The mechanism for charge separation within the cloud is not completely understood, but appears to involve the imparting of a negative charge on falling large ice or graupel particles as they collide with and impart a positive charge on convectively rising, small ice and/or water droplets. This process tends to accumulate negative charge at cloud base and positive charge at cloud top. Within the cloud, however, there can be regions having charge of either polarity. Intracloud strokes are then simply strokes between some oppositely charged pair of regions within a cloud, and so occur in random directions. In contrast, return strokes occur whenever the breakdown gradient between some point in the cloud and a point on the earth below is exceeded. Most of the strokes from cloud base, with its usual negative polarity, transfer negative charge to the ground and are termed negative return (NR) strokes. Similarly, those from some positive region of the cloud, often from the extended anvil top of a large cumulonimbus cloud, transfer positive charge to ground and are termed positive return (PR) strokes. The latter type usually contributes only about 10% of all return strokes.