Let us now discuss the manner in which a lightning discharge occurs. When a charged cloud passes
over the earth, it induces equal and opposite charge on the earth below. Fig. shows a negatively
charged cloud inducing a positive charge on the earth below it. As the charge acquired by the cloud increases, the potential between cloud and earth increases and, therefore, gradient in the air increases.
When the potential gradient is sufficient (5 kV*/cm to 10 kV/cm) to break down the surrounding air, the lightning stroke starts. The stroke mechanism is as under:
(i)As soon as the air near the cloud breaks down, a streamer called leader streamer or pilot streamer starts from the cloud towards the earth and carries charge with it as shown in Fig. (i). The leader streamer will continue its journey towards earth as long as the cloud, from which it originates feeds enough charge to it to maintain gradient at the tip of leader streamer above the strength of air. If this gradient is not maintained, the leader streamer stops and the charge is dissipated without the formation of a complete stroke. In other words, the leader streamer will not reach the earth. Fig. (i) shows the leader streamer being unable to reach the earth as gradient at its end cloud not be maintained above the strength of air. It may be noted that current in the leader streamer is low (<100 A) and its velocity of propagation is about 0·05% that of velocity of light. Moreover, the luminosity of leader is also very low.
(ii)In many cases, the leader streamer continues its journey towards earth [See Fig. (ii)] until it makes contact with earth or some object on the earth. As the leader streamer moves towards earth, it is accompanied by points of luminescence which travel in jumps giving rise to stepped leaders. The velocity of stepped leader exceeds one-sixth of that of light and distance travelled in one step is about 50 m. It may be noted that stepped leaders have sufficient luminosity and give rise to first visual phenomenon of discharge.
(iii) The path of leader streamer is a path of ionization and, therefore, of complete breakdown of insulation. As the leader streamer reaches near the earth, a return streamer shoots up from the earth [See Fig. (iii)] to the cloud, following the same path as the main channel of the downward leader. The action can be compared with the closing of a switch between the positive and negative terminals; the downward leader having negative charge and return streamer the positive charge. This phenomenon causes a sudden spark which we call lightning. With the resulting neutralization of much of the negative charge on the cloud,any further discharge from the cloud may have to originate from some other portion of it.
The following points may be noted about lightning discharge:
(a) A lightning discharge which usually appears to the eye as a single flash is in reality made up
of a number of separate strokes that travel down the same path.
The interval between them
varies from 0·0005 to 0·5 second. Each separate stroke starts as a downward leader from
the cloud.
(b) It has been found that 87% of all lightning strokes result from negatively charged clouds
and only 13% originate from positively charged clouds.
(c) It has been estimated that throughout the world, there occur about 100 lightning strokes per
second.
(d) Lightning discharge may have currents in the range of 10 kA to 90 kA.