A tempest is a tropical storm with major rain and high winds. You might have heard of these storms by a different name: a hurricane. Hurricanes are common in the Caribbean, the setting of The Cay. The hurricane in the novel is not just a natural phenomenon, though. Sure it's a storm, but it's also a metaphor that represents the violence of the natural world. We learn that nature can be pretty terrible, battering away at the island and the people on it.
Rain was now slashing into the hut, and the wind was reaching a steady howl. The crash of the surf was sounding closer; I wondered if it was already beginning to push up toward our hill. (15.9)
Notice how verbs such as "slashing" are used to describe the tempest, emphasizing its violent nature. Phillip is sheltered from the brutal storm by Timothy's body, while Timothy gets sliced and diced:
Timothy had been cut to ribbons by the wind, which drove the rain and tiny grains of sand before it. It had flayed his back and legs until there were very few places that weren't cut. (15.39)
Timothy selflessly gives his life to protect another person in the face of unstoppable violence. Who else in the book sacrifices like Timothy does? (Hint: The book is set during World War II when the world was at war with Nazi Germany.)