If you stand motionless under an umbrella in a rainstorm where the drops fall vertically, you remain relatively dry. However, if you start running, the rain begins to hit your legs even if they remain under the umbrella. Why? Now I know the answer, it's because when you run horizontally, your legs hit the drops that are currently at your knees because they're moving perfectly vertically. But I don't know how to put this into physics terms, it just seems like common sense to me? Is it just the vectors will meet, since one is pointing horizontally and another vertically, and they will meet at the same place?