At his glorious and powerful appearance his thick clouds passed away, i.e. vanished, (as this word is oft taken, as Psalms 90:5,6 Isa 29:5 Habakkuk 3:10) being dissolved into showers of hail-stones, &c.
Related Commentaries of Psalms 18:12
Psalms 18:12
12 At the brightness that was before him his thick clouds passed, hail stones and coals of fire.