The ancient’s understanding of what lay beyond the dome of the sky was limited; it was certainly where heaven was and in addition a cloud was frequently met in the Old Testament (Exodus, Daniel for example) as a sign of God’s presence. With all respect to my Walsingham friends whose chapel of the Ascension has feet poking down from a ceiling, I do not really find this imagery helpful. Even Luke, writing in Acts does not really dwell on it - he is more concerned it seems to me with what is being said.
The question uppermost in the disciples’ minds is “What next?” Having seen and fully understood that Jesus is raised from the dead they want to know if he would now go on to fulfill the expected purpose of the Messiah, to restore the kingdom to Israel. The answer they are given is “It is not for you to know when this might take place and your job rather than asking these questions is to be my witnesses in the world, in fact to all the ends of the earth.” And when he had said this as they were watching he was lifted up and a cloud took him out of their sight. The disciples witnessed a change from Jesus’ physical presence with them to a spiritual one. They certainly felt that, Luke and doubtless others sought to explain this transformation in terms of things they knew using language that their hearers and readers would understand.
I am comfortable with that. It is a very human thing to do. I was looking recently at an old Japanese print that was seeking to explain the existence of air. The print contained a picture of fish in a bowl. The artist and scientists of those times had no concept of the composition of air, which came very much later, but they understood that fish inhabited a medium and were able to describe our place in an invisible emptiness as being something like this - something they could see and touch, like fish in water.
The transformation that happened there on Mount Olivet is quite beyond us and I am happy that it is: Paul writing in 1 Timothy 3:16 puts the untouchable ethereal nature of Jesus well:
He was revealed in flesh
Vindicated in Spirit
Seen by angels
Proclaimed among the Gentiles
Believed in throughout the world
And taken up in glory.