Albert Barnes Commentary Psalms 147:8

"Who covereth the heavens with clouds, Who prepareth rain for the earth, Who maketh grass to grow upon the mountains." — Psalms 147:8 (ASV)

Who covers the heaven with clouds - Clouds that are designed to convey refreshing rain to the earth. The reasons for praise here stated (Psalms 147:8–9) are derived from the goodness of God as exhibited in his providential arrangements for the good of man.

Who prepares rain for the earth - By causing it to be taken from the sea, carried by the clouds, and conveyed through the air to the places where it is needed, and then gently sprinkled on the earth. Compare the notes at (Psalms 104:13): He watereth the hills from his chambers. See also (Job 5:10), note; (Job 28:26), note; (Job 36:27–28), notes; (Job 38:28), note; (Job 38:37), note.

Who makes grass to grow upon the mountains - Which would be barren but for the rain. Who conveys the water thus to the very tops of the mountains, and causes it to descend on their sides, so that even the mountains are clothed with verdure and beauty. Compare the notes at (Psalms 104:14): He causeth the grass to grow for the cattle.