Positive streamer
From Wikipedia, the free encyclopedia
A positive streamer is the bit of a lightning bolt that rises from the ground before the lightning strikes, often determining the path of the cloud-to-ground lightning. It is essentially composed of a column of ionized air formed by the flow of electrons down into the ground target. Eventually, the positive streamer grows vertically upward until it meets a nearby step leader, at which point the lightning strike proper can begin along this path. A positive streamer that failed to connect to a downward step-leader can be seen in the left side of this photo: http://www.erh.noaa.gov/er/lwx/lightning/lgtng-hits-tree.jpg
