If you stand still in the rain, and it falling straight down, you'll of course get wet. But if you grab an umbrella, then that will block the rain that's falling on you. Driving in a convertible in the rain has a similar effect, where the windshield will act as an umbrella. As you drive faster, the direction the rain travels with respect to you becomes more and more horizontal. So if you're driving really fast, the rain will only come right onto the windshield. If you go slower, some of the rain can still fall into the car, and you would still get wet.
I'm not too sure on the exact details of what speed you'd have to go to remain completely dry, but since the rain is always falling down, I'd imagine you would still get a tiny bit wet while driving. Unfortunately, I've never driven in a convertible, so I haven't been able to test this out.
I have never done this so I don't know, but my guess is that you are moving at sufficient speed that raindrops are caught by the back window of the car, in much the same way that if you throw an apple into a moving train the apple will hit the back of the cabin you throw it into. It's a matter of inertia. Meanwhile, the raindrops that WOULD hit you get blocked by the windshield.
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