While most lightning is initiated by downward leaders, either negative or less frequently positive, upward propagating discharges, both negative and positive, are possible, but rare.
Upward discharges almost always occur from towers, tall buildings, or mountain tops. In addition, they are thought to occur only when there is a rapid change in the charges aloft, most likely due to a very recent lightning strike. Upward-developing discharges are usually branched upward.
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