In chapter 7 of The Great Gatsby, why does Fitzgerald emphasize the heat?

The conditions of the heat and the effect on the train passengers are meant to foreshadow the coming events in the hotel room. Typically, intense heat causes emotions, such as anger and passion, to boil over. The passengers on the train have varying emotional reactions to the heat. Some are irritable, while one woman seems stunned and helpless. This foreshadows Tom and Gatsby's showdown, as well as Daisy's reaction to it.

The unbearable heat on the train causes tempers to flair and passions to rise, and this will be true later in the day, as well. Heat is often used to symbolize intense emotion, such as passion.