Albert Barnes Commentary Psalms 78:14

"In the day-time also he led them with a cloud, And all the night with a light of fire." — Psalms 78:14 (ASV)

In the day-time also he led them with a cloud — That is, the cloud was the visible symbol of his presence, and its movements determined the way in which they were to go. It was God who led them, and who adopted this manner of doing it, so that they had always with them, by day and by night, a visible proof of his presence. There was that with them which could not be ascribed to any natural causes, and which, therefore, demonstrated that God was with them, and that as long as they followed the cloud and the pillar of fire they could not err. See Exodus 13:21; Exodus 14:24. They had the less excuse, therefore, for rebelling against him.

And all the night with a light of fire — A column—a pillar—which stood over the camp, and which was a symbol of the divine presence and guidance. The cloud would not be visible by night, nor would the fire be a good guide by day; and hence, the form of the symbol was changed. The same thing, however, was intended by both, and together they were standing proofs of the presence of God.