Now an ionized path a few centimeters in diameter connects earth with cloud. The return stroke is highly luminous. Occasionally there are repeated strokes in the same channel, giving the appearance of flickering, but usually only the first is branched. It is possible to tell the difference between strokes with downward and upward propagating leaders. When the return stroke lights the pathways taken by the stepped leader, downward branching indicates that the stepped leader originated in the clouds.
Perspective can cause difficulties in interpretation, however.
Some observations suggest upward branching when, in reality, a portion of the flash propagated toward the observer, creating the illusion of upward branching. A small percentage of strokes, however, have been observed to lower positive charge to the ground. They have been observed in winter storms and seem to prefer strongly sheared environments, where the wind speed and direction change rapidly with height. The National Severe Storms Laboratory has documented positive strokes emanating from the forward and rear portions of thunderstorm anvils and from the main storm turret during severe thunderstorms. NSSL has also observed positive strokes in the gentle rain region behind squall lines.