What is Lightning?
Lightning is a spectacular phenomenon of nature. It is caused by the build up and discharge of electrical energy between positively and negatively charged areas. Mostly, this occurs either within a cloud, or between a cloud and the ground.
Inside a thundercloud the electrrical charges are separated, warm updrafts of air sweep the particles upwards leaving the cloud bottom with a negative charge. The ground beneathe has a positive charge, and therefore a lightning stroke occurs, that is a breif current of negative charge which travels to the ground.
All this starts in the thundercloud, where the charges are somehow separated. No one knows for sure how this happen, although there are many theories about it. One theory is that of the water droplets becoming negatively charged and dropping to the cloud bottom, being heavier than the other particles in the cloud. Then, as more charges separate, the cloud becomes highly charged that the electrical forces nearby tear the air molecules apart, making more charged fragments. So, since the ground has fewer negative particles than the bottom of the cloud, there is an attraction. Electrons are dragged down to the ground, hitting molecules in their way, creating more charged fragments. all of these new particles are dragged down too, creating an electrical avalanche.
This theory would continue unabated except for the postitive charges left behind. The positive charges pull the electrons back up, then the electrons pull them back down, creating a stop-start effect.This is intially called a 'stepped ladder', so called for its start-stop motion. the step ladder moves quickly (240 miles/second) to reach the ground in a short time (5/1000 of a second). when the lead end gets gets near the ground, it may draw a streamer (stream of positive charges) up to it.
The first changes occur near the ground, the light, mobilized negative charges accelerate along the ionized air, causing it to glow very brightly, and with a bluish-white color. The air near the ground glows first,, but as the particles move faster, the air farther up starts to glow. The negative particles flow from cloud to ground, but the light flows from ground to cloud, at an amazing 61 000 miles per second. The heated air expands explosively, causing a shockwave, which we call thunder.
Lightgning doesn't neccessarily happen between cloud and ground, but if two ends of a cloud are charged oppositely lightning can, and does, occur within a cloud.
The typical type of lightning is called forked lightning. This occurs when the lightning channel is blown by the wind during a multiple discharge, and each stroke is placed a short distance from eachother, giving it a snake-tongue like effect. Two other types of lightning are bead lightning and sheet lightning. Bead lightning occurs when the lightning seems to break up into beads the persist for as long as one second, and it is also called chain lightning. Sheet lightning is when the flash is obscured by clouds, and seems to come from everywhere, covering the sky like a sheet.
Lightning is attracted to tall objects simply because they are easier to reach. The Empire State Building and tall trees are often hit. The lightning rod, invented by the infamous Benjamin Franklin, was simply an idea he had from this basic knowledge. By putting a metal rod on the top of a building, and then driving a stake connected to it by a conductor into the ground, the rod would be hit instead of the building, and the charge would travel down the conductor, to the stake, and into the ground.
