Or HaChaim on Exodus 14:19/אור החיים על שמות י״ד:י״ט

1 ויסע מלאך האלוקים, The angel of the Lord travelled, etc. This verse is in perfect agreement with what we explained on 13,21 that the Israelites enjoyed the presence of three distinct clouds which accompanied them and served their specific needs. 1) The cloud which travelled ahead of them to serve as a moving pathfinder indicating the direction the Israelites were to take. 2) The cloud which protected them from the heat of the sun. 3) the pillar of fire which lit up the way for them at night. In our verse G'd informs us of what He did on that particular night (of the 21st of Nissan). The angel of the Lord referred to in our verse as "travelling" was the cloud which normally travelled ahead of the Israelites showing them the way and which is described in Deut. 1,33 as travelling in front of the Israelites. In this instance this cloud moved behind them. The Torah also informs us that the cloud whose daytime function it was to protect the Israelites from the sun's heat also now took up its position behind the camp of the Israelites. In other words the two clouds normally in front of the Israelites moved behind them. The reason the second cloud was needed in that position was in order to darken the atmosphere in front of the Egyptians so that they could not know exactly where the Israelites were encamped.; this is why the Torah says: "the cloud remained (in its position) and (the other cloud) lit up the night. Seeing that the camp of the Israelites was brightly illuminated by the pillar of fire, the other cloud had to blot out that light from in front of the Egyptians. [The term "angel of the Lord" is used to tell us that the angel assumed the form of a cloud, etc. Ed.]