In "All Summer in a Day," Bradbury develops a dreary tone through imagery, word choice, and conflict.

One of the primary images in the story, of course, is the endless rain. It has been raining for "thousands and thousands of days," and every single gray day melts into the next. Margot, the only child who can accurately remember the sun, stands apart from the group. She is described as a frail shadow of the child she used to be:

[She] looked as if she had been lost in the rain for years and the rain had washed out the blue from her eyes and the red from her mouth and the yellow from her hair. She was an old photograph dusted from an album, whitened away, and if she spoke at all her voice would be a ghost.

The children hate Margot for her memories and for the possibility she has of returning to Earth and a constant sun. Margot misses the brief appearance of the sun due to their cruelty. After the other children frolic in its warmth, they return to their school with "their hands at their sides, their smiles
vanishing away." All of these images create a mood of a dismal existence.

Bradbury also chooses specific verbs that contribute to this same mood. Consider the following sentences:

A thousand forests had been crushed under the rain and grown up a thousand times to be crushed again.

And once, a month ago, she had refused to shower in the school shower rooms, had clutched her hands to her ears and over her head, screaming the water mustn’t touch her head.

They surged about her...

(All bold added for emphasis.)

The repetition of these strong and negative verbs increases the sense of desolation.

The primary conflict occurs between Margot, the only child who remembers the sun, and the rest of the children. They hate her for her knowledge of a warmth and light that they have never known. The sun here can be seen as a metaphor for goodness and insight; they have never experienced these influences, and they therefore hate that which they do not know. Eventually, the children understand that Margot spoke the truth, but they have hidden her away and stolen the experience from her. Thus, even the resolution itself is somber and forlorn. Margot is robbed of the experience that she desperately longs for.

Ray Bradbury creates a leitmotif that expresses repeatedly the idea of rain with recurring phrases; this repetition generates the major atmospheric effect, or mood, of his story. It is an oppressive mood of grey anxiety and cynicism. Here is an example of the use of leitmotif:

It had been raining for seven years; thousands upon thousands of days compounded and filled from one end to the other with rain, with the drum and gush of water, with the sweet crystal fall of showers and the concussion of storms so heavy they were tidal waves come over the islands.

The monotony of this ever-present rain that has killed forests and flowers and any vegetation is rather overpowering. The effect of the grey atmosphere and unceasing rain is reflected in the children's behavior, as well. They bully the one girl who has come from Ohio and seen the sun and remembered it. To the other children, she has committed "the biggest crime of all." So, in their envy and cynical doubt of Margot's truth about the sun, the children lock her in a closet, causing Margot great anxiety. Only they get to enjoy the sun's powerful rays and joyous light and warmth. 

In her imprisonment, Margot suffers her worst oppression and anxiety as she is denied the vision of a sunny sky, a vision for which she has long been anxious; she has always remembered and yearned for it. She is also prohibited from the added satisfaction of erasing the cynicism that looms over her from other children who are skeptical of her description of the sun. Certainly, too, the behavior of these other children underscores the narrator's tone of cynicism with regard to human nature.

Bradbury uses lyrical language to convey a mood of longing and loss in this story of a Venus where the sun only emerges once every seven years. This mood is reinforced by the personality of the main character, Margot, a sensitive, melancholy little girl whose soul's sadness seems reflected in the ever present rain. The sun in this story becomes the metaphor for all our longings and desires. 

Bradbury doesn't just say it rained all the time, but describes the rain: "the sweet crystal fall of showers and the concussion of storms so heavy ... A thousand forests had been crushed." Likewise, Bradbury lingers over descriptions of the sun. It is like "gold" or a "lemon crayon," "flaming bronze" and a "warm iron." 

Bradbury repeatedly uses similes and poetic language to describe this sun and this world. Rather than hurtle us forward from event to event in this story, Bradbury encourages us, through his description, to stop and to experience being drenched in what it is like to be on this imaginary Venus. Only two things happen in terms of plot: the sun comes out and Margot, who longs so deeply to see it, is locked away in a closet by the other children. The rest is the longing mood Bradbury evokes.