Discussing Lightning and Thunder

Lightning bolt! (Photo credit: Wikipedia)

With all the storms in the weekend forecast, why not talk about lightning and thunder?  Local WCCO-TV news ran a story on this yesterday, but in my humble opinion, it didn’t tell us much.  Some of it even missed the mark.  So here we go!

The question was asked:  Why does thunder crack and sometimes roll?

Many reasons determine the sound of thunder.  Distance is a key component.  As the sound waves from thunder travel toward you, they travel through different densities of air, different temperatures, and fade.  Lower tones travel the farthest.  You tend to hear the low rumble of distant thunder, therefore.

Also the length of a lightning bolt affects the sound you hear.  All thunder is the result of lightning and some lightning can travel over 60 miles.  You literally hear different parts of the lightning bolt at different times.  This can also give thunder and rumbling, rolling sound.  Different parts of the sound travels through different densities of air, for example.

Where you are in the storm matters, too.  Generally we hear the first strikes of thunder the loudest and clearest, especially if the thunder precedes any rainfall.  Without rain impeding the sound waves, the thunder tends to be more sharp and clear.

And if you happen to be right next to a strike, you get the loud crack that sounds instantaneous because the sound has very little distant to travel and distort.  You hear the loud crack.  If you still have your wits about you, you can often still hear the thunder roll away in the distance, too.

Two errors in the WCCO story.  One minor and the other a bit more signficant.

First, lightning and thunder rarely occurs in the winter not because there isn’t as much moisture in the air, but because there isn’t as much static electricity in the air.  The turbulent rising and falling of rain and especially ice in thunderstorms creates a charge (usually a negative charge) in the cloud that is discharged against a positive charge elsewhere, even in another cloud.   It is a matter of turbulent storm dynamics, not moisture, that causes lightning and thus thunder.  It does happen in the winter, but rarely, because the storm dynamics are not right.

May Storm Over Linden Hills

(I heard someone once say that lightning is rare in hurricanes, too.  Is that true?  I was told hurricanes expend energy in circulating winds that inhibit tall thunderstorm development.  I don’t know if this is true.  Does anyone from Florida or Louisiana read this post?  My dear friend Sandy can answer this for me.  I’ll get back to you.)

The second and more silly error is how the difference in time between a lightning strike and the sound of thunder was explained.  They explained that every five seconds counted after seeing a lightning flash and hearing thunder counted for a mile of distance, which is essentially accurate.  If you can count thirty seconds, for example, the storm is about 6 miles away.   The meteorologist said this was true because light travels five times faster than sound.  This is incorrect.   (Sorry, Mike Augustyniak…but it is ok.)

Light travels at a speed of 186,000 miles per second or almost 700 million miles an hour.  (We learned this in school, right?)  Sound travels about 768 miles per hour, right?  I’m not a math wiz, but light travels many millions times faster than sound.  And if you think about it, it doesn’t matter anyway.  If light travels “five times” faster than sound, how would counting to five measure that.  Where is the constant?  If I counted really slowly and only got to three, would that mean light traveled three times faster than sound?  Nope.  The logic behind the incorrect answer is incorrect as well.

So there you have it, my little brush up on lightning and thunder.  Please reply with all corrections kindly.

Related articles

• Why are we getting thunder and lightning? (metofficenews.wordpress.com)
• Flash Facts About Lightning (nationalgeographic.com)
• Frequently Asked Questions About Lightning (nssl.noaa.gov)