Lightning, Thunder & The 30-30 Rule

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Whoa! We’ve been having a lot of thunderstorms this summer (it’s a good thing I enjoy them!) and I thought I’d bring up this tip I published way back in 2006, it’s a good one to know when things are rockin ‘n rollin outside…

FlashThe theory is that you can determine how close it struck to where you are by counting in seconds after you see lightning until you hear thunder. Divide by 5 for miles, divide by 3 for kilometers.

For Example:

  • You see lightning
  • You count 12 seconds and then you hear thunder

The lightning struck and the storm is approximately 4 kilometers or 2.4 miles away (12 seconds / 3 = 4 kilometers away; or 12 seconds / 5 miles = 2.4 miles away).


The speed of light travels faster than the speed of sound (that’s why we see the lightning first) and it takes approximately 5 seconds for the sonic boom to travel one mile.

If you count less than 30 seconds: Practice the 30-30 rule:

Take appropriate shelter when you can count 30 seconds or less between lightning and thunder.

Remain sheltered for 30 minutes after the last thunder.

Neat to know, whaddya think?

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13 Comments to “Lightning, Thunder & The 30-30 Rule”
  1. Pamela says:

    We always counted seconds for miles when we were kids.

    this means that it is a heck of alot closer than we were thinking.

    How I love to set on the patio and watch and listen to a thunder storm.

  2. Charles says:

    It’s all bull. You can have lightning in your own yard, and the clap of thunder is sometimes immediate, sometimes VERY delayed, and sometimes it never comes at all. Haven’t you ever had the kind of storm where you see a flash of lightning that is NOT followed by ANY thunder? I have, many times. The two are NOT *necessarily* related!!!

  3. Cynthia Long says:

    I live in AZ and we have frequent summer storms…lightening can travel 10 miles from the storm and you wouldn’t hear any thunder! Just be careful!

  4. Barbara says:

    Charles, you need to go back to science in school. If you don’t hear thunder it is either too far away or your deaf. “Lightning is an atmospheric discharge of electricity accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms.[1] In the atmospheric electrical discharge, a leader of a bolt of lightning can travel at speeds of 60,000 m/s (130,000 mph), and can reach temperatures approaching 30,000 °C (54,000 °F), hot enough to fuse silica sand into glass channels known as fulgurites which are normally hollow and can extend some distance into the ground.[2][3] There are some 16 million lightning storms in the world every year.[4]”From Wikipedia

  5. Mrs. D says:

    Thunder is caused by moving molecules. Sound waves travel by the transfer of energy from molecule to molecule. The disturbance of the molecules is caused by the release of the static electrical charge (lightning). The loudness of the “boom” of thunder depends on the amount of energy being transferred via the “wave” of energy and how efficient it is at getting the wave rolling.

    It is true that lightning can strike quite a distance from the storm center. I have heard it said that as long as you can hear the thunder it is an indication that you should move to a safe place. I’m not sure that’s because if your having a picnic or the kids are swimming it might take that long to gather all your kids and stuff together or what, but seems to be a pretty safe rule. Thunderstorms can move very quickly as those who live out on the open plains can tell you.

    I have never been struck by lightning, but it does make the hair on the back of my neck stand up. We’ve had some bad thunderstorms in our neck of the woods and I just play it safe. If the sky is dark and I hear thunder I gather up and get in a safe place.

  6. Jim says:

    “Heat” lightning is silent.

    Cloud to cloud lightning is often silent.

    Finally, having been in the midst of some scary storms (including just missing or being missed by two tornadoes) if the lightning is close enough, you’ll hear a snap or pistol shot followed by a sizzle followed by the smell of ozone. It’s easy not to call the snap-sizzle “thunder” since it doesn’t really sound like the rolling, booming, thundering sound.

    • Pat says:

      Jim: You are absolutely correct on this one! Snap sizzle lightening is real scary. You can almost “feel” it 1 second before it comes. Drop to the ground in a ball. I once had “ball lightening” come through the living room with my grandson on the sofa and exit through the opposite window. Unlike anything else I’ve seen & I’ve had some really close calls.

      • Carol says:

        My mother related a story of her baby brother lying on the floor in their living room. A ball of lightning entered through the front door and rolled across the baby and exited through an opposite window. She said her mom got a wet cloth and just kept bathing his face with it and silently praying that he would awaken. He did, and lived a long life. In my early years, Mother was very nervous when storms gathered.

      • Phoebe says:

        Immediately crouch or kneel steady, then wrap your arms around your head.

        Do NOT put your hands on the ground. Do NOT lie on the ground.

        Do not drop to the ground in a ball – crouch or kneel.

  7. RonNasty64 says:

    Coming up next: “The 5 Second Rule”…

  8. chloe says:

    I’ve only just discovered this 30-30 rule, my boyfriend told me about it then I googled it. I’m in the North west of England and theres a thunderstorm at the moment and I don’t like it but its probably nothing compared to what you guys have! I timed 8 seconds between the lightening and the
    under before so I’m presuming it might have been between one and two miles away! It makes sense to me so I believe its true! Don’t know where charles is coming from, he should read up on these things like I have! I consider myself lucky to live somewhere where we don’t have really bad weather conditions!

  9. Lee B says:

    Thunderstorms are cool. I like them.
    I am currently in central uk witnessing one.

    So you scientific guys, why the total difference in lightening (sheet, fork, ya Da ya Da) ?? X

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