The Stranger

I think the heat gives a sense of detachment and numbing sensation. Meursault is quite indifferent to his mother's death and feels awkward that the women at her funeral are showing more emotion than he is. The heat is uncomfortable much like the funeral is uncomfortable: he really just wants to escape both.

I agree with Aslan. However, I'd also say that there's a connection with the explosion of light later when he kills the Arab. Like that this is a foreshadowing of his repressed emotions getting ready to crack or explode open.