Your car’s thermometer is probably lying to you.

It is a common trope of social media in Central Florida. Whenever it’s unusually hot or unusually cold, we post a picture of the digital readout of our car’s thermometer along with an exaggerated lamentation of how we’re having to endure the indignity of weather that is not 70 degrees and sunny.

And yet, we are victims of a grand deception. Your car thermometer is a liar. It lies. It lies to you in a lying fashion which is forged in deceit, falsehood, and alternative facts. But it’s not your car’s fault. Electronic thermometers like these have been lying to us for ages. When I was growing up back in the paleozoic era, there were no thermometers in vehicles but we did rely on the giant digital thermometer at the bank in town. And it, too, was probably lying to us back then.

So first, let’s start off with some petty semantics. Your car thermometer is not a thermometer. It is a thermistor. The word is a portmanteau (not unlike Utilidors or Innoventions or Meteottractions) derived from the words “thermal” and “resistor.” The thermal part is pretty obvious. The resistor part is all about electricity… the electrical current passing through it behaves differently depending on what the temperature is. After a predetermined amount of Science™ is applied to the whole shebang, you can use that change in behavior to determine what the temperature is. Now here’s the funny part… thermistors are more accurate than the typical mercury-based thermometer. So how does the thermometer or thermistor or whatever in your car or bank end up being so darn wrong all the time? The answer lies, of course, in that grand tenet of real estate: location, location, location.

Credit: NWS

The National Weather Service keeps its thermometers boxed away in containers that are well-ventilated, painted with at least three coats of white paint to deflect sunlight, are about four feet off the ground (usually grassy areas), and look like some sort of high-tech beehive. You car thermistor is located under the hood of (in my case) a black car that sits in direct sunlight all day. It’s probably located right near the grill of the car so it gets airflow but also possibly direct sunlight on itself. And it’s maybe a couple of feet directly off of the pavement which, itself, can be as much as 60(!) degrees hotter than the air temperature on a hot sunny day. Even those thermistors on the electronic sign at your local bank, which might be well off the ground, are still usually sitting in direct sunlight and might be on something painted a dark color and not so well ventilated.

It’s no wonder your car’s thermometer lashes out by lying to you. It’s not bad… it’s just been put in a bad position.

1 comment on “Your car’s thermometer is probably lying to you.

  1. Pingback: May was hot. Thank you for coming to my TED Talk. – Meteottractions

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