The cloud is the opposite of the abyss. (Prov. 8,28). It's greatness is a sign of the power of God.
The Lord reveals himself without making himself known completely: the cloud manifests the divine glory by covering it. God assumes the form of a cloud, symbol of isolation and solitude when He meets Moses on Mount Sinai. There is a relation between silence-cloud-God (cf. Ex.24,15-18; 33,9; Psalm 99,7).
His tent is a dense cloud (Psalm 18,12); Yahweh's chariot is made up of clouds (Psalm 104,3).
The cloud is a sign of the presence of God in a known or unknown moment.
In Gn.9,13 Yahweh's theophany is accompanied by a cloud; the same thing happens in Ex.19,9.
The cloud accompanies the journey of the Jews across the desert (Ex.13,21f; 14,19f), showing that God is constantly ahead of his people.
On Ascension day, a cloud shields Jesus from the apostles (Ac.1,9).