Lightning may be described as a form of visible electric discharge when rain clouds meet or when a rain cloud meets the earth. It appears as a brilliant arc, sometimes several kilometres long, stretching between the discharge points. The discharge also sets off a sound wave that is heard as thunder.
To understand the phenomenon called lightning, we must first look into some basic facts regarding electricity. Things become electrically charged either positively or negatively. Two things with opposite charges are generally attracted to each other. That is to say, a thing with positive charge has a natural attraction for something with a negative charge. As the charge becomes greater, the attraction too becomes stronger. At a certain point, the resistance that holds the charges apart, whether it is air, glass or an insulating substance is broken down by the two charges. Then, to relieve the strain of the `meeting` and to equalise the electric charges in the two bodies, there is a discharge.
This is what happens in the case of lightning, though it is not clearly understood how rain clouds become charged. Most rain clouds are negatively charged at the base and positively charged at the top. A cloud containing innumerable drops of moisture may get opposite charged, either positively or negatively as the case may be, with respect to another cloud or the earth. When the pressure becomes strong enough for it to break down, a flash of lightning occurs.
There are some interesting facts that seek to dispel some myths associated with lightning.
1. Firstly, that lightning never strikes the same place twice is a myth. Photographic evidence shows that skyscrapers and other tall structures may be struck many times in the course of a single storm.
2. Secondly, it is said that the safest place to stay in a thunderstorm is under a tall tree. This is not true at all. Trees, because of their height, are more susceptible and are, therefore, actually dangerous during violent electric storms. The safest place for a person caught outdoors in a thunderstorm is either inside a metal-bodied car or lying flat on the ground in the open.