Q How does thunder form?
— Hazel Foster, Madison, Wis.
A Amanda Gumber, research assistant, Space Science and Engineering Center at the University of Wisconsin-Madison:
Thunder is formed by the intense heating produced by lightning. The thunder you hear is made up of vibrations that travel as sound waves through the air until they reach your ear.
When lightning occurs, a large discharge of electricity causes the surrounding air to vibrate as it passes through the atmosphere to the surface.
When air molecules experience an increase in temperature, they expand. The faster these air molecules heat up, the faster they expand.
In the case of lightning, the surrounding air is being heated to temperatures hotter than the surface of the sun, in a fraction of a second.
This expansion of air occurs so rapidly that it begins to forcefully push the surrounding air particles, creating more vibrations and compressing the air in front of it.
As the air begins to cool, it rapidly contracts, which creates the initial cracking sound of thunder. This air column continues to vibrate, forming sound waves that make up the rumbling sounds of thunder.
Since light travels faster than sound, we see lightning before we hear the thunder it’s producing.
If you see lightning and then hear an immediate crack of thunder, that means the lightning struck nearby.