Sunday, 10 July 2016

Understanding Lightning Phenomena


Lightning is an electric discharge in the form of a spark or flash originating in a charged cloud. Lightning is a very long electrical spark, ‘very long meaning greater than about 1 kilometer. Most lightning is generated in thunderstorms and is characterized by a length of 5 to 10 km, at the extreme about 100 km.



Thunder clouds are charged, and that the negative charge centre is located in the lower part of the cloud where the temperature is about – 50 °C, and that the main positive charge centre is located several kilometres higher up, where the temperature is usually below – 200 °C. In the majority of storm clouds, there is also a localized positively charged region near the base of the cloud where the temperature is 0 °C.

Figure below shows such a cloud located above an overhead transmission line.



Electric Fields of about 1000 V/m exist near the centre of a single bipolar cloud in which charges of about 20 C are separated by distances of about 3 km, and indicate the total potential difference between the main charge centres to be between 100 and 1000 MV. The energy dissipated in a lightning flash is therefore of the order of 1000 to 10,000 MJ, much of which is spent in heating up a narrow air column surrounding the discharge, the temperature rising to about 15,000 °C in a few tens of microseconds.

Vertical separation of the positive and negative charge centres is about 2 - 5 km, and the charges involved are 10 - 30 C. The average current dissipated by lightning is of the order of kilo-amperes. During an average lightning storm, a total of the order of kilo-coulombs of charge would be generated, between the 0°C and the -40 °C levels, in a volume of about 50 km3.

Under the influence of sufficiently strong fields, large water drops become elongated in the direction of the electric field and become unstable, and streamers develop at their ends with the initiation of corona discharges. Drops of radius 2 mm develop streamers if the electric field exceeds a 9 kV/cm which is much less than the 30 kV/cm required initiating the breakdown of dry air.

When the electric field in the vicinity of one of the negative charge centres builds up to the critical value of about 10 kV/cm, an ionised channel or streamer is formed, which propagates from the cloud to earth with a velocity that might be as high as one-tenth the speed of light. Usually this streamer is extinguished only at a short distance from the cloud.




Forty micro-seconds or so after the first streamer, a second streamer occurs, closely following the path of the first, and propagating the ionised channel a little further before it is also spent. This process continues a number of times, each step increasing the channel length by 10 to 200 m. Because of the step like sequence in which this streamer travels to earth, this process is termed the stepped leader stroke.

When the stepped leader has reached to within 15 to 50 m of the earth, the field intensity at earth is sufficient for an upward streamer to develop and bridge the remaining gap. A large neutralising current flows along the ionised path, produced by the stepped leader, to neutralise the charge. This current flow is termed the return stroke and may carry currents as high as 200 kA, although the average current is about 20 kA.

Following the first or main stroke and after about 40 ms, a second leader stroke propagates to earth in a continuous and rapid manner and again a return stroke follows. This second and subsequent leader strokes which travel along the already energised channel are termed Dart Leaders.

What appears as a single flash of lightning usually consist of a number of successive strokes, following the same track in space, at intervals of a few hundredths of a second. The average number of strokes in a multiple stroke is four, but may be as many as 40. The time interval between strokes ranges from 20 to 700 ms, but is most frequently 40-50 ms. The average duration of a complete flash being about 250 ms.


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