If blocking precipitation was implemented, checking each raindrop's position individually as it's moving would be a terrible way to do it. The next step up would be checking how far a raindrop has to fall as it's created, and set a lifetime for it or something. There's probably some clever way to generate some data structure for these lifetimes using the bricks, so you effectively get a function mapping any set of x-y coordinates to how long a raindrop spawned there has to fall, but I don't know of one off the top of my head.