Lightning sometimes happens between clouds. suggest a possible way that a discharge can occur between clouds

Most lightning happens inside a cloud, but sometimes it happens between the cloud and the ground. A build up of positive charge builds up on the ground beneath the cloud, attracted to the negative charge in the bottom of the cloud. The ground's positive charge concentrates around anything that sticks up-trees, lightning conductors, even people. The positive charge from the ground connects with the negative charge from the clouds and a spark of lightning strikes.