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Question What causes the sound of thunder?

Answer

Thunder is caused by the rapid expansion of the air surrounding the path of a lightning bolt.

Monsoon storm producing a forked lightning bolt from the Red Hills Visitors Center at Saguaro National Park in Arizona.  Pete Gregoire, photographer, NOAA Weather in Focus Photo Contest 2015.  NOAA Photo Library.

From the clouds to a nearby tree or roof, a lightning bolt takes only a few thousandths of a second to split through the air. The loud thunder that follows the lightning bolt is commonly said to come from the bolt itself. However, the grumbles and growls we hear in thunderstorms actually come from the rapid expansion of the air surrounding the lightning bolt.

As lightning connects to the ground from the clouds, a second stroke of lightning will return from the ground to the clouds, following the same channel as the first strike. The heat from the electricity of this return stroke raises the temperature of the surrounding air to around 27,000 C° (48,632 F°). Since the lightning takes so little time to go from point A to point B, the heated air has no time to expand. The heated air is compressed, raising the air from 10 to 100 times the normal atmospheric pressure. The compressed air explodes outward from the channel, forming a shock wave of compressed particles in every direction. Like an explosion, the rapidly expanding waves of compressed air create a loud, booming burst of noise.

A massive cloud presages a thunderstorm above Groom, a tiny community along old U.S. Route 66 in the Texas panhandle. Carol M. Highsmith, photographer, 2014. Prints & Photographs Division, Library of Congress.

Because electricity follows the shortest route, most lightning bolts are close to vertical. The shock waves nearer to the ground reach your ear first, followed by the crashing of the shock waves from higher up. Vertical lightning is often heard in one long rumble. However, if a lightning bolt is forked, the sounds change. The shock waves from the different forks of lightning bounce off each other, the low hanging clouds, and nearby hills to create a series of lower, continuous grumbles of thunder.

Lightning. Oklahoma, 2009. National Severe Storms Laboratory Collection, NOAA Photo Library.

Thunder Fun Facts

  • To judge how close lightning is, count the seconds between the flash and the thunderclap. Each second represents about 300m (984.25ft).
  • Thunder is not only heard during thunderstorms. It is uncommon, but not rare, to hear thunder when it is snowing.
  • Lightning does not always create thunder. In April 1885, five lightning bolts struck the Washington Monument during a thunderstorm, yet no thunder was heard.
Building line of cumulonimbus thunderstorms. View is from behind storms during early stages of development.  National Severe Storms Laboratory Collection, NOAA Photo Library .

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