Lightning strokes are classified into different types according to their own characteristics.  The two most common types are cloud-to-ground lightning and cloud-to-cloud lightning.  Cloud-to-ground lightning is lightning discharge between a cumulonimbus cloud and the ground.  Of all types of lightning, cloud-to-ground lightning poses the greatest threat to people and facilities on the ground.

Cloud-to-cloud lightning is the discharge between areas of cloud without the discharge channel reaching the ground.  For most of the time, it occurs between oppositely charged portions of the same cloud but sometimes it takes place between two separate clouds.  Since the discharge channel of cloud-to-cloud lightning may be obscured by the cloud, it may or may not be visible to an observer on the ground.  One may therefore be surprised to learn that cloud-to-cloud lightning is, in fact, the most frequently occurring type of lightning.  Roughly speaking, it occurs around ten times more often than cloud-to-ground lightning.

Obviously, cloud-to-cloud lightning does not pose a threat to life and property on the ground.  However, this type of lightning takes place in the sky and sometimes passes through the clear air between clouds, therefore it is of great practical interest to those concerned with the safety of aircraft.