The sequence of luminous processes that are involved in a typical negative CG flash is shown in Fig. 2. The stepped leader initiates the first return stroke after it propagates downward in a series of discrete steps. The stepped leader is itself initiated by a preliminary breakdown within the cloud, although there is no agreement about the exact form and location of this process. High-speed photographs show that leader steps are typically 1 ps in duration, tens of meters in length, and that the pause time between steps is 20 to 50 ps. A fully developed stepped leader can effectively lower 10 C or more of negative charge toward the ground in tens of ms. The average downward speed of propagation is about 2 x lo5 m/s. The average leader current is between 100 and lo00 A. The leader steps have peak pulse currents of at least 1 kA. During its progression toward ground, the stepped leader produces a downward-branched geometrical structure.