“Then they will see
The Son of Man
Coming in clouds
With great power
καὶ τότε ὄψονται τὸν Υἱὸν τοῦ ἀνθρώπου ἐρχόμενον ἐν νεφέλαις μετὰ δυνάμεως πολλῆς καὶ δόξης.
This is almost word for word in Matthew, chapter 24:30, and in Luke, chapter 21:27. Mark said that they would all see or experience the Son of Man (καὶ τότε ὄψονται τὸν Υἱὸν τοῦ ἀνθρώπου) coming in the clouds (ἐρχόμενον ἐν νεφέλαις) with his great power (μετὰ δυνάμεως πολλῆς) and glory (καὶ δόξης). The clouds were the common place where theophanies in the Old Testament occurred, as Yahweh often appeared in a cloud on a mountain. The Son of Man could be a reference to Jesus himself.