The section of the book that this question is referencing begins by telling readers that the date is Friday, August 21. In most editions of this book, the storm sequence is in chapter 35; however, the Project Gutenberg edition that you can read online has the storm chapter being chapter 32.
The second paragraph describes to readers that the weather is becoming quite ominous due to the atmosphere being "charged with vapours" and filled with electricity. The descriptions that follow tell readers about cumulus clouds, so it would seem that the storm is realistically nothing more than a thunderstorm that generates large amounts of lightning, thunder, and high winds.
As the men struggle on the raft, they are bombarded by high winds and large waves, but the narrator also tells readers that the amount of electricity in this particular storm is not normal. Readers are told that the electrical storm has so much electricity in it that the narrator's hair "bristles" when he stands on an insulated stool. The men's bodies carry enough charge that they worry about shocking each other if they touch.
The men attempt to ride out the storm, but their efforts are futile. The chapter ends on a cliffhanger by telling readers that the men hear the sound of the water crashing on rocks. The next chapter tells readers that the raft was smashed and that Hans heroically got the men to a shelter that might somewhat resemble a cave. I would hesitate to call it a cave, however, because readers are told that the men were sheltered from the rain by a few "overhanging rocks."