The weather during the this chapter is extremely hot and stifling. It is uncomfortable to be outside and everyone is looking for some relief. Here are a few quotes from chapter 7 to illustrate this:
"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."
"The straw seats of the car hovered on the edge of combustion."
“Hot!” said the conductor to familiar faces. “Some weather! hot! hot! hot! Is it hot enough for you? Is it hot? Is it . . . ?”
"The room was large and stifling, and, though it was already four o’clock, opening the windows admitted Only a gust of hot shrubbery from the Park."
As you know, a symbol is an object (or sometimes a person) that represents a larger idea. In this case, the brutal, oppressive, hot weather is symbolic of the atmosphere between the characters. Tom and Gatsby are edgy and ready for a confrontation. It is like the heat wave that is broken by a sudden storm. At least that's one interpretation of it : )
It has been observed over the ages that the hotter the weather the shorter the temper. Heat-or intense heat-seems to bring out the worst in some people. Homicide rates -- in fact, crime rates in general -- are scientifically known to increase in the summer, as the discomfort associated with hotter temperatures is directly correlated to violent behavior. And so it is in Chapter 7 of F. Scott Fitzgerald's The Great Gatsby. The intrigues that have revolved around Gatsby and the Buchanans, as well as between Gatsby and the organized crime with which he is associated, are literally and figuratively reaching their boiling points.
Nick Carraway, Fitzgerald's narrator, notes early in this chapter that the weather has turned particularly hot: "The next day was broiling, almost the last, certainly the warmest, of the summer." The discomfort associated with this intense heat is reaffirmed throughout the day. As Nick rides the train, the conductor, repeats, "Some weather! Hot! Hot! Hot! Is it hot enough for you? Is it hot? Is it … ?" Fitzgerald's purpose in emphasizing the heat is to further establish the setting for the events that will occur. Tom and Daisy Buchanan, both now engaged in illicit liaisons, the latter with the titular figure of the novel, have invited Gatsby and Nick to their estate in East Egg. The tension is thick enough to cut with a knife, as it is presumed that each of the characters has some inkling as to what has been going on between and around them. Fitzgerald sets the stage for the type of 'parlor games' that prefigure climactic developments yet to come. Soon after arriving at the Buchanan's mansion, Nick and Gatsby are greeted by Tom as described in the following passage:
"Tom flung open the door, blocked out its space for a moment with his thick body, and hurried into the room. ‘Mr. Gatsby!’ He put out his broad, flat hand with well concealed dislike. ‘I’m glad to see you, sir…. Nick….’"
Tom and Gatsby appear headed for a major confrontation, and lurking in the background is the latter's relationship to Meyer Wolfsheim and the underworld figures with whom he associates. The intense heat of the day prefigures the tensions that permeate the scene in Chapter Seven.