"The Lord preceded them, by day in a pillar of cloud to show them the way, and by night in a pillar of fire to give them light, so they could march by day and by night." Exodus 13:21
"Anyone who wanted to consult the Lord would go out to the Tent of Meeting, outside the camp. Whenever Moses went out to the Tent, the people would all stand up and every man would stand at the door of his tent and watch Moses until he went into the Tent. And whenever Moses went into the Tent, the pillar of cloud would come down and station itself at the entrance to the Tent, while the Lord spoke with Moses. The people could all see the pillar of cloud stationed at the entrance to the Tent and the people would all stand up and bow low, each at the door of his tent.
The Lord would talk to Moses face to face, as a man talks to his friend, and afterwards he would come back to the camp, but the young man who was his servant, Joshua son of Nun, never left the inside of the Tent." Exodus 33:7-11
The painting, A Pillar of Cloud by day, thus references both God’s powerful guiding presence of the Israelites in the desert, and God's continued revelation through created beauty (in this case, the beauty evident in the painting).