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:

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