External air temperature reading inaccurate

Here's thread Outside temperature reading incorrect. | PriusChat

 

i've had the same on all three of my prius, none with flaps. somedays, i can be driving for over half an hour, and it will read in the 80's on a 94 degree day.

 

This article explains it well

 

but the article says it will read hotter than actual temp on hot days

 

Depends on the actual situation but most of our travel these days is on BLACK asphalt and if the sun is shining the temp above the road surface can be 20 degrees hotter than the "official" temp......or more.

Every car I've ever owned with an outside temp display can't be trusted.
It IS useful in the winter time though to note when it gets around 35F and you can expect icing on the road.

 

None of my cars have had an accurate thermometer.

For an ICE car. An EV doesn't throw off as much waste heat to skew the sensor.

 

How does the reported outdoor air temp compare to the IAT (Intake Air temperature)? It wasn't on your chart.

 

IF......the sensor is properly located AND the vehicle is moving.....the "waste heat" shouldn't make any difference.
But in ALL cases, the road makes "waste heat" which is impossible not to measure.

 

None of my cars (including an Outback) have ever read accurately in the first 10-15 minutes of operation.

All of them were accurate outside of that time range. I never really paid attention to whether the errors were consistently high or low.

 

Outside air temps are measured with a simple, cheap thermistor.
They’re VERY accurate.
They’re NOT very responsive, IIRC.

Plus...how often are they being polled?
How often is the display in the cabin bring updated?

Do what everybody else does.
Look at your phone.
Those temperatures use (mostly) ASOS and NWS data which also use thermistors, but since they’re already outside then thermal inertia works both ways and you get a more or less accurate reading, depending on how far you are from the sensor.

 

How does a weather app know the temperature inside a particular garage with any reasonable degree of accuracy?

In my environment, I'm not the least bit surprised to see that much actual thermometer difference between my garage and the nearest major official weather station that would be reporting to the apps.

 

IIRC my phone displays the local ASOS (airport) data.
Rest assured, it’s “fairly” (very) accurate but air temperatures are going to vary quite a bit even in the same small town because the source of much of the Earth’s heat is pretty far away - and that heat is being inequality distributed.

 

Which is what I actually meant by my cars not being accurate. Beyond that time, I just use the display to see shifts in temperature.

The car's measurement of fuel consumed is also accurate. The computer knows fuel flow rates and time injectors are open. So it knows how much is used, even if it doesn't display it.

Cars are very poor platforms for measuring out side temperatures.
From How to Measure Temperature Correctly | WeatherWorks
1. Place the thermometer 5 feet above the ground (+/- 1 ft.). A thermometer too low will pick up excess heat from the ground and a thermometer too high will likely have too cool of a temperature due to natural cooling aloft. 5 ft. is just right.

2. The thermometer must be placed in the shade. If you put your thermometer in full sunlight, direct radiation from the sun is going to result in a temperature higher than what it should be.

3. Have good air flow for your thermometer. This keeps air circulating around the thermometer, maintaining a balance with the surrounding environment. Therefore, it is important to make sure there are no obstructions blocking your thermometer such as trees or buildings. The more open, the better.

4. Place the thermometer over a grassy or dirt surface. Concrete and pavement attract much more heat than grass. That is why cities are often warmer compared to suburbs. It is recommended to keep the thermometer at least 100 ft. from any paved or concrete surfaces to prevent an erroneously high temperature measurement.

​

5. Keep the thermometer covered. When precipitation falls, you do not want your thermometer to get wet as that could permanently damage it. A Stevenson screen is a great place to store thermometers and other instruments as they provide cover as well as adequate ventilation. If you can’t get one, a simple solar radiation shield is adequate.

I'll add to 5 that a wet thermometer is actually measuring the temperature of the water, which could vary from the air.

A parked car's thermometer is violating three of those rules, depending on where the thermistor is mounted.

A Scangauge does, and it is a basic variable the ECM is monitoring, so the data is readily available for any other gauge type application. The Scangauge would display an 'ambient' temperature when the engine wasn't a running.

IIRC, the air intake temperature was always higher than the air's. The senor is part of the airflow sensor, which is mounted downstream of the engine air filter box, and can be sitting over the engine itself.

 

the only time i see a difference in asphalt temp is on a busy highway, and the later in the day, the hotter. it comes down fairly quickly after getting off.

i don't notice much difference on sunny side roads.