John Calvin Commentary Psalms 18:12

"At the brightness before him his thick clouds passed, Hailstones and coals of fire." — Psalms 18:12 (ASV)

At the brightness, etc. The Psalmist again returns to the lightnings which, by dividing and, as it were, cleaving the clouds, lay open the heaven; and, therefore, he says that the clouds of God (that is to say, those which he had set before him as a token of his anger, to deprive people of the enjoyment of the light of his countenance) passed away at the brightness which was before him. These sudden changes affect us with a much more lively sense of the power and agency of God than natural phenomena which move on in one uniform course. He adds that hail-storm and coals of fire followed; for when the thunder separates and rends asunder the clouds, it either breaks out in lightnings, or the clouds resolve themselves into hail.