Hailstones, like rain, come down from the “air” (sky), where the angel had poured the current bowl. John had earlier mentioned “flashes of lightning, rumblings, peals of thunder” (Revelation 16:18), all reactions up there to the outpouring of the bowl onto the air. But the reaction up there produced results down here: “Great hailstones…fell from heaven on people.” Not just the air (heaven, sky) and not just earth responded (Revelation 16:18), but the two come together with the air (recall the rumblings/storm) generating and dropping enormous hailstones on the earth where people dwell.
21 And great hailstones, about one hundred pounds each, fell from heaven on people; and they cursed God for the plague of the hail, because the plague was so severe.