The most important element of weather in chapter 7 is the desciption of the HEAT of the day. Fitzgerald says the day was "broling... the warmest day of the summer." He explains that the sun was "simmering" and on the edge of "combustion" and that the ladies were "perspiring" with the "deep heat." The continual references to heat mirror the heating up of the emotions of the novel to this climatic chapter. Daisy is boldly telling Gatsby she loves him at the same time that Tom is on the phone with Mrtyle. Tom is aggravated to see the rather bold behavior and talk of Daisy in regards to Gatsby and sees how she feels about him. He demands that they all head to the city for a change of scenery in the hopes of cooling her emotions and behavior.
At the same time, the emotions of Wilson and Myrtle are heating up and Wilson will eventually have Myrtle locked up in their rooms while he prepares to leave town with her in order to cool her emotions towards "the other man."
Once they are in the cool of the hotel, the emotional temperature only soars, and Tom, Gatsby, and Daisy have their huge blow-up. Gatsby demands that Daisy say she never loved Tom, and Daisy just can do it. The whole scene is infuriating to all three of them. As tensions run about as high as they can, Daisy and Gatsby storm out into the overbearingly hot weather and get into the accident that kills Myrtle.
The actual heat of the weather that day clearly mirrors the heat and emtions of all of the main characters as the action of this chapter plays itself out.