h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I´ve been wondering about one thing for some time now, maybe someone here can help me.
My question is: Is the windshield of a car driving in constant rain hit by more raindrops per time as if it were standing? Does it depend on the angle of the windshield? Does it depend on the angle of the falling raindrops? My physics teacher couldn´t tell me, maybe you can

i think that a car windshield will get hit by more rain drops when driving than when sitting still.
when you are sitting still you can clear your windshield with the intermittent cycle but when driving in the same conditions you have to have your wipers on constantly and the faster you go the quicker you need the wipers to go to keep the windshield clear. Does that make sense???

Makes sense to me, Slim.

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Makes sense to me, Slim.

Anyway, welcome to the Guide from one of the Assistant Community Editors (ACEs) - you are already finding out there's lots to do here - places to go, people to meet etc.

If you have any problems/queries etc, feel free to contact either me directly or any of the ACEs - you can leave a note at the ACEs home page.

Have fun

Yes, more rain does hit the windscreen when you're moving. Consider the two extremes:

If the windscreen is horizontal (that is, perpendicular to the rain), having it moving doesn't make any difference at all; you're moving the target area but since it's raining over a large area this doesn't have any effect.

If the windscreen is vertical (parallel to the rain), however, no rain at all will hit it if it's not moving. If it is moving, the rain no longer appears (from the windscreen's point of view) to be moving parallel to its surface, and a large amount will hit.

In between these two extremes you'll find the real answer: rain does hit a non-moving windscreen, but it also hits a moving one a fair bit more.

The situation is analogous to changing the direction of the rain, so that it's hitting the windscreen directly on as opposed to at an angle. This leads to the windscreen presenting a larger target from the point of view of the rain, and therefore it gets hit a fair bit more.

You might ask what speed you should drive at in order to arrive at your destination as dry as possible; this would take quite a bit more answering, I think...

26199

Ah! I've found the actual original 'Windshields in the rain' forum - I first saw your question in the one about dollars and pounds - don't ask me how it got there, I suspect the h2g2 data may be a little flaky..

Let's start with a stationary car:
The number of raindrops hitting the windshield increases, as the rain gets more perpendicular to the glass. In other words, if the wind is blowing the rain so that it falls at an angle that is at 90 degrees to the windshield, you have the maximum number of drops hitting, and if it is blown parallel to the glass, hardly any actually hit (although you will obviously see smears, because the wind is constantly changing, etc. etc.) The function desribing how the rate of raindrops varies according to the angle will probably be something based on a cosine curve.
Now for a moving car:
This isn't any different from the stationary example; however, you have to subtract the car's velocity from that of the wind to get the effective wind velocity, then use the same calculation as above. Note that there is also a 'shadow' effect; if the car is driving slowly enough and the wind is blowing strongly from behind, then the effective strike angle becomes negative, and the rain all falls on the roof, so you won't get any on the windshield.
To answer the 'is it better to be moving' question, you would need information about the downward velocity of the rain and the direction and speed of the wind. I remember seeing a similar problem answered once about whether one should walk or run through a rain shower to get the least wet. I seem to remember that the answer was 'It depends', so this is actually quite a complex little problem!
I think I might go and work out all the formulae now. Would you like me to post them if I finish?

Sounds good to me. I was planning on writing a computer simulation, but I haven't got around to it yet...

26199

Aah, thank you very much, guys!! I think your answers quite helped opening up my eyes to this problem! Whew, I can´t believe that this question could stir you up so much! On your behalf, 26199, if your computer simulation is finished, tell me about the result! (as long as you´re not considering the wipers )