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Lightning, Thunder & The 30-30 Rule

Posted By Tipnut On July 16, 2010 @ 3:17 am In Household Tips,Notes & Tidbits | 13 Comments

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 [1]:

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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[1] Practice the 30-30 rule: http://ec.gc.ca/meteoaloeil-skywatchers/

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