Was wondering the same thing a few weeks ago as we were shooting at 600 (could no longer see the steel at 1000).
I read the "long answer" Brian referenced, and his response at the end of that thread sure seemed to make sense (although, I didn't feel like checking his math...calculus was a loooong time ago for me).
We do get 5" per hour rainstorms here in Florida, even more than that during routine summer storms (we call 'em "frog-drowners"
).
At first blush, you'd swear nothing could get through 1000 yards of that, and not hit a drop.
But as I learned a loooong time ago, numbers don't lie.