Perhaps more than facial expressions, the sun is an apt indicator (and perhaps, predicting device for us, much like Punxsutawney Phil). However, also like Punxsutawney Phil, these predictions are vague and hard-to-read. Depending on its intensity, the sun either makes Meursault sleepy, angry, happy, or resentful. Or Dopey. Or Sneezy. For a guy with a limited range of emotions to begin with, this is quite extensive. It’s almost as though Meursault is using the sun as an excuse to justify every feeling he has. And the murder he commits.
So let’s take a look at this murder bit. Just as Meursault is about to turn around, to leave the beach altogether, we hear this line: "But the whole beach, throbbing in the sun, was pressing on my back." "But," he says. He would have left, but the sun was too intense. The sun "[makes him] move forward" toward the spring (and therefore, toward the Arab).
What kind of guy lets the weather dictate his actions? As we’ve seen many times before, Meursault is a "path of least resistance" kind of guy. He’s also mentioned that his "physical needs often [get] in the way of [his] feelings." We see these both at play here; it’s easier for Meursault to step towards the cool water and away from the sun, and his feelings of apprehension (probably about the impending showdown at high noon) are inhibited by his physical need to cool off.
It’s also perfectly reasonable to claim that Meursault is like an element of nature himself. After all, he claims at the end of the text that he’s found a kinship with world – that it is so much like himself, a "brother," really. Additionally, if all living beings are made equal by death (which Meursault argues at the end of The Stranger), then he is just a creature of the world himself; it makes sense, then, that he’s subject to his physical surroundings. We shouldn’t think of him as any sort of higher level being – just as an animal with physical needs, pains, and desires.