The judgment of God is described as a theophany—an appearance of God—and riding on the clouds indicates his power. We also find the description of clouds as God's vehicle in other parts of Scripture (see Psalm 18:10–14 and Psalm 104:3). In a vision of Daniel, the Son of Man is associated with clouds (Daniel 7:13).
1 An oracle concerning Egypt. Behold, the LORD is riding on a swift cloud and comes to Egypt; and the idols of Egypt will tremble at his presence, and the heart of the Egyptians will melt within them.