Golding's island, I would have said, is presented as a natural paradise, with extreme weather conditions, but, most significantly of all, it is regularly seen through the eyes of the boys themselves as something ominous, and rather threatening.
Early on in Chapter 1, Golding sets up the oppressive heat of the jungle:
...the jungle was a bath of heat. He was clambering heavily among the creepers and broken trunks when a bird, a vision of red and yellow, flashed upwards with a witch-like cry; and this cry was echoed by another.
"Witch-like cry", I always think, is a simile which must be meant to be heard through Ralph's ears: he is the one who makes the comparison in his head to a witch. And it has a sort of ominous, horror-movie energy about it which lays the ground for the beast to come later on.
The island, with its pink rocks, its candle-buds, and its deep lagoons is a place of magic. But there's also a sense that that magic can have some bite, some danger to it:
Some act of God— a typhoon perhaps, or the storm that had accompanied his own arrival— had banked sand inside the lagoon so that there was a long, deep pool... The water was warmer than his blood...
It's a glorious warm pool, like a hot bath. Yet the way it is presented draws our attention to its origins in a scary storm, and makes an odd comparison between water and blood. And that sense of beauty tinged with danger is, I would say, the novel's atmosphere.