Commentary Par XXIII 40-45

The third simile of the canto compares the swelling lightning bolt, escaping from the cloud that can no longer contain it, and falling, against its nature, downward, to Dante's mind, swelling with its rapt vision of Christ, escaping from its 'container,' and becoming other than it had been.

Dante's meteorology (for this phenomenon Steiner [DDP Steiner.Par.XXIII.40-42] cites Albertus Magnus [Meteor. I.iv.7) held that lightning resulted when contention between fiery and aquatic elements within a cloud resulted in the fiery part becoming too large and bursting the edges of the acqueous envelope, as it were.  Theorists of the phenomenon were hard pressed to explain why this excess of fire should, only in this instance, fall downward rather than follow its natural inclination up.