Benjamin Franklin performed his famous experiment of flying kites in thunderclouds in 1745. People used to consider lightning as an ‘Act of God’. He also proved that the lightning stroke is because of the discharge of electricity. He invented lightning rods for fixation on tall buildings and grounding for protection from lightning strokes.
Fundamentally, lightning is a manifestation of a very large electric discharge and spark. Lightning is a huge spark, which is due to the electrical discharge taking place between the clouds, within the same cloud, and between the cloud and the earth. A large number of discharges occur between or within clouds than to earth and enough discharge terminates on earth and results in serious hazards.
In an active thundercloud, the larger particles usually pass a negative charge and smaller carriers are positive. Thus the base of the thundercloud generally carries the negative charge and the upper part is positive; with the cloud being electrically neutral. Lightning discharge to the earth is usually initiated at the fringe of negative charge centers.
There are various theories explaining the phenomenon of lightning. As a result of certain atmospheric processes that take place during thunderstorms, charges are accumulated in clouds and equal and opposite charges are induced in the earth beneath. According to other theories, the positive as well as negative ions in the air attach themselves to the small dust particles and small drops of water suspended in the air and because of polarization by induction, they get charged to a certain potential under the storm conditions. Thus a cloud gets charged either positively or negatively. whenever such a cloud passes over the earth, it induces opposite charges in the earth below.
To the human eye, the lightning discharge appears as a single luminous discharge. Although at times branches of variable intensity may be absorbed which terminate in mid-air. while the luminous main channel continuous Zig-Zag path to the earth. Lightning discharges require the puncture of air between the cloud and the earth. The stroke is initiated in the region of the negative charge sector where local field intensity[(30 Kv peak/cm)]atmospheric air or [(10 Kv/cm)]in the presence of water droplets.
charge centers in the cloud. pilot streamer and stepped leader propagates earthward. outward branching of streamers to earth. Lowering of charge into space beneath the cloud.
The current in the streamer is of the order of 100A and the speed of the streamer is 0.15m/µs.
process a is almost completed. the pilot streamer is about to strike Earth. The leader steps are of the order of 50 m in length and are accomplished in about a microsecond.
Heavy return streamer discharge of negatively charged space beneath earth. This is the most dangerous condition.
once the stepped leader has made contact with the ground it is believed that a power return stroke moves very fast up towards the cloud through the already ionized path by the leader. This streamer is very intense where the current varies from 1 KA to 200 KA and the speed is about 10 % of that of light. About 55kwh of energy is dissipated in one or more strokes that make discharge.
First charge center completely discharged, development of streamers between charge centers within clouds.
This instant gives rise to lightning flash which is observed by us. There may be another cell of charges in the cloud near the neutralized charged cell. This charged cell will try to neutralize through this ionized path. This streamer is called the dart leader. Discharge between two charge centers. Dart leader propagates to ground along the original channel, dart leader is about to strike. The velocity of the dart leader is about 3% of that of light. The effect of the dart leader is much more severe than that of the return stroke.
The discharge current in the return streamer is relatively very large but as it lasts only for a few microseconds the energy contained in it is small therefore this streamer is called the cold lightning stroke while the dart leader is called the hot lightning stroke because even though current in this leader is relatively smaller but it lasts for some milliseconds and so the energy contained in it is relatively larger. Thunderclouds contain as many as 40 charged cells and a heavy lightning stroke.
fig: Generalized wave shape of the Lightning stroke
Surges caused due to lightning discharge are generally of nonoscillatory nature with a very steep wavefront and have a gradually decreasing tail. The wave is designated by
- maximum or crest value
- the front time t1 which is the time to rise to the peak value and
- the duration t2 which is the time from 0 to the point where the voltage or current has decayed to one-half its peak or crest magnitude.