than nitrogen or oxygen.


    Reminds me of the question that always trips up my students in the physical chemistry lab...What is more dense, dry air or humid air?


    Dry air is more dense, again this is due to water molecule being lighter than nitrogen, N2 or oxygen O2.

    John

  9. #9
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    Quote Originally Posted by Alan Rutherford View Post
    That's true about the height of the thunderstorm, but now that we live on the fringe of the lightning capital of the country I've noticed that the sound comes from the whole length of the lightning bolt, including where it hits the ground, if it does. I saw a lightning strike on a neighbor's tree from inside my car about 100 yards away a few years ago and I assure you there was no delay waiting for the sound to travel down from the cloud. Part of me was wishing for instant replay but another part says once was enough.
    Having been in both barns and small planes struck by lightning, I can affirm that the instantaneous and unexpected combined flash and bang (to put it mildly) may require a moment or two to recover from. I recall as a young fella, working furiuosly to get a couple of loads of square bales into the neighbors' barn as a massive prairie storm (Greg Brown: "Clouds roll in from Nebraska; dark chords on a big guitar" says it best, musically) rolled in. I was up in the peak stacking when a bolt hit the (grounded to the lightning rod system) cupula just above me - I was maybe 15 ft away from the terminus of the strike. Holy shit. Not too different from the percussion you feel if you've ever been right next to a black powder cannon being fired - except then you typically know it's coming, so you're a little less likely to need a change of underwear after.

  10. #10
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    It was a joke about a cat not a question of scientific inquiry............
    Last edited by Rob Damon; 04-09-2021 at 1:02 PM.

  11. #11
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    I agree, I said it backwards. It is five seconds per mile.
    Bill D

  12. #12
    Supposedly there's a story -or maybe a movie even as I don't remember the particulars, but how it goes is some guy was underwater when he heard an explosion... he then surfaced, and heard "another" explosion, so his story was there was 2 explosions-- but was eventually determined that it was just one explosion due to sound traveling nearly 4-1/2x faster in water than thru air, so he heard the same explosion twice...
    ========================================
    ELEVEN - rotary cutter tool machines
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    THREE- make that FOUR now - fiber lasers
    ONE - vinyl cutter
    CASmate, Corel, Gravostyle


  13. #13
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    Lightning in real time: https://map.blitzortung.org/#1.26/-3.1/-110.8

    Turning on the display of the number of strikes in each area right now shows over 7,000 recent strikes in the Gulf and the Tallahassee-Pensacola Florida area, quite a few in Alabama, a bunch in Oklahoma-Texas-Arkansas, and nowhere else in the world even close. Not much right here but we've been listening to the rumble.
    Last edited by Alan Rutherford; 04-09-2021 at 3:30 PM.

  14. #14
    I was in a limestone quarry (aggregate mine) once for 'a shot'. They back off 6ft from the active cliff face and drill a row of 12" holes 6ft OC spanning perhaps a 1/2 mile of the face, tie det cord to a rock and drop it to the bottom of each hole (~200'), then fill it with liquid explosive, 'splice' the det cords together, and clear the area. Objective is no rubble larger than a human head (all goes into crushers). Safety first :: there is even a lookout for low flying aircraft.

    I was located on the mine floor, about 1/3 to 1/2 of a mile back from face - "FIRE".

    Cliff face crumbles - dust rising - no sound. You can see a disturbance coming on the ground :: sand particles kicked up.

    Buh-whuump! ...Seems like a sound (but it's not). Body feels a sharp thump.

    Turned to Facility Electrician, "That wasn't as bad as" BOOM!! "I thought." Check eardrums for bleeding.

    First shock arrived at the speed of sound in limestone. Second arrived at speed of sound in air.

    No cats were harmed in the making of this aggregate. ...nor airplanes.
    Last edited by Malcolm McLeod; 04-09-2021 at 3:22 PM.

  15. #15
    High school physics. We had a guy set off a large aerial mortar firework on one side of the river and had five people measure from the flash to the sound with stop watches. We threw out the two most extreme times and averaged the middle three. Then distance per second x time in seconds down to the hundredths. We came up within 30 feet of what the US Coast and Geodetic survey map had as the distance on a topographical map. Not far off for a group of high school kids and a 1.4 mile wide river. At the 4th of July a little town along the river has an impressive fireworks display for such a small town. We went down to the river to watch one night. The problem is that we were 7 miles south. The sound took over a half minute to get to us.

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