Thunder

Thunder

     There is a myth about thunder that has been around since at least 1955 when I first heard it.  I never heard the myth again until about 3 months ago when a TV weatherman tried to explain thunder to some elementary students.  He explained the thunder in exactly the same manner as I had heard the first time.  The TV weatherman explained that the lightning would create a vacuum.  When the lightning ended, the air would rush in to fill the vacuum causing the sound of thunder. The man demonstrated this by clapping his hand.  The explanations were so similar that I suspect that it is in the literature somewhere.  However, the explanation is wrong.  The correct explanation is that the surge of electrons associated with lightning will heat the air within the lighting stroke to approximately 30,000 degrees C or 54,000 degrees F within a tiny fraction of a second.  This is about five or six times hotter than the surface of the sun.  This quick and extreme heating causes an explosive expansion of the air.  That is the thunder you hear. 

     When a lightning bolt strikes within a few tens of feet, you would hear a very loud pop or bang.  This is the pop of the big electrical spark.  This loud pop or bang is immediately followed by the rumble.  The rumble has two causes.  One is that the sound from the part of the lightning bolt that farther away will arrive later than the sound from the part that is closer.  The second cause is the echoes of the thunder.

    Another myth about lightning and thunder it that a sizzling or crackling sound sometimes heard just before a close lightning strike is the sound of lightning striking something nearby.  The sizzling or crackling sound is caused by Saint Elmo’s fire.  Saint Elmo’s fire is electrical discharges from tall objects such as trees, roof tops, etc. that sometimes occur during thunderstorms.  Just before a lighting strike, the electrical field between the cloud and the ground increases to the point where electrical sparks will begin discharging from tall objects.  These sparks are the Saint Elmo’s fire and these sparks is the source of the sizzling sound.

Fredmet.