The Uncontrollable

The Stranger is all about inevitability. We assume that Meursault had no control over his "accidental murder" of another man. He cannot control his actions during the act, and is equally helpless during the trial, when his fate is being decided by more strangers.
Camus represents this helplessness by using the heat as Meursault's foremost thought and motivating factor. While we can influence behavior of other humans and manipulate objects, the weather is one thing that humans are unable to alter. In particular, heat is very common. Meursault's reaction to it shows how he is different from "normal" people who would just live with it or turn on a fan or strip or something.
When he is about to leave to shoot the arab, Meursault thinks about the extreme heat and decides that no mater what he does, it amounts to the same thing. While this could mean that he will eventually die no matter what, I think it also means that he will be hot if he goes inside of stays outside. As he is about to pull the trigger, the glare from the arab's knife intensifies the heat, pushing Mearsault over the edge. Of course, he could do nothing to prevent the heat. In this situation, the heat represents his lack of control over what happened. Later on, while at trial, Meursault again notices the heat inside the courtroom. He is just sitting there while his attorney makes his case for him, against another attorney he has never met before, to be judged by a group of people he has never seen before and finally to be sentenced by another complete stranger. (On a side note, I think that this could be an alternate meaning for the book's title. Maybe my next post will be about that.) Again, the heat marks a situation in which Meursault has no control over his future.
To conclude, I think that heat has two roles in this novel. First, it represents the uncontrollable, reinforcing the argument that Meursault was fated to kill a man. Heat also brings out the contrast between Meursault and the average human being, who will react much differently and with less passion than Meursault does. (Another side note-- isn't it interesting how the thing he cares most about in the world is the temperature? More than his girlfriend or friends or mother.)