Late July through early August is usually our hottest days of the year in the Tri-State, but if you're looking for an accurate reading of the temperature outside, pay no attention to your car's thermometer.
Usually on hot summer afternoons, the temperature reading you see on the dashboard is often off by 10° or more for a few reasons.
Firstly, your car does not have a built-in thermometer. Instead, it uses an instrument called a "thermistor" to measure heat. Unlike a thermometer, which expands and contracts mercury for temperature readings, a thermistor uses an electrical current to evaluate heat that is added or removed.
Under most circumstances, both instruments can be fairly accurate - but the real problem is the thermistor's location on your car.
Most automakers place the thermistor on the front of the car just behind the grille. This location exposes the instrument's readings to re-radiated heat from the road surface. The asphalt on most city roads is great for absorbing solar radiation which allows the thermistor to generate a reading exaggerated from the actual air temperature.
Here in the Storm Team 44 Weather Center, we receive our temperature readings from local airports in the area. The data is measured in a controlled location 6-and-a-half feet above a grassy surface.
But not all is lost. Your car can provide a better representation of nighttime temperatures when the added heat from the sun is gone.
Thermistors are also more accurate on a cloudy day for the same reason, and when traveling at higher speeds compared to idling in standstill traffic.