In its simplest terms, lightning is a big spark that occurs whenever the potential gradient between two regions in the atmosphere exceeds the breakdown strength of the intervening space. Lightning can be classified using the pair of regions that terminate the spark, i.e. cloud-to-air, cloud-to-ionosphere, cloud-to-cloud, etc. The most common types of strokes are the intracloud and the cloud-to-ground or return stroke; intracloud strokes occur approximately twice as often as return strokes. (Many feel that the intracloud events do not differ significantly from cloud-to-ground events in their basic processes. That is, both involve leaders followed by return strokes. However, for simplicity in all the following, we will use the term return stroke to mean only cloud-to-ground return strokes.)