Jamieson, Fausset and Brown on Job 36:30

30. light--lightning.

it--His tabernacle (Job 36:29). The light, in an instant spread over the vast mass of dark clouds, forms a striking picture.

spread--is repeated from Job 36:29 to form an antithesis. "He spreads not only clouds, but light."

covereth the bottom--roots.

of the sea--namely, with the light. In the storm the depths of ocean are laid bare; and the light "covers" them, at the same moment that it "spreads" across the dark sky. So in Psa 18:14, 15, the discovering of "the channels of waters" follows the "lightnings." Umbreit translates: "He spreadeth His light upon Himself, and covereth Himself with the roots of the sea" (Psa 104:2). God's garment is woven of celestial light and of the watery depths, raised to the sky to form His cloudy canopy. The phrase, "cover Himself with the roots of the sea," is harsh; but the image is grand.