charge locations that show that a triggered discharge can The leaders in upward-initiated lightning are usually positive (Fig. 1 , category 2). Positive upward leaders show a continuous luminosity that is modulated in a fashion similar to positive downward-stepped leaders. Negative upward leaders (category 4) exhibit a stepped behavior that is similar to negative downward-stepped leaders. Positive upward leaders often enter the cloud and produce only a more or less continuous flow of current, of the order of 100 to 1000 A, at ground. In about half of the upward initiated events, however, the continuous current is followed by a sequence of dart leaders and return strokes that are similar to those following first strokes in natural CG discharges that are initiated by negative downward-moving leaders. A detailed review of the measurements that have been made prior to 1987 on upward-initiated lightning is found in [26]. Lightning that is artificially initiated, as discussed in a companion paper in this issue, begins with an upward leader from the triggering rocket.