A short compendium of information about lightning.
A typical lightning flash lasts about a quarter of a second and consists of 3 or 4 individual discharges called strokes. Each stroke lasts a few ten thousandths of a second, although the visual appearance is longer. The "flicker" sometimes observed in lightning is due to seeing the actual strokes making up the flash.
A lightning stroke begins with a faint predischarge, called the leader, which goes from the cloud to the ground. The leader establishes a path for the highly luminous return stroke (what you really see) which propagates from the ground up to the cloud. The first stroke of a flash is usually preceded by a "stepped leader", so called because it appears to progress in discrete steps (about 100 segments, each 50 m long) from cloud to ground. The subsequent strokes are preceded by a "dart leader" which smoothly follows the path of the previous return stroke (and is about 10 times faster).
Thunder is formed from the shock wave formed by the rapid heating of the air along the path of the return stroke, which reaches some 30,000 degrees K. The sound of thunder varies depending on how far you are from the various parts of the stroke. The sound from a part of the stroke farther away will get to you later and be fainter.
The following table gives data for normal cloud to ground lightning discharges bringing negative charge to earth. Representative values are given, as well as a range of typical values. Data is taken from Uman, Lightning, which in turn takes the data from a variety of sources.
|Length of Step||50 m||3 - 200 m|
|Time interval between steps||50 uSec||30 - 125 uSec|
|Averave Velocity of propagation of stepped leader||150 km/sec||100 - 2600 km/sec|
|Charge deposited on stepped-leader channel||5 C||3 - 20 C|
|Velocity of Propagation||2000 km/sec||1000 - 21,000 km/sec|
|Charge deposited on dart-leader channel||1 C||0.2 - 6 C|
|Velocity of propagation||80,000 km/sec||20,000 - 160,000 km/sec|
|Current rate of increase||10 kA/uSec||<1 - >80 kA/uSec|
|Time to peak current||2 uSec||<1 - 30 uSec|
|Peak Current||10-20 kA||-110 kA|
|Time to half of peak current||40 uSec||10-250 uSec|
|Charge transferred (excluding continuing current)||2.5 C||0.2 - 20 C|
|Channel Length||5 km||2 - 14 km|
|Energy dissipated||100 kJ/meter|
|Number of strokes per flash||3-4||1 - 26|
|Time interval between strokes||40 msec||3 - 100 mSec|
|Time duration of flash||0.2 sec||0.01 0 2 sec|
|Charge transferred including continuing current||25 C||3 - 90 C|
The best book on lightning I've run across:
Martin A. Uman
c. 1984, Dover Publications, Inc. New York
Copyright 1997, Jim Lux / lfacts.htm / Back to Lightning / Back to home page / Mail to Jim