An electric discharge between cloud and earth, between clouds or between the charge centers of the same cloud is known as lightning.

  • Lightning is a huge spark and takes place when clouds are charged to such a high potential (+ve or -ve) with respect to earth or a neighboring cloud that the dielectric strength of neighboring medium (air) is destroyed. 

  • There are several theories which exist to explain how the clouds acquire charge. The most accepted one is that during the up rush of warm moist air from earth, the friction between the air and the tiny particles of water causes the 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 drops of water they contain. 

  • The charge on a cloud may become so great that it may discharge to another cloud or to earth and we call this discharge as lightning. The thunder which accompanies lightning is due to the fact that lightning suddenly heats up the air, thereby causing it to expand. 

  • The surrounding air pushes the expanded air back and forth causing the wave motion of air which we recognize as thunder.

  • When a charged cloud passes over the earth, it induces equal and opposite charge on the earth below. Figure 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 Figure. 
  •  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. 

(ii) In many cases, the leader streamer continues its journey towards 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 traveled 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 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 09005 to 05 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.

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