MadSci Network: Earth Sciences |
Lightning is basically just the result of static electricity. You can build up a static charge on your body, say, by shuffling your feet across the floor. This causes electrons an excess of electrons to build up on your body (or maybe they leave and go to the floor? it's all the same in the end), which than then create a spark if you get close to something. Lightning is similar. Thunderstorms build up because of something called "convection". You probably know that hot air rises and cool air falls. When the sun heats the air near the ground, it tends to rise, but the higher it goes the more it cools off, which eventually causes it to fall back down again. This circulation is called convection. If the air carries with it moisture, you form thunderclouds. The exact mechanism of how and why lightning occurs isn't really well understood. Here's one theory: as the convection process goes on, warmer water droplets going up bump into cold ice crystals going down. Just like rubbing your feet on the floor builds up a static charge on your body, so do these collisions between the water drops and ice crystals build up a static charge in the cloud, which eventually makes a spark - just a really big one. But this is only one theory. The mechanism behind the spark itself is better understood, and is something called "dielectric breakdown". Basically a dielectric material is one that is also a good electric insulator, i.e., electrical current flows very poorly through it. The example you may be familiar with is rubber. Air is also a good insulator. With an insulator, you can build up a very large static charge on one side of the insulator, and it will not flow through to the otherside UNLESS something catastrophic happens. This is dielectric breakdown, where basically the stresses on the atoms from the electric field of the static charge become large enough to pull electrons off the atoms - this then allows electrical current to flow, and so the static charge blasts through. So when enough charge builds up in a thundercloud, the air undergoes dielectric breakdown, allowing the electricity to flow to the ground, or perhaps another cloud. The speed of lightning is simply the speed at which this breakdown takes place - about 1/3 of the speed of light. Lightning bolts are crooked because air itself is irregular, especially in a storm where winds cause turbulence, etc. Even at the molecular level, the air molecules aren't all lined up, but in random positions relative to one another. So there's no reason to think the dielectric breakdown will occur is a straight line. The color occurs because the air through which the lightning passes is heated to a very high temperature. Just like an electric stove glows red when you push electricity through the burner, the air glows when you push electricity through it. However, the lightning has a tremedous amount of energy, and heats the air to much higher temperatures than a stove (tens of thousands of degrees). Higher temperature means it glows with a "hotter" color of the rainbow, in this case purple-white instead of red.
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