Most cars have a display for outside temperature, and I have been wondering how it was measured...
In other words, where exactly is the sensor and how reliable is it?
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Generally, the outside temperature sensor is located under the front of the hood near the bottom of the car. Although the sensor itself is accurate, the location causes the sensor to pick up heat from the road surface. Thus, it will usually read several degrees higher than the air temperature. This is especially noticeable when driving on high speed highways for which the road surface not only picks up heat from the sun but from car tires.
It varies a lot, but in general it is placed somewhere where it will get as accurate a reading as possible - avoiding engine heat etc.
In most of mine it has been low down behind the front bumper, but I did have one car that had the sensor below the driver's door.
Misplacement may lead to the effect @Barry pointed out, but in any case, many are calibratable, and the real uses for them are to indicate when you are approaching extremely hot or cold conditions (which could require a change in behaviour or fluids)
A proper thermometer setup is shielded from sunlight, several metres away from any heat source (eg: concrete, car engine, metal/glass that's been sitting in the sun), and a raised a couple of metres off the ground.
Nothing in a car will ever be accurate, no matter how hard they try. There are just too many variables, especially when you think about sitting in traffic with engine heat radiating out the bottom of the car.
You should assume the reading is out by at least 5°F, and possibly 10°F or even more. Usually the real temperature will be lower than the reading but I wouldn't rely on that.
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