This long-exposure photograph (left) shows a triggered lightning event. The initial copper wire burn glows green, while nine subsequent return strokes are more purplish. Southwest Research Institute scientists plotted acoustic data (right) measured at the array that clearly show the unique signatures of the nine return strokes (RS) associated with the triggered lightning event. The 'curved' appearance of the RS signatures is associated with sound speed propagation effects. A secondary acoustic signature after the first RS (b) is the result of an electric current pulse associated with the return stroke.