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What Is Thunder?

People throughout history have tried to figure out the origins of the mighty blasts that resonate through the sky during a lighting storm. The Greeks believed they came from Zeus hurling his lightning bolts, believers of the Shinto faith thought that the god Raijin was beating on his drum, and Norse mythology claimed that thunder was created when Thor rode his chariot across the sky. Today, thanks to more scientific methods, researchers know exactly how thunder is formed and, unfortunately, none of the explanations involves supernatural beings. So, what exactly is thunder?

First comes lightning

Lightning bolt ripping through sky with arcing tendrils against dark clouds
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Thunder and lightning always occur together because thunder is actually a byproduct of lightning. Two things need to happen to make lightning. First, there needs to be moisture in the air. Second, the air closer to the ground needs to be warmer than the air higher up in the atmosphere.

According to physics, warm air rises. As the warm air rises, the moisture in the air cools and forms clouds. The clouds get bigger and bigger as they collect more moisture. At the very top of the clouds, temperatures can dip below freezing and form ice crystals. As more warm air comes up through the cloud, the ice and water molecules bump into each other and, much like rubbing a balloon on your head, create static electricity. Over billions and billions of collisions, energy builds up until the cloud can’t contain it anymore and a lightning bolt is shot down to the ground.

Massive energy

Lightning striking group of trees
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A little bit of static electricity won’t do much harm. It can make your hair stand on end, stick a balloon to the wall, or give you a little shock when you touch a metal door knob. But when so many small charges are put together, they can let loose more than a billion volts of electricity. The amount of energy is so enormous that a lightning bolt can heat the air around it to temperatures of 50,000 degrees Fahrenheit. That’s more than five times hotter than the surface of the sun!

The heat and speed from a lightning strike is enough to vaporize just about anything in its path. When a tree is struck by lightning, all the water in the tree is instantly vaporized. The steam creates so much pressure that it can blow the entire tree apart.

Here comes the boom

Distant lightning bolt arcing from dark clouds into the ground
Credit: John D Sirlin/ Shutterstock

When a lightning bolt is released, the air around it becomes superheated. When things are heated, they expand and need more space. Lightning heats the molecules in the air so extremely and immediately that the expansion is almost instantaneous. The air pressure around the lighting bolt can increase up to 100 times its original amount in a matter of milliseconds. The compressed air explodes outward, creating a shockwave in every direction and making a loud boom.

Thunder and lightning always occur together. Wherever there’s one, there's the other. Sometimes the lightning is so far away that the sound of the thunder fades away before you can hear it. In the case of heat lightning, it occurs so high up in the atmosphere that you can’t hear the booms from the ground. You can only see the flashes in the clouds.

How far away is the storm?

Lightning bolts striking in various places on hills
Credit: lucien_kolly/ Unsplash

Even though thunder and lightning are formed almost simultaneously, you will always see the lightning before you hear the thunder. That’s because the speed of sound is much slower than the speed of light. It’s so different that it could be compared to a snail versus a rocket, but even that wouldn’t do it justice.

You can determine how far away a storm is based on how many seconds there are between when you see the lightning flash and when you hear the thunderclap. Light travels at around 186,000 miles per second, which is basically instantaneous between any two points on Earth. Sound travels at approximately 1,000 feet per second depending on air temperature and humidity. That means that when you see a lightning strike, you can determine how far away the storm is by counting the seconds until you hear the thunder. One second is equal to about 1,000 feet, and five seconds is about equal to a mile.

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