Although you aren’t literally “struck” by thunder, it is definitely one of the most striking parts of a lightning storm! But how exactly does thunder form from lightning?
During a lightning strike, the air surrounding the bolt of lightning is heated to about 39,000 °C! This sudden increase in pressure and temperature caused by the lightning causes the air immediately around the lightning to “explode” all along the length of the lightning.
Did you know? The average bolt of lightning is more than six times hotter than the surface of the sun!
This explosion of air is called a shock wave — when the air expands faster than the speed of sound. The expanding air then rapidly slows down to become an ordinary sound wave, which we hear as thunder.
Did you know? At extremely short distances, thunder can cause damage to structures — for example, by breaking glass!
The sound of thunder occurs in distinct phases, matching the different parts of a lightning strike. During a cloud-to-ground strike, a channel of negative charge (stepped leader) travels downward and connects with a channel of positive charge (streamer) reaching up from the ground. Following the connection, the electrical charge will actually move upward (return stroke) to the cloud before returning to the ground again. The initial snapping sound of thunder comes from the upward streamer and the crack (the most distinct part of a thunderclap) occurs during the return stroke. This is followed by rumbling, as the thunder reverberates and echoes off distant hills, buildings, and the cloud itself. Usually, you can’t hear thunder if it originated much further than 15 kilometres away from you.
Did you know? Since light travels much faster than sound, you can use the time between seeing lightning and hearing thunder to calculate how far away the lightning strike was. The distance works out to about one kilometre for every three seconds!
The Science of Thunder. http://www.lightningsafety.com/nlsi_info/thunder2.html
Awesome Slow-motion lightning! http://www.youtube.com/watch?v=_X_7YRVGvtA