Why Your Car Thermometer Reads 110 F When the Heat Index Is 102 F
Car thermometers, asphalt, and shadeless concrete patios are not measuring the same thing the weather service measures. Here is what each is actually telling you.
Your car's thermometer is reading the temperature of the sensor, which is mounted in the front bumper or grille. After driving for any length of time in the sun, that sensor is sitting on a pile of hot metal, sometimes inches above asphalt that is 30 F hotter than the air. The number you see is biased high by anywhere from 5 to 20 F.
The NWS reports air temperature measured in shade, at 4 to 6 feet above the ground, in a ventilated screen that lets air flow freely but blocks direct sun. This standardization is why a weather station at the airport reads 98 F while your car at the same moment shows 113 F. Both are correct measurements of different things.
Asphalt surface temperature can run 40 to 60 F hotter than air temperature in full sun. At an air temperature of 90 F, an asphalt parking lot surface can hit 145 F. This matters for pets walking on it (paw burns occur in 60 seconds above 130 F surface), kids playing barefoot, and anyone working on a dark roof or sealed driveway.
Sun versus shade can swing the felt heat by 10 to 15 F. The NWS Heat Index assumes shade. Add 10 to 15 F when you are in direct sun. This is the most common reason people feel worse than the heat index number suggests, they are reading the shade number while standing in the sun.
Indoor temperature in an unconditioned space climbs more slowly than outdoor temperature but holds longer. A south-facing room with no AC and uninsulated walls can be 5 to 10 F hotter than the peak outdoor temperature that day, and stay that way 4 to 6 hours after sunset.
The cheapest accurate reading is a $10 mercury or alcohol thermometer hung in shade, with airflow, at chest height, away from any building wall (which radiates heat). For surface temperatures, an infrared thermometer (under $30) reads asphalt, sidewalk, car interiors, and roofs in a half second, the readings are eye-opening.
If you only have a car thermometer, subtract about 5 F if you have been driving more than 10 minutes with the AC on, and 10 to 15 F if you have been parked in the sun. The result will be closer to actual air temperature, though never as accurate as a weather station.