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.

Fitzgerald uses weather, specifically contrasting rain, gloom, and darkness with sunshine, light, and happiness throughout the book. Five significant quotes dealing with the weather in F. Scott Fitzgerald's The Great Gatsby include Nick’s thoughts at the beginning of the book:

And so with the sunshine and the great bursts of leaves growing on the trees—just as things grow in fast movies—I had that familiar conviction that life was beginning over again with the summer.

This is an allusion to how Gatsby will attempt to restart his life to include Daisy. It is also a commentary on how the sunny day makes Nick feel positive about his own outlook on life as he embarks on his post-college adulthood.

Other significant quotes dealing with the weather occur during the scene in which Gatsby has asked Nick to arrange a tea party with Daisy. Gatsby is excited by the prospect of seeing Daisy, but the weather is ominous and forebodes the ultimately negative outcome of their renewed relationship. Fitzgerald writes,

The day agreed upon was pouring rain.

Despite the “pouring rain,” Gatsby is nervous about appearances. He wants everything to look perfect when Daisy sees his home and his grounds. Even in the rain he sends a gardener over to mow Nick’s lawn. Gatsby deludes himself into believing that the rain will stop, which implies his self-delusion about how he can steer his life to realize his dreams.

One of the papers said they thought the rain would stop about four. I think it was "The Journal."

The rain is symbolic of several things. First, as noted, it foreshadows the rain that will fall on Gatsby’s relationship with Daisy and, in fact, on Gatsby’s life as a result of his obsession with her. In more mundane terms, it also signifies that it will ruin their tea party that day. However, the rain stops briefly.

"It’s stopped raining."

"Has it?" When he realized what I was talking about, that there were twinkle-bells of sunshine in the room, he smiled like a weather man, like an ecstatic patron of recurrent light, and repeated the news to Daisy.

The news that the rain has stopped prompts joy for both Daisy and Gatsby, but it is short-lived. Finally, after Gatsby has died, Nick thinks about him in terms of rain, which is a symbol again of the rain or heartache that fell on his brief life.

Dimly I heard someone murmur, "Blessed are the dead that the rain falls on," and then the owl-eyed man said, "Amen to that," in a brave voice. We straggled down quickly through the rain to the cars.

One quote that would be worth using:

And so with the sunshine and the great bursts of leaves growing on the trees, just as things grow in fast movies, I had that familiar conviction that life was beginning over again with the summer. (Chapter I, pg. 4)

Another quote that is worth considering is when Nick goes to the Buchanans' house for the first time and a breeze is blowing through the room until Tom shuts the windows:

A breeze blew throught he room, blew curtains in at one end and out the other like pale flags, twisting them up toward the frosted wedding-cake of the ceiling, and then rippled over the wine-colored rug, making a shadow on it as wind does on the sea. (Chapter I, pg. 8)

And then on the hot day when the group goes to the hotel to escape the heat, the description of the heat segues the discussion from the weather to marriage:

As Tom took up the receiver the compressed heat exploded into sound and we were listening to the portentous chords of Mendelssohn's Wedding March from the ballroom below.

"Imagine marrying anybody in this heat!" cried Jordan dismally. (Chapter VII, pg. 127)

And one description of the weather references the day Gatsby last spent with Daisy before he left for war:

On the last afternoon before he went abroad, he sat with Daisy in his arms for a long, silent time. It was a cold fall day, with fire in the room and her cheeks flushed. (Chapter VIII, pg. 150)

And for a final quote, a description of the day on which Gatsby tells Nick that he think Daisy never loved Tom, despite what she said just the day before:

It was dawn now on Long Island and we went about opening the rest of the windows downstairs, filling the house with gray-turning, gold-turning light. The shadow of a tree fell abruptly across the dew and ghostly birds began to sing among the blue leaves. There was a slow, pleasant movement int he air, scarcely a wind, promising a cool, lovely day. (Chapter VIII, pg. 152)

Hope these help you key into a few places where Fitzgerald uses the weather to indicate the mood of the characters or events, or perhaps the tone of the day for the characters and their lives. Great question!

