God’s throne in heaven, like Sinai, flashes with lightning, crashes with thunder (Revelation 4:5). Those throne phenomena are repeated at crucial seams in the book of Revelation, with additions.
After the seventh seal, an angel throws fire to earth, which reproduces the heavenly throne on earth (8:5)thunder, sounds, lightning, with the appropriate addition of an earthquake.
After the seventh trumpet, the same phenomena appear in heaven (11:19), this time with a hailstorm added.
The scene comes to a climax after the seventh bowl (16:18-21). but this time the earthquake is unprecedented and the hailstones are 100 pounds apiece.
There’s an intensification of the throne phenomena here, but something more. The throne moves from heaven to earth, and as it does it shakes earth. But as it moves, it also brings hail. As James Jordan has suggested, hailstones are, cosmologically, fragments of the firmament. As the throne descends, the barrier between heaven and earth cracks and crumbles.
When it’s all said and done, the heavenly city descends. It can get all the way to earth because the firmament boundary has already been opened. By the end of Revelation, you don’t need doors in the sky to get to heaven, because the walls of heaven have been broken down and heaven has come to earth.