# External Thermometers on Cars
**URL:** https://boards.straightdope.com/t/external-thermometers-on-cars/191417
**Category:** Factual Questions
**Created:** [July 28, 2003, 12:24pm UTC](https://boards.straightdope.com/t/external-thermometers-on-cars/191417 "2003-07-28T12:24:31Z")
**Posts on this page:** 12
**Page:** 1
### Author:  [@Achilles](https://boards.straightdope.com/u/Achilles)
#### Post date: [July 28, 2003, 12:24pm UTC](https://boards.straightdope.com/t/external-thermometers-on-cars/191417/1 "2003-07-28T12:24:31Z")
How do they make a thermometer that will register the external temperature of the air, when a car is moving.
I can imagine that they would use a “dry bulb” of plastic around the thermometer, to keep it dry and to protect against wind chill. How do they prevent the air in the bulb from getting colder with evaporation ? Is there a vacuum in the bulb ?
😕
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### Author:  [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)
#### Post date: [July 28, 2003, 12:34pm UTC](https://boards.straightdope.com/t/external-thermometers-on-cars/191417/2 "2003-07-28T12:34:32Z")
“Wind chill” only affects moist opbjects, like your skin. There will normally be very little moisture adhering to the thermometer sensor (which is likely to be a thermistor or less likely a thermocouple), except after a heavy morning dew. Even then, the water will evaporate away shortly.
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### Author:  [@Achilles](https://boards.straightdope.com/u/Achilles)
#### Post date: [July 28, 2003, 12:47pm UTC](https://boards.straightdope.com/t/external-thermometers-on-cars/191417/3 "2003-07-28T12:47:36Z")
What about snow and ice ? Would they not affect the readings ?
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### Author:  [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)
#### Post date: [July 28, 2003, 12:55pm UTC](https://boards.straightdope.com/t/external-thermometers-on-cars/191417/4 "2003-07-28T12:55:42Z")
Usually the sensor isn’t exposed directly to the elements, so snow and ice aren’t normally a problem. If the sensor _did_ get covered with ice somehow, then yes, this would affect the reading somewhat. These thermometers aren’t known for their accuracy in any case, being off by as much as +/- 2 [sup]o[/sup]F.
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### Author:  [@Achilles](https://boards.straightdope.com/u/Achilles)
#### Post date: [July 28, 2003, 12:57pm UTC](https://boards.straightdope.com/t/external-thermometers-on-cars/191417/5 "2003-07-28T12:57:47Z")
Oh. I didn’t know that. I don’t have a car, and when on my bike in the winter, my nose acts as a perfect thermometer.
Thanks. What a cool board. Any question answered so quickly !!
🙂
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### Author:  [@Balthisar](https://boards.straightdope.com/u/Balthisar)
#### Post date: [July 28, 2003, 1:02pm UTC](https://boards.straightdope.com/t/external-thermometers-on-cars/191417/6 "2003-07-28T13:02:38Z")
I’d conjecture that “wind chill” only affects things that are warmer than the ambient temperature. Once your body is frozen solid to the ambient temperature, it’s not going to get colder despite how much we keep throwing water on you and letting the wind get at you. The wind chill AFAIK works by reducing the pressure around a liquid (your surface moisture), encouraging it to expand/vaporize. But to aquire this higher energy level it needs to take it from your body, where it’s plentiful compared to the environment. Once the body’s the same temperature as the environment, the further heat loss would be nil to negligible.
So, for the car thermometer, I’d guess that after having been parked overnight it’s already at ambient temperature and it likely to always be at ambient temperature (except see below). So the dew wouldn’t matter, and there’s no wind-chill to speak of.
Exception: my previous vehicle owner’s manual stated the following, but my current vehicle’s manual (different manufacturer) makes no mention of it, so it could be a different location or implementation: after the engine is completely warmed and during a brief turn-off period, the external temperature reading may indicated warmer than the actual temperature. This is because the sensor starts picking up the engine heat. The temperature will indicate correctly after a few minutes of travel.
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### Author:  [@scr4](https://boards.straightdope.com/u/scr4)
#### Post date: [July 28, 2003, 3:55pm UTC](https://boards.straightdope.com/t/external-thermometers-on-cars/191417/7 "2003-07-28T15:55:26Z")
Does “wind chill” really include evaporative cooling? I thought it was simply the convective cooling rate of an object that’s at body temperature.
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### Author:  [@McDeath\_the\_Mad](https://boards.straightdope.com/u/McDeath_the_Mad)
#### Post date: [July 28, 2003, 4:01pm UTC](https://boards.straightdope.com/t/external-thermometers-on-cars/191417/8 "2003-07-28T16:01:50Z")
Where are these sensors usually placed?
I have added my own to my pickup. The forst spot was in one of the front wheel wells, but the ambient temperature from the engine threw it off. I have then moved it to the inside the wall of my box, but it seems to get hotter as I drive, so I thing the ambient temperature from the exhaust is effecting it.
MtM
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### Author:  [@3waygeek](https://boards.straightdope.com/u/3waygeek)
#### Post date: [July 28, 2003, 4:39pm UTC](https://boards.straightdope.com/t/external-thermometers-on-cars/191417/9 "2003-07-28T16:39:51Z")
Depends on the manufacturer, **McDeath**. For example, BMW places them in a front brake duct (usually the left for left-hand drive cars). The moving air keeps engine heat from building up, and as we’ve already established, it doesn’t cause a wind chill effect.
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### Author:  [@Ender\_Will](https://boards.straightdope.com/u/Ender_Will)
#### Post date: [July 28, 2003, 5:38pm UTC](https://boards.straightdope.com/t/external-thermometers-on-cars/191417/10 "2003-07-28T17:38:34Z")
[hijack] Anyone know if I can find a picture or some such showing how to replace the thermometer on a BMW? It’s still attached to the wiring and registers the temperature just fine, but is swinging free at the moment, and I can’t find any obvious place that it goes. Off to investigate the brake duct!
[/hijack]
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [July 28, 2003, 6:06pm UTC](https://boards.straightdope.com/t/external-thermometers-on-cars/191417/11 "2003-07-28T18:06:13Z")
Am I the only one who read the thread title as “External Thermometers on **Cats** ”? Lemme tell you, I was disappointed when I opened the thread.
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### Author:  [@Walloon](https://boards.straightdope.com/u/Walloon)
#### Post date: [July 28, 2003, 6:24pm UTC](https://boards.straightdope.com/t/external-thermometers-on-cars/191417/12 "2003-07-28T18:24:59Z")
> [@](#):
>
> \*Originally posted by Balthisar \*
> \*\*I’d conjecture that “wind chill” only affects things that are warmer than the ambient temperature. Once your body is frozen solid to the ambient temperature, it’s not going to get colder despite how much we keep throwing water on you and letting the wind get at you. \*\*
No, the premise is wrong. As long as blood is coursing through your skin’s capillaries (i.e., as long as the skin is alive and not frostbitten), your skin temperature will never get anywhere near below freezing.