The extremely high beach serve that drops back down almost vertically — forcing the receiver to look up into sun and wind.
The skyball is the serve everyone on the beach talks about and nobody ever explains. Instead of pace it trades on hang time: the ball climbs so high that it falls back almost vertically, and the receiver has to track it overhead, straight into blazing sun or wind. With a low sun it turns into a genuine weapon; under an overcast sky it is usually a gift. To be honest about the evidence: this is practical knowledge from the sand rather than research — there are no solid numbers on it. The typical receiving error is backpedalling and losing sight of the ball on the way. Better to move sideways under the flight path early, shade your eyes with a hand, and solve the position with your feet rather than by reaching. And if you are the one serving, check the wind first — on this ball it matters more than how hard you hit it.