Cloud To Ground
A common type of lightning discharge between a cloud and the earth occurs as a flow of electrons from the base of a thundershower to the surface. The base of the cloud carries a negative charge, inducing a localized positive voltage on the
Earth's surface
Fig. 4-7. Four types of lightning stroke: intracloud (A), cloud-to-ground (B), intercloud (C), and ground-to-cloud (D).
Earth's surface
Fig. 4-7. Four types of lightning stroke: intracloud (A), cloud-to-ground (B), intercloud (C), and ground-to-cloud (D).
surface beneath (at B in Fig. 4-7). The charge concentration on the ground can get so great in some locations that people's hair stands on end.
When the voltage builds up sufficiently and the air can no longer maintain effective insulation, a streamer of electrons, carrying negative charge, begins to probe outward and downward from the base of the cloud. This is called a stepped leader. The electrons seek the path of lowest resistance to the ground, in the same way that a river finds the easiest route from a mountain to the sea. There are many dead-end paths that have good conductivity for a short distance, but then terminate. As the stepped leader approaches the area of positive charge at the surface, the air dielectric breaks down completely.
When the stepped leader is about 100 to 200 m (330 to 660 ft) from the ground or the object about to be stricken, a flashover of positive charge takes place. The flashover meets the downward moving leader a short distance above the ground. This point is called the point of strike. The electrical circuit is complete; the stepped leader has generated an ionized, highly conductive path from the cloud base to the ground. A massive discharge immediately follows, beginning at the ground and progressing upward to the cloud. (Although the individual electrons travel downward, the discharge as a whole takes place in an upward direction.) This is the return stroke, and it is responsible for the visible flash. The diameter of a typical return stroke is only a few centimeters, but it is as hot as the surface of the sun. Fires can be started within the few milliseconds (ms) during which the current flows.
After the return stroke, another leader may move downward from the cloud base. Because an ionized, conducting path has already been established by the first stroke, this leader moves much faster than the original leader, and is called a dart leader. As the dart leader reaches the point of strike, there is another return stroke. A third leader may follow, causing a third return stroke. The whole process can be repeated several times.
A complete lightning stroke lasts about 20 ms if there is only one return stroke. The flash can last as long as 500 ms if 10 or 15 return strokes occur. The greater the number of return strokes, the brighter is the flash and the louder is the resulting thunder. A multiple stroke presents a much greater fire hazard than a single stroke, and is also more deadly.
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