From Glossary of Meteorology
On an annual basis, negative charge is lowered to the ground in about 95% of the flashes. The remaining flashes lower positive charge to the ground. This type of lightning flash, which can be contrasted with an intracloud flash or cloud flash, consists of one or more return strokes. The first stroke begins with a stepped leader followed by an intense return stroke that is the principal source of luminosity and charge transfer. Subsequent strokes begin with a dart leader followed by another return stroke. Most of the strokes use the same channel to ground. The time interval between strokes is typically 40 μs.