מדרש אויף שמות 40:36

(Ibid. 21) "And the L–rd went before them by day with a pillar of cloud": We find there to have been seven clouds: here, (Numbers 14:14) twice, (Ibid. 9:19), (Exodus 40:36), (Ibid. 37), (Ibid. 38) — Seven clouds: four on four sides, one above, one below, and one going before them, lifting what was low, and lowering what was high, as it is written (Isaiah 40:4) "Let every valley be raised, and every mountain and hill be lowered. Let the rugged ground become level and the ridges become a plain." And it would kill the snakes and scorpions before them and sweep and sprinkle before them.

(Ibid. 21) "And the L–rd went before them by day with a pillar of cloud": We find there to have been seven clouds: here, (Numbers 14:14) twice, (Ibid. 9:19), (Exodus 40:36), (Ibid. 37), (Ibid. 38) — Seven clouds: four on four sides, one above, one below, and one going before them, lifting what was low, and lowering what was high, as it is written (Isaiah 40:4) "Let every valley be raised, and every mountain and hill be lowered. Let the rugged ground become level and the ridges become a plain." And it would kill the snakes and scorpions before them and sweep and sprinkle before them.

And the Lord went before them by day (Exod. 13:21). You find that there were seven clouds of glory in all. And the Lord went before them by day in a pillar of cloud (ibid.) was the first; and Thy cloud standeth over them (Num. 14:14) was the second; In a pillar of cloud (ibid.) was the third; That the cloud tarried (Num. 9:22), the fourth; And the going up of the cloud (Exod. 40:36), the fifth; And whenever the cloud was taken up (ibid., 37), the sixth; and For the cloud of the Lord was upon the Tabernacle (ibid., v. 38) was the seventh. Four of the clouds were at each of the four sides; one was above them, another below them, and the seventh cloud preceded them. The last of the clouds lowered the high places, raised the low places, burnt the serpents and the scorpions, and sprinkled the road (to lay the dust) before them.