C HARGES IN CLOUDS In olden days lightning was considered as an atmospheric flash of supernatural origin. It was believed to be the great weapon of Gods. But today the phenomenon of lightning can be explained scientifically by elementary principles of physics. Atmospheric electricity arises due to the separation of electric charges.
In the ionosphere and magnetosphere strong electric current is generated from the solar- terrestrial interaction. In the lower atmosphere the current is weaker and is maintained by thunderstorm. There are ice particles in the clouds, which grow, collide, fracture and break apart. The smaller particles acquire positive charge and the larger ones negative charge.
These charged particles get separated by updrafts in the clouds and gravity. The upper portion of the cloud becomes positively charged and the middle negatively charged, leading to dipole structure. Sometimes a very weak positive charge is found near the base of the cloud. The ground is positively charged at the time of thunderstorm development.
Also cosmic and radioactive radiations ionise air into positive and negative ions and air becomes (weakly) electrically conductive. The separation of charges produce tremendous amount of electrical potential within the cloud as well as between the cloud and ground. This can amount to millions of volts and eventually the electrical resistance in the air breaks down and lightning flash begins and thousands of amperes of current flows. The electric field is of the order of V/m.
A lightning flash is composed of a series of strokes with an average of about four and the duration of each flash is about seconds. The average peak power per stroke is about watts. During fair weather also there is charge in the atmosphere. The fair weather electric field arises due to the existence of a surface charge density at ground and an atmospheric conductivity as well as due to the flow of current from the ionosphere to the earth’s surface, which is of the order of picoampere / square metre.
The surface charge density at ground is negative; the electric field is directed downward. Over land the