Astronomers last week saw a sky sizzling with lightning ? on another planet. Images sent by Nasa's Cassini spacecraft, during its close approach to Saturn, show swirling thunderstorms and lightning in the alien atmosphere.Updated: Feb 20, 2006 00:16 IST
Astronomers last week saw a sky sizzling with lightning — on another planet. Images sent by Nasa's Cassini spacecraft, during its close approach to Saturn, show swirling thunderstorms and lightning in the alien atmosphere. Brilliant flashes of electric energy snaking across a dark sky have always fascinated man. But it wasn't until 1752, when Benjamin Franklin famously flew his kite in a thunderstorm, that scientists realised the electrical nature of lightning.
During thunderstorms, air currents rise and fall rapidly, and the resulting friction creates electrical charges in clouds. We still don't know why clouds acquire negative charges at the bottom and positive on top. It's believed that falling water droplets and ice pellets carry charged electrons to the lower portion of a cloud, building a negative charge there, while a positive charge builds on top. These charged particles attract their opposites and lightning occurs within a cloud, between clouds, between a cloud and the air around it, or on the ground below. In fact, it's misleading to say that lightning ‘strikes the ground', because in many ways it's the ground ‘striking' the sky. The positive charges on the ground produce a ‘charge separation' with negative particles on the cloud's bottom and electricity flows from cloud to ground as lightning, though it may appear that the ‘sky strikes first'.
Lightning packs enormous power into each bolt: from 10 million to two billion volts, at almost 300,000 amperes (your household wiring, by contrast, carries just a few tens of amperes). These currents induce momentary magnetic fields powerful enough to stop the human heart and heat the air along the lightning's path to 8,000 degrees Celsius. Hence the lightning deaths that make news. The hot air expands explosively, creating a shock wave that we hear as thunder: a sharp clap if the path is short, or a long, rolling rumble if it's longer.
Because light travels faster than sound (186,300 miles per second as against 1,088 feet per second) we see lightning before we hear thunder. When you next see it, count the seconds before the thunder and divide it by five (given a mile equals five seconds) to know the distance to the cloud. But remember the traditional advice if caught in a thunderstorm: avoid sheltering under trees, as they act as lightning conductors, and ensure you are not the tallest object on the ground or you'll be its flaming pathway.
Scientists simulate lightning in labs, making it strike the same spot hundreds of times a day to see how they could deflect the real thing away from power lines and installations. Latest research uses lasers to trigger lightning strikes: scientists shoot laser beams at thunderclouds, hoping to second-guess Mother Nature and channel the electric discharge towards a safe location.
First Published: Feb 20, 2006 00:16 IST