Cloud Charge Transfer

Uman "The usual cloud-to- ground discharge probably begins as a local discharge between the small pocket of positive charge at the base of the cloud (the p region) and the primary region of negative charge (the N region) above it. This local discharge frees electrons in the N-region that previously had been attached to water or ice particles. These electrons overrun the p-region, neutralize its small positive charge, and then continue on their trip to the ground. " This description is based upon the tripolar model of charge buildup.

The Stepped Leader

Following a charge transfer event in the lower part of the cloud, the released electrons proceed to the ground. Uman "The vehicle by which these electrons move from the cloud to the ground is called a stepped leader... it moves to the ground in rapid, luminous steps that are about fifty yards long. Each step occurs in less than one-millionth of a second, and the time between each step is about one fifty-millionth of a second. The stepped leader, moving at a velocity of about 200 miles per hour, takes about one-hundredth of a second to travel from the cloud to earth."

" Measured photographically, the stepped leader is between one and ten yards in diameter. It is thought, however, that most of the current flows down a narrow conducting core that is less than one inch in diameter, and that the large, photographically obvserved diameter is due to a luminous electrical corona surrounding the conducting core."

Upward-moving Discharges

When the stepped leader approaches the ground, carrying some five coulombs of charge, a large positive charge is induced below it and an upward-moving discharge some 30-50 meters long comes up to meet it. The position of the upward-moving discharge actually determines what ground point the lightning will hit, so lightning rods are used to initiate this discharge to offer some control over the strike. When contact is made with the stepped leader, a violent, high-current discharge travels to ground. This highly luminous discharge then travels back up the leader in the return stroke.

The Return Stroke

After the stepped leader and the upward-moving discharge initiate the first violent, luminous discharge near the Earth, "this high luminosity (and the high current) then moves up the leader channel and out its branches at somewhere between one-half and one-tenth the speed of light. This movement from ground to cloud is called the return stroke and is actually the dazzling display we recognize as lightning. The eye is not fast enough to resolve the movement of the return stroke, and so it seems as if all points of the channel become bright simultaneously. " Uman

The Dart Leader

After the return stroke, the lightning flash may be ended, but "most flashes contain three or four strokes - some as many as twenty or thirty." If enough charge is available in the cloud to produce another stroke, a continuous leader called a dart leader moves down the return stroke channel from the previous stroke, depositing negative charge along its length. "Dart leaders generally deposit less charge than stepped leaders do, and, as a result, subsequent strokes generally lower less charge to ground and have smaller currents than first strokes. " Uman