Thunder and Lightning

Cue (NZERS), Issue 3, 15 July 1944, Page 13

Thunder and Lightning

Lightning is a high potential discharge of electricity in the atmosphere.

If a fountain pen barrel is rubbed vigorously by a dry cloth it • will attract small pieces of paper. The energy expended through rubbing the pen barrel has been converted into static electricity — or electricity at rest — thus producing an electric field about the object. When the pen barrel is brought close to small, light pieces of paper, an electric field of the opposite kind is induced in the paper. Because unlike fields attract one another —as do unlike magnetic fields — the pieces of paper are attracted to the pen. When higher potentials are involved and where suitable conditions exist, the electric charge will discharge across an air gap producing a blue-violet coloured spark accompanied by a crackling noise. Enormous potential charges are thought to be formed in the atmosphere by air friction. This energy is held in the atmosphere strata or cloud formation. • The earth, of course, remains on a low potential — normally taken as zero ; or negative — in comparison to charges held in the atmosphere. -» , . > : V ■ Under certain favourable conditions such as (1) when a sufficiently high potential has been built up or, (2) when a conducting path formed by a reduction of resistance has presented itself, this high potential charge will arc across. Resistance reduction is caused by the electrified strata moving nearer to a low potential strata or ionization, thus making the air conducting. This arcing across can take place between one atmosphere

strata and another, or, between an atmosphere strata and the earth. Such high pressure discharge is called lightning. It may be visible as a sheet of light or as a thin, jagged flash following an irregular path, and its length may be measured in miles. A lightning flash .can involve a potential of as much as 200 million volts. An American, Benjamin Franklin, by flying a kite in a thunderstorm, obtained sparks from the end of the string, thus becoming the first to demonstrate that lightning was electrical. This was about 1752. Thunder is the result of lightning and therefore occurs after it. During the lightning flash the heat developed in the path of the discharge is intense. This causes rapid expansion of the air in the area, reducing the pressure in the immediate discharge path to almost zero. The heating is momentary and the air, therefore, rushes in again to destroy the . partial vacuum. The air currents in the direction of this partial vacuum - coming from all directions meet with a loud report commonly referred to as a thunder-clap. The rolls or peals of thunder are caused by echoing in the clouds, which gradually diminishes in intensity until all the energy is dissipated. It is interesting to note that the distance of a lightning flash can be calculated roughly by noting the time lapse between the flash and the thunder-clap, and reckoning every five seconds as a mile. The flash is seen instantaneously, but sound travels at only 1100 feet per second. <

PSS/B/M63/3.M./6-44.