Have you ever been confused about the readout on your car thermometer? Many people see their dashboard readout record super high temperatures, especially during the summer here in Dallas, Texas, and then get surprised when the local news stations report a figure several degrees lower.
Surely that can’t be right? Well, sorry to say, but your car thermometer is not accurate. Whether it’s ultra-hot or freezing cold, it’s likely that the figure on your dashboard doesn’t match up with the real temperature outside. But why is that?
Your Car Thermometer Is Not Actually a Thermometer
First things first, your car thermometer isn’t actually a thermometer at all — it’s a thermistor. Whereas a thermometer measures the temperature by using a liquid that expands and contracts, like mercury, a thermistor is a little bit different. It tells the temperature by measuring the changes in electrical current caused by heat.
In most cars, the thermistor is located toward the front of the car, usually somewhere near the grille. Thermistors tend to be positioned lower down, and this is the main reason why they can give inaccurate readings. As anyone who has ever walked along a beach barefoot on a sunny day can attest, the ground absorbs heat much more easily than the air and therefore gets warmer than the ambient temperature. Since the thermistor is positioned close to the ground, it’s measuring this higher temperature.
This issue is made worse by the fact that most roads are colored black. Black is the color that best absorbs heat, so the thermistor is tricked by the higher temperature of the asphalt.
The Car Temperature Is Likely To Be Different From the Outside Temperature
Another reason why car thermometers aren’t always right is that they are also affected by the surrounding temperature of the car itself. Being placed under the hood of the car, they will pick up some of the temperature of the engine, and there’s no way of factoring out these extra degrees. That means when you’ve been driving for a while and the engine has started to heat up, the thermometer is always liable to be slightly off, regardless of the outside temperature.
At nighttime, it’s a different story. With less heat radiating from the ground, your car thermometer will more likely to give you an accurate reading. The same goes for when there’s a decent amount of cloud coverage or when you’re driving fast, since the radiating heat will have less time to actually reach the thermistor before you’re away.
So, while your car thermometer will give you a fairly accurate readout, you shouldn’t accept it as the gospel truth. In the summer Dallas heat, it’s likely to be off by at least a couple of degrees, so take the reading with a grain of salt. Until engineers come up with a whole new way of measuring the temperature, you’ll just have to accept that weather stations have your car beat when it comes to recording the Dallas heat.