Sometimes outside car thermometers are so inaccurate that they feel like random number generators. They're basically the worst feature of the car, next to the car's infotainment systems. That's because they're not actually thermometers, they're thermistors.
What's the difference? Unlike a thermometer, which measures heat, a thermistor measures the changes in electrical current after heat is added or removed. Thermometers and thermistors do really similar things, so this isn't why your car is so bad at measuring outside temperatures (but we're still appalled and kind of upset that it isn't a thermometer).
The real reason is the thermistor's placement. Most thermistors are at the front of the car behind the car's grille (which is normally between the two headlights).
This makes the reading a lot less accurate, especially on hot, sunny days, because it also picks up on the heat radiated from the road. Measurements are most accurate when you're travelling at quick speeds and at times when the sun isn't hitting the road, such as at night and during cloudy weather.
Your car thermistor can still be helpful in measuring below-freezing or freezing temperatures during cold weather, but it's important to note that it isn't that precise. You can still use it to look at changing temperatures in different areas, but we recommend having a good weather app in the meantime.