Fitzgerald uses the weather throughout the novel to amplify the mood of the narrative. As Nick arrives in West Egg in the first chapter, the season is late spring, moving into summer, and Nick observes that "with the sunshine and the great bursts of leaves growing on the trees . . . I had that familiar conviction that life was beginning over again with the summer." Nick sees his move as a fresh start after completing his education and war service; his intention is to become a success on Wall Street.

Gatsby's parties at his mansion next door to Nick's bungalow rage throughout the summer, with much of the action taking place outdoors on his lawn and beach. Guests swim and boat in the day and mingle outdoors at night.

On the day that Gatsby reunites with Daisy in chapter five, the weather reflects the mood. Neither Nick nor Gatsby know how the surprise reunion will turn out, and the uncertainty and heightened emotions are reflected in the way that the rain comes and goes throughout the day. The weather alternates between downpours and drizzles as Daisy and Gatsby work through their emotions of sorrow, regret, hope, and joy. Once they have reached a stable emotional state and leave Nick's bungalow for Gatsby's mansion, the skies clear briefly, and then a gentle rain begins again when Nick leaves them alone together.

In the novel's climactic chapter seven, the weather is brutally, punishingly hot and humid. Nick calls it "broiling." The tension between Gatsby and Tom Buchanan builds throughout the day as they drink gin rickeys and ale to try to cope with their barely concealed anger. When (counterintuitively) Gatsby, Daisy, Nick, Tom, and Jordan head into Manhattan and take a parlor room at the Plaza, it is so hot that Daisy insists they "open another window." The ensuing showdown between Tom and Gatsby is as superheated as the weather.

The day of Gatsby's funeral is another rainy day, deepening the sorrowful mood as Nick tries vainly to find people to come to pay their respects. He and Owl Eyes attend, and "the rain poured down his thick glasses and he took them off and wiped them to see the protecting canvas unrolled from Gatsby's grave."

And lastly, when Nick runs into Tom Buchanan in Manhattan, it is late October. Just before he leaves West Egg to return to the Midwest, Nick observes that "the big shore places were closed now." With the autumn comes Nick's decision to close the door on his experiment with life in the East.

Gatsby is so incredibly nervous about his long-awaited reunion with Daisy, and the "pouring rain" just gives him one more thing to be nervous about besides Nick's lawn, the flowers, the tea, and so on.  At 3:58, he panics, believing that Daisy is not coming (though she was invited to arrive at 4:00).  When Nick tries to reassure Gatsby, "He sat down miserably, as if [Nick] has pushed him [...]."  It seems that the rain is echoing the hopelessness that Gatsby feels; he worries that things won't be perfect, and he is anxious that Daisy won't come.  The rain both sets the mood and mimics his own: it is gloomy and depressing outside and Gatsby seems determined to be gloomy and depressed inside.  When Daisy arrives, Gatsby acts with a "strained counterfeit of perfect ease," making everything feel even more awkward and odd.  The rain seems to dampen spirits and also to foreshadow the way the relationship between Daisy and Gatsby might, finally, end.  

Chapter Five of The Great Gatsby describes the day when Gatsby is reunited with Daisy after five years of separation, and the weather reflects Gatsby's changing mood throughout this chapter in a kind of pathetic fallacy. Pathetic fallacy is a literary device in which nature is given emotional attributes, and often adds to the atmosphere or mood of the passage.

In Chapter Five on the day Daisy is supposed to come to Nick's for tea, it is "pouring rain". Gatsby is very anxious because he wants everything to be perfect for Daisy's arrival, and he also seems very distracted. Nick describes Gatsby as "pale," with "dark signs of sleeplessness beneath his eyes." This description links to the image of the cloudy sky on this rainy day.

