Exodus 19:11, 18, 20

and be ready by the third day, because on the third day the LORD will come down on Mount Sinai [in the cloud] in the sight of all the people.

Mount Sinai was wrapped in smoke because the LORD descended upon it in fire; its smoke ascended like the smoke of a furnace, and the whole mountain quaked violently.

The LORD came down on Mount Sinai, to the top of the mountain; and the LORD called Moses to the top of the mountain, and he went up.