# Rain detecting windshield wipers

**URL:** <https://boards.straightdope.com/t/rain-detecting-windshield-wipers/209583>\
**Category:** Factual Questions\
**Created:** [October 26, 2003, 12:12am UTC](https://boards.straightdope.com/t/rain-detecting-windshield-wipers/209583 "2003-10-26T00:12:46Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![Ringo](https://avatars.discourse-cdn.com/v4/letter/r/779978/32.png) [@Ringo](https://boards.straightdope.com/u/Ringo)\
**Post date:** [October 26, 2003, 12:12am UTC](https://boards.straightdope.com/t/rain-detecting-windshield-wipers/209583/1 "2003-10-26T00:12:46Z")

</div>

Today it rained in Houston, but I had to get about and I discovered something I didn’t know about my car. When the rain started as a sprinkle, I put the wipers on intermittent.

Suddenly the rain picked up, and so did the wipers, to regular speed. When it became intense, the wipers ramped up to match. Then, when I pulled up to a covered ATM, they stopped.

When I pulled out again, into a light drizzle, they started up on intermittent again.

Nifty.

So, how do they work?

---

<div class="post-metadata">

**Author:** ![Nanoda](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Nanoda](https://boards.straightdope.com/u/Nanoda)\
**Post date:** [October 26, 2003, 12:40am UTC](https://boards.straightdope.com/t/rain-detecting-windshield-wipers/209583/2 "2003-10-26T00:40:37Z")

</div>

Knowing what kind of car you’re driving would help. Ones I’ve made were two closely spaced wires, but I bet most car rain detectors work like [this](http://www.i-car.com/html_pages/about_icar/current_events_news/advantage/advantage_online_archives/2003/050503.html). Check your rear-view mirror for wires.

---

<div class="post-metadata">

**Author:** ![DarrenS](https://avatars.discourse-cdn.com/v4/letter/d/977dab/32.png) [@DarrenS](https://boards.straightdope.com/u/DarrenS)\
**Post date:** [October 26, 2003, 12:40am UTC](https://boards.straightdope.com/t/rain-detecting-windshield-wipers/209583/3 "2003-10-26T00:40:58Z")

</div>

Apparently, [Infra-red light](http://auto.howstuffworks.com/wiper4.htm).

---

<div class="post-metadata">

**Author:** ![Ringo](https://avatars.discourse-cdn.com/v4/letter/r/779978/32.png) [@Ringo](https://boards.straightdope.com/u/Ringo)\
**Post date:** [October 26, 2003, 1:46pm UTC](https://boards.straightdope.com/t/rain-detecting-windshield-wipers/209583/4 "2003-10-26T13:46:59Z")

</div>

I hope I never have to replace my rear-view mirror. Upon inspection, it (BMW) appears to use the infra-red sensor system.

For our non-clickers, from **DarrenS** ’s link above:

> [@](#):
>
> The sensor projects infrared light into the windshield at a 45-degree angle. If the glass is dry, most of this light is reflected back into the sensor by the front of the windshield. If water droplets are on the glass, they reflect the light in different directions – the wetter the glass, the less light makes it back into the sensor.
> 
> The electronics and software in the sensor turn on the wipers when the amount of light reflected onto the sensor decreases to a preset level. The software sets the speed of the wipers based on how fast the moisture builds up between wipes. It can operate the wipers at any speed. The system adjusts the speed as often as necessary to match with the rate of moisture accumulation.

_Très_ nifty.

I’m guessing, **Nanoda** , that your two wire system compares rates of deflection of the (exposed?}wires. Is that even close?