What Causes The Sound Of Thunder? - Library Of Congress

Question What causes the sound of thunder? Listen to this page

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°). The rapid rise in temperature creates a rapid increase in the air pressure as well, rising to 10 to 100 times the normal atmospheric pressure. Under such pressure, the heated air explodes outward from the channel, compressing the surrounding air.  As the heated air expands, the pressure drops, the air cools, and it contracts. The result is a shock wave, with a loud, booming burst of noise sent in every direction.

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 (984.25 ft).
• 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.

Published: 6/17/2021. Updated 9/4/2025. Author: Science Reference Section, Library of Congress