Riding on a swift cloud shows God's control and power over creation (Psalm 104:3). He is coming to judge Judah's enemies (see Deuteronomy 33:26; Psalms 18:9–10; 68:4; 97:2–3).
The idols of Egypt tremble because they are weak and powerless, unable to help (see Isaiah 40:18; 45:16; 46:1).