Luke 9:34

And while he said these things, there came a cloud and overshadowed them: and they feared as they entered into the cloud.

A cloud ... As Dummelow said:

(This was) the visible glory, which according to Jewish ideas, manifested the divine presence. It is the same as the pillar of cloud and fire in the wilderness, the cloud that filled Solomon's temple, and the visible glory, which according to the rabbis, rested upon the ark, and was called the "Shechinah."[6]

It was certainly no ordinary cloud, which explains the fear of the apostles.