Near the beginning of the chapter, Fitzgerald incorporates heat imagery to mirror the explosive situation that Nick is about to witness. As Nick rides the train, he comments that the
straw seats of the car hovered on the edge of combustion (120).
The image lends itself well to the situation with Gatsby, Daisy, and Tom. It is just a matter of time before someone "explodes."
Likewise, as Nick, Gatsby, and Tom linger at the Buchanan household, Fitzgerald contrasts Tom's stifling, overheated personality to Gatsby's effortlessly "cool" one. He is "astounded" when Daisy praises Gatsby right in front of him and immediately suggests going into the city (125).
The heat has made Daisy and Tom irritable with each other, and the change of setting to the confining nature of the hotel room does not help. In the end, the broiling nature of not only the day but also of the overheated room imitate Tom's rising, uncontrollable temper and drive Gatsby to a confrontation when he otherwise might have remained calm.