I agree with the previous post. The constant presence of the rain is the main feature of the setting that Bradbury emphasizes throughout the early parts of the story.
Bradbury successfully grabs readers' attention as the story begins. He uses a quick dialogue exchange between the students to begin the story.
"Do the scientists really know? Will it happen today, will it?"
The single-word question-and-answer format allows readers to quickly read those lines, and that creates a sense of excitement and curiosity for readers. We are instantly drawn into the mysterious moment that the kids are waiting for. Then we are told that they are waiting to see the sun. That seems like such a huge letdown. What could be so exciting about seeing the sun? Then Bradbury drops a daunting detail on readers.
It had been raining for seven years . . .
Wait. What? Seven years of constant rain? The rest of that paragraph then drills into a reader's mind just how oppressive seven years of rain would be.
. . . 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. A thousand forests had been crushed under the rain and grown up a thousand times to be crushed again.
Bradbury continues to provide readers with details about how persistent the rain is on Venus. He tells readers that all of the children in the story are nine years old, and none of the children can even remember a day without rain. None of the children (except Margot) remembers what the sun looks like.
They were all nine years old, and if there had been a day, seven years ago, when the sun came out for an hour and showed its face to the stunned world, they could not recall.
The only experience that most of the children have with the sun is the stories that they read in class about what sunlight looks like and feels like.
What's great about this story is how Bradbury takes the common phenomena of sun and rain and uses them to make a foreign location feel so much more foreign. Placing this story on Venus is foreign-feeling, but readers could have a hard time relating to what life on another planet might be like. But taking away from readers something as simple as the sun, and replacing it with endless rain, makes Venus completely relatable and completely foreign at the same time. Without the seven years of rain, the setting is just some distant hypothetical. The rain makes experiencing the setting much more concrete for readers.