Exod. 19:16   On the morning of the third day there was thunder and lightning, as well as a thick cloud on the mountain, and a blast of a trumpet so loud that all the people who were in the camp trembled. 

Exod. 24:15   Then Moses went up on the mountain, and the cloud covered the mountain. 

Exod. 24:16   The glory of the Lord settled on Mount Sinai, and the cloud covered it for six days; on the seventh day he called to Moses out of the cloud. 

Exod. 24:18   Moses entered the cloud, and went up on the mountain. Moses was on the mountain for forty days and forty nights.