I recently solved a question related to the problem below, but am having trouble getting an intuition for the problem. Suppose an automobile is traveling at a constant horizontal velocity, u, and it's raining. There is no wind, so the raindrops do not have an initial horizontal velocity, just a vertical one. However, when the rain reaches the window, it is "given" a horizontal velocity. This horizontal velocity, as measured relative to a point on the ground, is the same as that of the automobile's. However, if we take our reference frame to be a point on the automobile, then the horizontal velocity of the rain should be zero, no? Thus the rain would only have a vertical velocity, and should not appear to be slanted from the perspective of someone sitting inside the car. So essentially, my question is: Why does vertically falling rain make slanted streaks on the side of a window?