A lightning strike or lightning bolt is an electric discharge between the atmosphere and an object. They mostly originate in a cumulonimbus cloud and terminate on the ground, called cloud to ground lightning. A less common type of strike, called ground to the cloud, is upward propagating lightning initiated from a tall grounded object and reaches into the clouds. About 25% of all lightning events worldwide are struck between the atmosphere and earth-bound objects. The bulk of lightning events are intra-cloud or cloud to cloud, where discharges only occur high in the atmosphere. Lightning strikes the average commercial aircraft at least once a year, but modern engineering and design means this is rarely a problem. The movement of aircraft through clouds can even cause lightning strikes.