In Fitzgerald's stylish hands, heat functions to shape plot and test character. His acute recognition of the role of atmosphere in both furthering conflict and testing character is illustrated by his unwavering use of detail from first to final draft. From the beginning of these scenes to the end, we are made to feel the relentless heat as clearly as we see the green leather seats in Gatsby yellow car. Fitzgerald's revision adds more than degrees to the hot day. Heat serves to parallel the acceleration of conflict between Gatsby and Tom.
Heat gives their conflict a further sense of inevitability.
Fitzgerald does not miss his many opportunities to remind us that the heat of the moment is testing his characters, wearing away the outer veneer they wear so well, and revealing them as they struggle in a hot situation. Man versus nature while man versus man. The weather takes its toll on character's moods. It is a small detail. Its significance is that so many small details like this one were included that our perception of the scene and the characters is altered.