How does the stormy weather in chapter 9 reflect the plot? What implication is Golding making by employing such violent descriptions of nature?
At the beginning of chapter nine, Golding describes the build up of the storm and the way that "masses of gas piled up" and a gigantic storm is clearly waiting to explode. This mirrors the point in the plot of the story as it is clear that something terrible is going to happen. Simon has a clear understanding now of what the beast is and this knowledge must be accompanied by some change. The build up of the storm mirrors the rising action towards some kind of climax.
As Ralph and Piggy go to Jack's party, the weather continues to build. Then comes the point of confrontation between Jack and Ralph about who is in charge and whether the conch has any more authority. At this point the thunder explodes violently, signaling the impending violence. The flash of lightning lights up their terrible dance of feigned violence and as the lightning and thunder become a constant backdrop, the dance too reaches a crescendo.
At this point, Simon stumbles out of the jungle and into the midst of the boys. In the middle of all the lightning and thunder and chanting, the boys lose all ability to reason and descend into madness. They beat and stab and bite Simon to death as the storm rages and crashes around them.
Only after they murder Simon does the torrential rain open up and cool things down. It acts as a signal that the rage and mania are over.
It is also worth noting that, by describing nature so violently, Golding is likely implying once again the connection between the violence and madness of the boys and the natural world. In several places in the story, Golding suggests that this connection is a natural one, that man is flawed and these sorts of actions are unavoidable when the trappings of civilization fall away.