At the brightness that was before him - From the flash - the play of the lightnings that seemed to go before him.
His thick clouds passed - or, vanished. They seemed to pass away. The light, the flash, the blaze, penetrated those clouds, and seemed to dispel, or to scatter them. The whole heavens were in a blaze, as if there were no clouds, or as if the clouds were all driven away. The reference here is to the appearance when the vivid flashes of lightning seem to penetrate and dispel the clouds, and the heavens seem to be lighted up with a universal flame.
Hail-stones - That is, hailstones followed, or fell.
And coals of fire - There seemed to be coals of fire rolling along the ground, or falling from the sky. In the corresponding place in 2 Samuel 22:13 the expression is, “Through the brightness before him were coals of fire kindled.” That is, fires were kindled by the lightning. The expression in the psalm is more terse and compact, but the reason of the change cannot be assigned.