halle1 (02/08/83)
Several people have asked me for the explanation of why lightning strikes up. As a few incorrect explainations have appeared here, I feel that it is proper to post it rather than mail it. During a thunderstorm there is often a large buildup of positive charge on the underside of the cloud. (Why it is usually positive I don't know.) When the potential difference between the cloud and the induced negative charge on the ground below becomes large enough, an electric field propagates from the cloud to the ground. If the field is of sufficient energy (Poynting vector of sufficient energy/area), then it will ionize things on its way, such as pollutants, water droplets, even air molecules. These little ion packets by themselves will not be seen, but instead provide a path for the lightning strike. As the field reaches the earth, a (relatively) high conductivity path is thus set up for an intense stream of electrons to travel upwards to neutralize the charge on the cloud. This electron beam jumps from ion packet to ion packet, resulting in a jagged path. The energy is so great that the air surrounding the beam becomes highly ionized and a hole is blown in the atmosphere. This hot plasma is what is seen as lightning. The channel sets up an LRC circuit. Basically, this means that the current overshoots what was necessary for neutralization, so another electron beam shoots down the channel, reionizing and causing another lightning bolt. This procedure repeats several times, with strikes alternating up and down, each weaker than the last, until the channel cools enough so that the conductivity is too low to permit the current to flow. Meanwhile, the channel has been drifting, kinking, bending, etc. This irregularity is the real cause of the jagged shape of a lightning bolt. Jeff Halle houxz!halle1