The weather is brutally, oppressively hot in chapter 7. Nick emphasizes that it is the hottest day of the year.

The extremely hot weather symbolizes the way events are heating up in the novel. Tom has suddenly discovered that Daisy is having an affair with Gatsby. It has been going on right under his nose, but it simply had never occurred to the snobbish, arrogant Tom that he would find a rival for his wife's love in a man he considers a low-class grifter. It dawns on Nick that Tom must now realize that Nick himself has known of the affair and not said a word to Tom about it.

Realizing that Gatsby is blandly assured that Daisy is going to go away with him, Tom insists they all drive to Manhattan. On the way in, he stops for "gas" (to see Myrtle), only to receive a second surprise. George tells him he has discovered Myrtle is having an affair, has locked her in their apartment, and is taking the two of them out west. He doesn't know that Tom is her lover, but the news nevertheless jolts Tom sharply. Tom, in a very short space of time, finds that has entire world, which he had thought secure, has been upended.

In Manhattan, the four main characters end up in a room in the posh Plaza hotel, which, in the days before air conditioning, is brutally hot. Here, amid the appropriately sizzling and unbearable heat, Tom and Gatsby have the showdown over Daisy that Tom wins.

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.

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 : )