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Have you ever wondered what causes thunder? Most people do at one time or another. Thunder is caused by the bolt of electricity produced in a lightning strike(hence thunder always follows lightening). Its deep rumbling and sharp cracks are produced as the air around a lightning bolt is super heated(up to 33,315°C) and rapidly expands as a result. The rapid heating and expansion causes a shock wave that we hear as thunder. The closer the lightning is, the louder the thunder clap will be.
What causes thunder perplexed ancient scientists for hundreds of years. The first known theory belongs to Aristotle, he speculated that it was caused by the collision of clouds. As late as the mid-19th century, the accepted theory was that lightning produced a vacuum. It wasn’t until the 20th century that a theory evolved that thunder must begin with a shock wave due to the sudden thermal expansion of the plasma in the lightning channel. The temperature inside the lightning channel varies from about 19,726°C to about 29, 726°. The average is about 20,100°C. This heating causes it to expand outward, plowing into the surrounding cooler air at a speed faster than sound would travel in that cooler air. The outward-moving pulse that results is a shock wave.
Unrelated to what causes thunder is a method for calculating the distance that a lightening strike is from your observation point. You can estimate how far away the bolt of lightning is by timing the interval between seeing the flash and hearing thunder. The lightning is approximately one kilometer away for every 2.9 seconds that elapses between the visible flash and the first sound of thunder. Thunder is rarely heard more than 20 kilometers away. It may be time to duck and cover if you see lightening and hear thunder at the same time.
The answer to what causes thunder has evolved from a frightened reference to the gods to a scientific understanding that is observable.
If you’d like more info on thunder, check out the National Oceanic & Atmospheric Administration (NOAA) Homepage. And here’s a link to NASA’s Earth Observatory.
We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.