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Why does thunder follow lightning?

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Helen Czerski Helen Czerski | 09:00 UK time, Tuesday, 12 April 2011

d ~ 262'425'600 km: day 102

Thunder sends some people rushing to the window to watch one of nature's most exciting performances, while others just hide under the bed until it's all over. And just imagine what it must have felt like for most of our ancestors, without the shelter of brick or stone houses. It's easy to see how they might have thought that this enormous noise signaled the approach of Thor, a fierce god known for waving a hammer around. So where does all that sound come from?

You probably know that lightning is a flow of electricity between clouds and the ground. But unless you're having a bad day involving water and a toaster, electricity is silent as it flows through the wires in your home. Lightening is different because there's no permanent "wiring" in the atmosphere - the thunderstorm has to make its own. This happens when a small amount of electric charge finds a path to the ground, ionizing the air along a tube just a few centimeters wide. This tube of air is the "wire", a conducting path for the electricity. Until that connection is made, the electrical charge in the cloud just keeps building up and so by the time the "wire" is ready, there's a vast amount of energy poised to pour along it. For about twenty millionths of a second, a 10,000 amp current rushes down towards the ground (the fuses in your house can't cope with anything greater than 13 amps). The air in that tube is heated up to about 30,000 degrees C - five or six times as hot as the surface of the Sun. As it gets hotter, it expands outwards so quickly that it thumps into the air around the lightning strike. This thump is the thunder. You're listening to the sound of cool air being struck by a plasma hammer much hotter than the surface of the sun. No wonder it's so loud!

The sound then races out sideways in a constantly widening circle, a bit like the ripple from dropping a pebble in a pond. Sound in air travels about 340 metres every second, so if you're a mile from the lightning, you hear the thunder start 4.7 seconds after you see the light. But if thunder comes from an event that lasted a tiny fraction of a second, why do we hear a rumble that goes on for many seconds? The answer is that sound travelling to us from higher up the lightning bolt takes longer to reach us than the sound that only has to travel sideways along the ground. You're hearing different parts of the lightning bolt as time goes on, starting from the ground and moving upwards. It's as if you could freeze the lightning bolt in time and then let your glaze sweep along it, from the bottom to the top.

I love thunderstorms, and I think that it's amazing that you can hear plasma hammering on our atmosphere whenever the right kind of storm is happening. But I will be watching from inside, where it can't hammer on me


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