In chapter 7 of The Great Gatsby, how does Fitzgerald use weather as a backdrop?

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.

In The Great Gatsby, how does Fitzgerald use weather to reflect the story's mood?

The extreme heat in chapter seven really seems to both reflect and add to the tension felt by the characters, and this certainly affects the mood of the book in similar ways as well. Daisy, for example, lacks perspective on the heat, wondering what they will "'do with [them]selves this afternoon . . . and the day after that, and the next thirty years.'" Jordan points out that "'Life starts all over again when it gets crisp in the fall,'" clearly connected the weather to the tense mood in the room.

Next, Gatsby picks up his drink and speaks with "visible tension" at the Buchanans's house, and they all "drank down nervous gayety" at lunch. At the same time, Daisy is "on the verge of tears" when she cries out that "'it's so hot.'" She also declares that "'It's too hot to fuss'" when Tom bickers with her over cigarettes. The tension rises higher and higher, with the temperature, as the group sets out for the city. Tom has realized that Daisy is cheating on him with Gatsby, and he is angry and resentful. Everyone is "irritable" from the heat (and from their alcohol, which they drank profusely to cut the tension, wearing off), and Tom drives "impatiently." The room they rent at the Plaza is "stifling" and Tom becomes "insulting" toward Gatsby as the tension rises between Tom and Daisy, who tells her husband that she'll make him a drink so that he "'won't seem so stupid to [him]self.'" The oppressive heat that makes everyone so uncomfortable heightens the tension and affects the mood all throughout the chapter.

Fitzgerald uses the weather to intensify the mood in pivotal scenes in the novel. One notable example appears in chapter five on the day of Gatsby's reunion with Daisy. The weather is uncertain: rainy as Nick and Gatsby make the preparations, drizzly as Daisy arrives and she and Gatsby make their awkward re-acquaintance, and still unsettled when Nick leaves them together at Gatsby's house where "the rain was still falling, but the darkness had parted in the west."

When the tension ratchets up in chapter seven in the hours leading up to the showdown between Tom and Gatsby, the weather is described as "broiling" hot. Nick describes the hotel room where the climactic scene occurs as "large and stifling" and offering little respite from the heat they've been struggling with all day.

On the day of Gatsby's funeral in chapter nine, it rains. Nick describes raindrops "splashing... over the soggy ground," the owl-eyed man wiping his glasses "outside and in," and someone at the funeral murmuring, "Blessed are the dead that the rain falls on."

How does Fitzgerald use weather to set the mood in Gatsby's meeting with Daisy?

In Chapter five, Nick has "arranged" for Daisy to come to his house for tea, so that Gatsby can meet her. Gatsby is incredibly nervous. The weather -- on the appointed day, it is pouring rain -- is meant to suggest Gatsby's inner state, of course, but also the inappropriateness of the rain -- on this of all days -- underlines something about the unreality of Gatsby's dreams about Daisy, something that becomes more uncomfortably clear as the chapter continues and  Gatsby shows off his house to Daisy. Here are a few details:

• Nick's lawn: Gatsby is afraid Nick's lawn is not well-kept enough for Daisy. Even though it is pouring rain, he sends a man over to cut it. It is a bit absurd. Later, when asked about how the lawn looks, it is as if he had forgotten about the whole thing!
• When Daisy shows up, Gatsby, out of embarassment, has gone out the back door and pretends to arrive after Daisy does. When Nick opens the door to him, “Gatsby, pale as death, with his hands plunged like weights in his coat pockets, was standing in a puddle of water glaring tragically into my eyes.” 
• Nick so fed up with Gatsy's behavior he tells him he is acting like a child and goes outside and stands under a tree to be out of the rain. After half an hour, the sun comes out, and he goes back into the house to find Gatsby changed: “He literally glowed; without a word or a gesture of exultation a new well-being radiated from him and filled the little room.” 
• Later, when Gatsby is showing Daisy around his house, Daisy looks out the window: “Come here quick!” cried Daisy at the window. The rain was still falling, but the darkness had parted in the west, and there was a pink and golden billow of foamy clouds above the sea. “Look at that,” she whispered, and then after a moment: “I’d like to just get one of those pink clouds and put you in it and push you around.” 

We never find out what happens between Gatsby and Daisy while Nick is out in the rain. But Daisy's enthusiasm for the clouds is concurrent with the general feeling of irrational happiness at the end the chapter. The weather can be seen to represent Gatsby's mood (rain means sad, sun means happy), but in another sense the weather has nothing to do with how Gatsby feels, and its apparent "symbolic" value can be seen as another attempt to assign meaning to things that in themselves have no significance. Of course what Daisy says about the clouds makes no sense, but that doesn't matter to Gatsby. As Fitzgerald says of him, “No amount of fire or freshness can challenge what a man will store up in his ghostly heart.” 

How does weather and setting description at the end of The Great Gatsby produce tone and mood?

After meeting Tom for the last time, Nick goes back to his house and notices that Gatsby's grass is as long as his own. Gatsby's yard and his mansion used to be well kept. Gatsby had tried to create a magical, glorious world around himself. This was part of his allure, his way of impressing Daisy. Now that the grass is long and Gatsby is gone, it shows that the allure is gone too. The image is depressing to Nick. 

During his last night at the house, Nick notices a curse word on Gatsby's steps. It is another reminder of the glory being gone. Nick erases the word. Nick gazes upon the water and imagines what the land looked like before all the houses and buildings were there. He says the houses "melt away until gradually I became aware of the old island here that flowered once for Dutch sailors’ eyes—a fresh, green breast of the new world." He is thinking of how North America (the new world) was once a tabula rasa in the eyes of European explorers. In other words, it was a blank canvas, a glorious natural world that seemed to evoke promise and dreams. In contrast, modern America offers the same promise, but the American Dream is difficult if not impossible for some. Gatsby's dream eventually failed. 

Nick associates this fresh, green, natural world full of promise with the green light at the end of Daisy's dock. Here, the color green suggests vibrant life, the flowering of plants and trees. "And as I sat there brooding on the old, unknown world, I thought of Gatsby’s wonder when he first picked out the green light at the end of Daisy’s dock." The mood is solemn, even depressing. Gatsby's green light represented the hope and potential of a dream. The dream ended in tragedy. The fresh, green land of old world America represented the "American Dream" to explorers and later, to immigrants. Like Gatsby, many of those immigrants did not see their dreams realized. This notion of dreams unrealized is part of Fitzgerald's critique of the American Dream. To those explorers who first came to America, the great green landscape looked full of promise. Fitzgerald questions if America has lived up to that promise.