Summer months in the Coachella Valley means a Facebook feed filled with pic after pic of dashboard thermometers. Sure, your phone said it officially hit 115 today, but your pal Javier wants you to know that his Toyota Corolla said it was 128 outside (he should also probably get gas as his gauge is damn near empty). So why the difference in temperature? Well, it’s mostly because Javier’s car thermometer is not actually a thermometer.
As the Weather Channel notes, cars use a thermistor instead of a mercury thermometer. While the liquid inside a thermometer expands and rises when it gets hot (and contracts and falls when it gets cold), a thermistor – which is typical pretty cheap to make – measures change in electrical current when the temperature changes. to a lower value when heat is removed and car makers place them in the worst possible location for accurate readings:
The real problem is where the thermistor is located on your car. Most automakers place the thermistor on the front of the car behind the grille. This location exposes the instrument’s readings to re-radiated heat from the road surface.
If you’ve ever walked barefoot on the beach or on a blacktop on a sunny day, you likely felt the re-radiated heat directly as your feet burned on the hot surface.
Roadways, or any blacktop for that matter, are great absorbers of incoming solar radiation. They heat up quickly, raising the temperature right at the surface, so temperatures over the blacktop are higher than over grassy or shaded surfaces.
This can result in a thermistor in a car showing a temperature 10+ degrees hotter than the actual air temperature. The same thing happens with those bank “thermometers” that are always laughably off.