In Chapter Nine of Lord of the Flies, Golding uses detail and imagery to describe a building thunderstorm over the island:
A steady current of heated air rose all day from the mountain and was thrust to ten thouand feet; revolving masses of gas piled up the static until the air was ready to explode.
Golding's scientific description at the beginning of Chapter Nine takes a didactic approach to the building storm, but as the boys gather on the beach, the author's tone toward the thunderstorm shifts from explanatory to dramatic and fearful. Golding uses weapon metaphors to portray the storm as dangerous and frightening; for example, the narrator compares the crack of thunder to "the blow of a gigantic whip" or uses the verb "exploded" to liken the rumbling to that of a bomb (152). Golding's use of imagery and detail portrays the storm as violent and uncertain and mirrors the tension and violence of Jack's tribal meeting and deathly dance. The storm itself becomes a dark metaphor for the building trouble of that fateful feast which will end in Simon's death.
I took it to be thunder or "heat thunder" since at the beginning of the chapter it alludes to that with the building clouds and increasing winds.