### Q.

I've noticed something while driving, and I want to find out if it's real or just my imagination. While sitting at a stoplight during a light rain, a given amount of drops seem to hit my windshield. I turn the wipers on and off to clear them, say, every five seconds. When the light changes and I pull away, it seems as if more drops are hitting the windshield, and I need to either turn the wipers on and off more frequently or turn them on and leave them on. My questions: Are more raindrops hitting the windshield while moving? In effect, is the windshield larger as it travels through not only space, but time, making it gather more drops? Does the angle of the windshield have any effect? Am I imagining this in the first place? I have considered pasting construction paper to ping-pong paddles and running through a light drizzle, then counting the drops. Writing to you seems easier. I just need to get an answer. It's been bugging me for years.

TOM: We all carry burdens in life. Yours is a particularly heavy one. We are happy to help.

RAY: This is a very straightforward calculus problem, one that we are completely incapable of solving.

TOM: But we do know that the answer is yes, your windshield does get hit with more raindrops per second if you're moving forward.

RAY: The simplest explanation is that when you're moving, you are encountering not only the raindrops falling from right above you, but you're also intercepting some of the raindrops that would have fallen in front of you - on your hood or in front of your car.

TOM: Since you brought up ping-pong paddles, let's work with that.

RAY: Imagine you hung a bunch of ping-pong balls from the ceiling in your garage to simulate a moment of a rain storm that's been frozen in time. So you have an even distribution of ping-pong balls - maybe they'e spaced one foot apart in every dimension.

TOM: Now, if you park your car in the garage, a certain number of those ping-pong balls will be in contact with your windshield, right? Let's just say there are five of them.

RAY: If this were a rainstorm, and those ping-pong ball drops were falling at the rate of one per second, you'd add another five drops to your windshield every second - drops that are falling straight down onto your windshield from the sky. You with me so far?

TOM: But when you start to move forward, you're not only getting those five drops every second that are coming from right above you, but because of your forward motion, your windshield also is colliding with additional drops that would have fallen in front of your car had you been sitting still.

RAY: You're sort of adding a third dimension. You have the height and width of your windshield to accept whatever raindrops fall. But you're adding "depth" by plowing through additional regions of raindrops.

TOM: It might be easier to imagine if you think of an airplane instead of a car. Sitting on the tarmac, the plane would collect a certain number of raindrops on its windshield - just like your car would. But if it were flying through that rainstorm at 550 mph, it would collide with a lot more of those drops, right?

RAY: Or to take it even a step further, imagine that the plane's windshield were missing completely. In which scenario do you think you'd get wetter? Sitting in that cockpit parked on the tarmac while it rained, or sitting there as the plane flew through the storm?

TOM: I think if I get on a plane and it has no windshield, I'm getting off and taking the train.

### Q.

My car is a 2005 Hyundai Sonata. The fan just started blowing air full blast whether the switch is set to On, Off, High or Low. I pressed all the buttons and turned my car off and on several times. The heat and air conditioning still work. Help!

TOM: You need a new resistor. Or you need to resist the temptation to use your car.

RAY: A resistor is a component that creates electrical resistance. If you think of electricity being like water running through a hose, a resistor is something that constricts the hose, making it narrower and allowing less water to pass through.

TOM: The speed of your fan is controlled with resistors. Let's say your fan switch has four settings: Off, High and two middle positions.

RAY: When the fan is set on full blast, there's no resistor involved. A full 12 volts of electricity is sent to the fan motor.

TOM: If you turn down the fan switch one notch, one resistor is then engaged. That reduces the amount of electricity sent to the fan motor, which reduces the fan speed. You switch the fan to the next-lower speed, and yet another resistor is engaged. And so on, until you have the fan switch set to Off, and there's no electricity getting to the fan.

RAY: My guess is that your resistors are fried, so you're always getting a full 12 volts to the fan, and it's always running at high speed.

TOM: There's a little box under the dashboard that's called, surprisingly enough, the fan motor resistor. It's actually a set of resistors, but the part is referred to as "the resistor."

RAY: You need a new one. Or a bunch of paperweights and magnets to keep things from blowing around in your passenger compartment.

Got a question about cars? Write to Click and Clack in care of this newspaper, or email them by visiting the Car Talk website at www.cartalk.com.