Phenomenon of lightning: An electric discharge between cloud and earth, between clouds or between the charge centres of the same cloud is known as “Lightning”. Lightning takes place when the clouds acquire charge. During the uprush of warm moist air from earth, the friction between the air and the tiny particles of water causes building up of charges. When drops of water are formed, the larger drops become positively charged and the smaller drops become negatively charged. When the drops of water accumulate, they form clouds and hence cloud may possess either a positive or a negative charge, depending upon the charge of water drops they contain. The charge on a cloud may become so great that it may discharge to another cloud or earth through air medium, after its electrical breakdown. Such a discharge is called as lightning.
When the charged cloud passes over the earth, it induces equal and opposite charge on the earth below. As the charge acquired by the cloud increases, the potential between cloud and earth increases and therefore the potential gradient in the air between them increases. When the potential gradient becomes more than the dielectric strength of the air medium, the electrical breakdown of the air takes place and the lightning stroke starts. The lightning stroke mechanism is as follows:
(i) As soon as the air near the cloud breaks down, a streamer called “leader streamer” or “pilot streamer” starts from cloud towards the earth and carries charge with it. So far the cloud feeds enough charge to maintain the gradient above the dielectric strength of air at the tip of the leader streamer; the leader streamer continues its journey towards the earth. (ii) As the leader streamer moves towards earth, it is accompanied by points of luminescence which travel in jumps giving rise to stepped leaders. The stepped leaders have sufficient luminosity and give rise to first visual phenomenon of discharge. (iii) As the leader streamer reaches near earth, a return streamer shoots up from the earth to the cloud, following the same ionized path as that of leader streamer. This phenomenon causes a sudden spark which we call lightning.