What causes lightning during a thunder storm?

Lightning storms are created in humid weather conditions because the static electricity cannot dissipate and builds up in the air until it is finally released in the form of lightning.

Lightning is produced in thunderstorms when liquid and ice particles above the freezing level collide, and build up large separated electrical fields in the clouds. Once these electric fields become powerful enough to overcome the resistance of the air, a giant "spark" occurs between them (as with static electricity) reducing the charge separation. The lightning spark, or bolt, can occur between clouds, between a cloud and the air, or (more importantly) between a cloud and the ground.