Charles Ellicott Commentary Exodus 19:18

"And mount Sinai, the whole of it, smoked, because Jehovah descended upon it in fire; and the smoke thereof ascended as the smoke of a furnace, and the whole mount quaked greatly." — Exodus 19:18 (ASV)

Mount Sinai was altogether on a smoke. —Hebrew, smoked, all of it. Some interpret this as “dense clouds, having the appearance of smoke.” But if the mountain burned with fire, as asserted (Deuteronomy 4:11), the smoke would be real.

The whole mount quaked greatly. —Compare Psalms 68:8, The earth shook, the heavens also dropped at the presence of God: even Sinai itself was moved at the presence of God. The expression is more suitable to an earthquake than to the vibration sometimes produced by very violent thunder.