How does Ray Bradbury develop the mood in "All Summer in a Day"?

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...

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.