Just before Daisy is supposed to arrive, the weather changes from rain to mist: "The rain cooled about half-past three to a damp mist, through which occasional thin drops swam like dew." Gatsby behaves impatiently, and moments before Daisy arrives, he nearly gives up and leaves. The misty atmosphere reflects his impatient mood, and he is acting cool like the weather outside: "He looked at his watch as if there was some pressing demand on his time elsewhere." In this quote, Gatsby is being cool, behaving as though he doesn't have time to wait around for Daisy to show up and doesn't even care.

Daisy arrives like a ray of sunshine on the wet day: "The exhilarating ripple of her voice was a wild tonic in the rain." A tonic is like a medicine, so this shows that having Daisy there lifted the dreary mood like a spoonful of medicine makes one feel better when one is sick.

When Nick takes Daisy into the house, Gatsby has disappeared, only to reappear again at the front door in a puddle of rain water. This constant reference to the rain maintains a mood of dreariness, and having Gatsby wet and standing in a puddle makes him less dashing than he would probably want to be on his first reunion with Daisy, adding to his discomfort and anxiety with the situation.

Gatsby almost gives up on wooing Daisy, and he tries to leave, but Nick persuades him to stay and talk to Daisy while he goes and stands under a the umbrella of a large tree for half an hour. By this time it is pouring again, which again reflects Gatsby's uncertainty, but when Nick feels he has left them alone for long enough, the sun comes out. Before Nick even goes back into his house, readers know from the change in weather that things are going well for Gatsby and Daisy. Sure enough, when Nick reenters his living room and announces that the rain has stopped, both Gatsby and Daisy are in cheerful moods that reflect the sunshine outside:

“It’s stopped raining.”

“Has it?” When he realized what I was talking about, that there were twinkle-bells of sunshine in the room, he smiled like a weather man, like an ecstatic patron of recurrent light, and repeated the news to Daisy. “What do you think of that? It’s stopped raining.”

“I’m glad, Jay.” Her throat, full of aching, grieving beauty, told only of her unexpected joy.

Fitzgerald uses atmosphere throughout the novel to reflect the mood of the characters, and it is especially apparent in this scene with the changing weather as a backdrop to Gatsby's changing emotions. 

The weather has symbolic significance at several points in The Great Gatsby. The most intense instance of this is in Fitzgerald's constant references to the heat in chapter seven. The point is made over and over again:

Hot!" said the conductor to familiar faces. "Some weather! Hot! Hot! Hot! Is it hot enough for you? Is it hot? Is it . . . ?"

My commutation ticket came back to me with a dark stain from his hand. That any one should care in this heat whose flushed lips he kissed, whose head made damp the pajama pocket over his heart!

Daisy even uses the oppressive heat as a reason or an excuse for them to make their ill-fated journey into the city:

"But it's so hot," insisted Daisy, on the verge of tears, "And everything's so confused. Let's all go to town!"

Her voice struggled on through the heat, beating against it, moulding its senselessness into forms.

Here, the heat operates and oppresses on both a literal and a symbolic level. It causes irritability and fractiousness in the characters even as it represents those feelings metaphorically.

Unlike the continuously oppressive heat in chapter seven, the rain which is almost as insistently referenced in chapter five comes and goes. It is an inauspicious beginning to Gatsby's relationship with Daisy. Nick refers to its behavior many times: pouring, slackening off, increasing, and sounding like voices. The rain finally stops when Gatsby appears. This might be merely fortuitous. According to Daisy, the weather forecast said the rain would stop anyway. However, the respite takes on an intentional, almost miraculous quality, symbolized by the complete transformation Nick sees in Gatsby.

He literally glowed; without a word or a gesture of exultation a new well-being radiated from him and filled the little room.

Before long, however, the rain resumes, stopping Gatsby from taking Daisy to see "the grounds and the swimming pool, and the hydroplane and the midsummer flowers." This respite from the rain, like Gatsby's achievement of his romantic dream, cannot last long.

