For those of you baffled by this "no water on Coruscant, so why does it rain?" thing: You're treating Star Wars as if it's science-fiction. It's not science-fiction. Although it is set in space, the story and imagery are clearly fantasy-based mythology. From explosions in space, to lightning being shot from people's fingers, to the scientifically improbable mechanics of a lightsaber blade, the entire saga defies logic and science. The characters and locations represent certain Jungian archetypes and are not in need of a scientific explanation for their appearance. Lucas set the saga in another galaxy in another time specifically so that he wouldn't have to explain things like this.

Lucas is steeped in film lore, so the Frankenstein connection someone mentioned was probably very much a factor when designing this scene. Plus, a perfect place (visually) for the Emperor to be seen is a lightning storm.

I think a more important question would be why do they have a 1950's-style diner in Coruscant? But, the answer would be the same. It's all about how these archetypes are translated visually into the minds of the audience. There is more written about this in the book "Mythmaking", which is basically a book about the making of AOTC.

I don't see the water thing as a goof or a plot-hole, but for those who still have a problem with it, keep in mind, we're not seeing the entire planet. We've only seen the capital city in the films, a very small part of a presumably large planet. Maybe there are lakes or oceans somewhere.

--InfoDroid