text: For cloud to ground lightning, it has to make contact with the ground in order for the current to flow.\n\nTo make a long story short, a path is created from the cloud to the ground by negative charges trying to meet with the positive charges on the ground (or building, human, whatever).  When they can finally meet, a channel is created for the negative charges to flow to the ground.  But, what we see as lightning is the the current flowing from the GROUND to the CLOUD.  So what we're seeing is light traveling upward, rather than down.\n\nBut to answer your question, lightning can't just 'strike' midair, it must have a 'connection' on the ground.  This is only for cloud-to-ground lightning, though.