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Q & A: When the Lightning Cracks! and the Thunder Booms!

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Most recent answer: 10/22/2007
Q:
What happens during a lightning storm?
- Evelyn Falasca (age 14)
S. Hamilton MA.
A:
Evelyn -

The first thing that you need for a lightning storm is a big cloud. In particular, you need a big cloud that's moving. As the cloud moves over the ground, it builds up a whole lot of electrical charge. This is the same as when you drag your feet on a carpeted floor. You build up charge that can shock you later. When one kind of charge builds up in one place, its first priority is to spread out again. In order to spread back out, the charge on the cloud will jump to the ground. (Since the Earth is really big, the charge can spread out there.) This is when we see lightning.

So how about thunder? Well, as the electricity of the lightning passes through the air, it ionizes the air molecules. When this happens, the air in that area gets very very hot. Hot air takes up more space than cold air, so this suddenly hot air expands very rapidly. When the air expands, it shakes the air molecules around it. Shaking air creates soundwaves, which we hear as thunder. Thunder is literally the sound of a giant explosion occuring all along the lightning strike.

But why do we hear a long rumble instead of just a loud bang? Well, the top of the lightning bolt is a lot farther away from us than the bottom (since it's a lot farther up). This means that the soundwaves coming from the top have farther to go and will take longer to get to us than the soundwaves coming from the bottom of the bolt. So we hear the sound of thunder from the bottom of the lightning bolt first and the sound from the top later.

-Tamara

(published on 10/22/2007)

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