Chapter 7 is the crucial chapter of the book in which Tom discovers that Daisy and Gatsby are having an affair, Daisy flaunts the affair in Tom's face, Gatsby tries to force Daisy to choose, and an accident happens as a result of all of the cheating. Behind the scenes in this chapter, Wilson (we find out in chapter 8) locked his wife up and she tried to escape to get to Tom when she thinks Tom is driving by their place. All of these details make for a very hot set of scenes indeed.
Fitzgerald uses the heat in the beginning of the chapter to foreshadow all of the conflict which closes the chapter. Here are some of the quotes early in the chapter that refer to the heat:
"The next day was broiling, almost the last, certainly the warmest of the summer... only the hot whistles of the National Biscuit Company broke the simmering hush at noon."
You refer to imagery specifically. Did you know that imagery can actual evoke any sensory detail? Here in this quote, the words broiling and warmest help readers experience the touch sense. Then the words simmering hush and hot whistles mix sight and sound senses together.
"The straw seats of the car hovered on the edge of combustion; the woman next to me perspired delicately for a while into her white shirtwaist, and then, as her newspaper dampened under her fingers, lapsed despairingly into deep heat with a desolate cry."
Fitzgerald is using the effects of the heat to portray a mood. This mood is one that feels insurmountable, full of despair, and difficult to move through.
These beautifully developed images indirectly cause the reader to experience the vivacious reality of living in heat.
As the scene moves to the Buchanans' house, the heat is referred to in character dialogue, narration, and character description. The butler cracks a joke about moving a dead body in the heat. The women do not want to move because of the heat. The request for cold drinks occurs. The characters are referred to with the words glistening and dampened to describe their skin and clothing. The appearance of the water even sounds hot.
A final worthwhile note is the fact that the heat is exactly what led them to decide to go to the city. The city excursion causes the downfall that occurs in this chapter.
Read enotes analysis in the link below for further commentary.
Heat is used as a device to create the atmosphere at the climax of the feelings of the main characters. Heat imagery is used all through the chapter up to the moment after Myrtle's death when the evening gets cooler. The images are visual, auditory and of the senses and help to create the feeling of opression and irresponsibility that that is permanent in the chapter.
When the weather gets colder at night, Gatsby is left "watching over nothing", meaning that his dream is over as the American Dream is over, too.-