Your car thermometer is wrong! Yes that is true. You might be thinking ,”Wait, huh, what? Scott, I know you are a meteorologist but I just paid and will be paying thousands of dollars for my new fancy car that has all the bells and whistles and you are telling me my car thermometer isn’t working??” Yes that is exactly what I am saying. No matter if you have a fancy Bently, an Audi, a BMW, Toyota or another make of car, every car thermometer has the same component as a really fancy, schmancy weather station.
That fancy, schmancy component is a thermistor. A therma-what? A thermistor is another way to say a temperature sensor. They are highly accurate sensors that produce electricity based on ambient temperature. You may be thinking, “Whew! I knew my car thermometer was accurate.” Well, the big problem with car thermometers, bank thermometers and billboard thermometers is that the environment surrounding the temperature sensor affects the temperature reading. This is where we get the problem with our car thermometer reporting really hot temps.
The National Weather Service goes through an extensive process of finding the proper place for weather stations to avoid re-radiated heat. The sensors are solar-shielded and are located at least ten feet away from trees, fences, buildings, concrete and asphalt. The weather stations are placed mainly on grassy surfaces or dirt, on a post five feet above the ground. This way the National Weather Service has a set standard and then a set constant for the housing and location of a weather station. This then helps improve accuracy of all the weather reports.
Car thermometers aren’t in this type of environment and therefore can’t follow the National Weather Service standard. Neither does your bank sign and those new, modern billboards.
Granted a car is a tough environment for a temperature sensor. The engineers designing the layout have to be concerned about engine heat, re-radiated heat from the asphalt, concrete, tires, etc. As far as billboards and bank thermometers, a lot of the sensors are mounted on roofs. You know that when you are up on a roof it is hot simply from the heat coming back off of the roof surface. Imagine what that heat does to the temperature reading.
You car thermometer will never be an accurate measure of the actual air temperature because of the environmental challenges but instead will “be in the range” of the temperature. The thermometer however will be accurate for its environment but that is as far as you can take that reading. So on days where your car is reading 110 degrees or even higher just know the temperature you are taking a picture of and posting to facebook is influenced by the concrete, the road you are driving on, the car itself and many other environmental factors. I’ll give you on cooler days the temperature will be off by maybe just a few degrees from the fancy, schmancy, weather stations sensors but on days where there is a lot of re-radiated heat don’t trust your temperature reading even if you are driving a shiny, new fancy car with all the bells and whistles.