In the first chapter, Nick describes the leaves coming on the trees, the sun shining, and says that he had "...that familiar conviction that life was beginning over again with the summer."  A little later in that chapter, when he enters the Buchanan house for the first time, he describes the soft breeze causing the curtains to gently billow like pale flags.  The conviction that Nick speaks of and the favorable weather at the beginning of the summer and the beginning of the story symbolizes the promise that lies ahead.  Just like there was so much promise in Jay's life.  Jay had the talent and the drive to become whatever he wanted to become.  Toward the end of the story, in chapter 7, the heat is intense and so is the tension between the characters in the story.  When Tom, Daisy, Jordan, Nick, and Jay are in the suite at the Plaza hotel, Fitzgerald dwells on the description of the extreme heat.  Tension among the characters rises with the heat until the explosion occurs that carries Daisy out of the hotel and Jay following her.  In the next chapter, the day after Myrtle was killed, the gardener mentions emptying the pool soon before the leaves fall and clog the drain.  This hint of autumn and dying leaves comes just before Jay is shot and killed while in his pool.  The weather follows the action in the story.  There is freshness and possibility at the beginning, then things heat up and explode, finally, there is death.

The weather is often reflective of the emotional tone of the narrative in The Great Gatsby.

• In Chapter One, Nick describes his..."familiar conviction that life was beginning over again with the summer," and, in a sense it does as he is introduced to the wealthy Buchanans and Jay Gatsby. When Nick approaches the Buchanan mansion, "on a warm, windy evening," he meets Daisy and Tom Buchanan and Jordan Baker, people whose miens seem artificial. There is a breeze sweeping through the Buchanan mansion as Daisy and Jordan recline upon white couches, in poses that remind one of the Muses. As the evening progresses, Nick begins to be swept away by the breezes of wealth, and he is seduced by the life of the rich and the person of Jordan Baker. 
  After Nick returns home, he pulls into his garage and notices that "the wind had blown off," the illusion is gone. But, he notices Mr. Gatsby out on his lawn under the "silver pepper of the stars." He stares at the green light at the end of Daisy's pier.
• In Chapter Four as Nick and Gatsby ride in his gorgeous car across the bridge to the city, "with the sunlight through the girders making a constant flicker upon the moving cars," Nick exults in "the mystery and the beauty of the world."
• In Chapter Five, it rains on the day that Gatsby reunites with Daisy, and their reunion is rather strained and tinged with a certain melancholy. As Nick leaves, Gatsby's shows a "faint doubt ...as to the quality of his present happiness."
• In Chapter Six, at Gatsby's party, Nick watches some of Gatsby's guests at the party and through "a pale thin ray of moonlight" between a director and his star, Nick notices the man stoop and kiss at the cheek of the actress, an act of little meaning, just as the beam of moonlight is temporal.
• In Chapter Seven, when Gatsby confronts Tom with Daisy's love for him, it is the hottest day of the summer: the weather "was broiling" and the room "was large and stifling." And, as the tension rises, the descriptions of the heat reflect this tension:

From the ballroom beneath, muffled and suffocating chords were drifting up on hot waves of air.

• Afterwards as they leave, Nick comments that "we drove on toward death through the cooling twilight."
• Also in this chapter, Gatsby stands outside the Buchanan house, keeping vigil in the night to be sure that Tom does not hurt Daisy, but he is really "watching over nothing." The night signifies the end of Daisy's relationship with Gatsby.
• In Chapter 8, there is an unreality to the dawn as the "grey turning, gold turning light" on a tree causes the shadow to make the "ghostly birds" sing among "blue leaves."
• It is the beginning of autumn in this chapter and Gatsby decides to use the pool, perhaps the retain yet some of summer, stopping time and perhaps revitalizing his relationship with Daisy. But, Wilson "nod[s] into the twilight" and kills Gatsby.