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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.

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.

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.

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.
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Library of Congress Web SiteFurther Reading
  • Engelbert, Phillis. Thunderstorms. In The complete weather resource, v. 2, Weather and phenomena. Detroit, UXL, c1997: p. 211- 242.
  • Keen, Martin L. Lightning and thunder. New York, Julian Messner, c1969. 94 p. (Juvenile literature)
  • Pfeffer, Wendy. Thunder and lightning. New York, Scholastic Reference, c2002. 32 p. (Juvenile literature)
  • Rakov, Vladimir A., and Martin A. Uman. Lightning: physics and effects. Cambridge, UK, New York, Cambridge University Press, 2003. 687 p.
  • Uman, Martin A. All about lightning. New York, Dover Publications, 1986. 167 p.

SearchFor more print resources...
Search on "lightning," " sound," or "thunderstorms," in the Library of Congress Online Catalog.

Lightning issuing from  a sky  full of dark clouds strikes the ground.
Twin bolts of lightning strike the ground. From the NASA Website.

Looking down on a circular cloud formation.
Near vertical view of thunderhead over
South America as seen from Apollo 9
. From
NASA JSC Digital Image Collection.

Large, light-colored clouds billowing  in the background, with blue sky above.  A farm is in the foreground.
Building line of cumulonimbus thunderclouds. From the NOAA Photo Library.

Bright orange and yellow clouds.
Globular mammatus clouds, often associated with thunderstorms and
severe weather.
From the NOAA Photo Library.

Picture of lightning striking the Eiffel Tower. Labeled, June 3, 1902, Taken by M.G. Loppe.
Lightning striking the Eiffel Tower. From NASA's SciJinks Web site.

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  July 24, 2013
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