|MadSci Network: Earth Sciences|
The processes involved in a lightning strike are pretty much the same whether the lightning travels from a cloud-to-ground, cloud-to-air or cloud-to-cloud. The lightning strike is a result of opposite electrical charges "meeting" and discharging. Different parts of a cloud will have different charge, setting up the possiblity of positive charge from one cloud being attracted to negative charge in another cloud or even in the clear air. You can read about lightning and lightning strikes at the National Weather Services Storm Prediction Center and at USATODAY.
Try the links in the MadSci Library for more information on Earth Sciences.