CHARLESTON, S.C. (WCBD) – The thermometer outside is reading 100° or higher in some back yards, but when you start the car it might read a completely different number.
The traditional thermometer uses liquid mercury to measure the temperatures outside. The mercury expands or contracts based on the heat surrounding it. A thermistor, which is found inside our cars, measures the temperature-based change in electrical current. While thermistors are normally considered to be accurate, the location on your car results in an inaccurate reading.
Most cars place the thermistor on the front of the car behind the grille. This placement would expose the reading to re-radiation from the surface of the road.
When meteorologists measure temperatures using a thermometer, that reading is taken out of direct sunlight, normally in a grassy area, and at about waist height. Roadway and blacktop heat up much quicker than the atmosphere around us, so readings from the car can be 10 degrees warmer than the actual air temperature. Bank – or digital signs – are also located on the blacktop, which is why they as well read warmer than the actual air temperature.
The car reading can be as much as 20 degrees off during the day, but at night or during a cloudy day that reading will be a bit more accurate because the sun is not heating the blacktop or the car itself.