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Why your car's outside-temperature reading may be wrong.
An interesting article from the Washington Post. (I'm not much of a fan of that publication, for a number of reasons, but they occasionally come out with some good articles).
Outside-air temperature readings, which are included in the trip-computers and/or video-screens on many newer vehicles, can often be off several degrees or more. I have noticed this myself. This article explains why.
Few industries integrate technology into their products faster than do automobile companies. One of those features, now ubiquitous in most cars, is the dashboard thermometer display. But the temperature reading on your vehicle’s dashboard is often misleading and not representative of the actual temperature.
In its most basic definition, temperature is a measurement of how fast gas molecules are moving or its average kinetic energy. Heat excites molecules, causing them to bounce around much more frequently, resulting in a higher temperature value. The most common way to measure this reaction is with a mercury thermometer, where the liquid mercury will physically expand and rise to a particular value when heat is added.
Your car is equipped not with a thermometer but with a thermistor. Thermistors work in a similar manner to thermometers, but rather than using a liquid like mercury, thermistors measure the change in electrical current as a result of heat added or taken away. Thermistors are quite convenient, since they are small, cheap to make and for the most part, accurate. And really, the problem is not that your car uses a thermistor but rather where that thermistor is placed.
Most vehicles have their thermistors on the front of the car, located behind the grill. This location makes the instrument’s measurements sensitive to reradiated heat from the road surface. If you’ve ever walked barefoot across sand or concrete that’s been exposed to direct sunlight, then you’ve experienced reradiated heat directly.
Surfaces such as roadways are great absorbers of the sun’s incoming radiation. And consequently, they heat up very quickly, creating a localized hot spot right at the surface. As you can imagine, the temperature of a road surface in the middle of a hot summer day is not a true representation of the air temperature. But the heat generated is real, and it’s this “extra” heat that is picked up by a car’s thermistor, artificially inflating the reading you see on your dashboard.
So in effect, your car’s thermistor is measuring the temperature of a very localized environment and not what we would consider the air temperature. The average or ambient temperature that we associate with your daily highs and lows is measured in a sheltered, controlled environment, to limit the effects of heating from nearby surfaces as much as possible.
Car thermistors provide a better representation of air temperature at night than day because the sun’s heat isn’t reradiating off the road surface any longer. And they’ll also be a truer reflection of air temperature in cloudy daytime conditions (vs. sunny conditions) for the same reason.
The artificial heating effect of the road is further minimized when you’re traveling at high speeds vs. stop-and-go traffic.
During winter storms, the thermistors can be a helpful gauge as to how far below freezing it is and changing conditions as you coast through various microclimates. But they are not reliable enough to accurately discern temperatures within a degree of freezing, which can be the difference between wet roads and black ice.
Despite their shortcomings, car temperature readings are fun to monitor and can give you a sense of how fast temperatures are changing. Next time we get a burst of heavy rain from a thunderstorm, take a look at how much your car temperature drops.
Here’s a nice video explainer on car thermometers by Brad Panovich, chief meteorologist for the NBC affiliate in Charlotte:
(The video won't post for some reason...you have to click on the article's link)
Rather than mount a thermistor behind the grille, many cars (including Lexus) now tap the air temperature sensor in the engine air intake (that is also used to adjust for air density when working with your mass airflow sensor to determine your fuel-air mix). It's a legitimate test point because it does affect your Engine Control Computer, and ultimately your engine power - but the error is relatively harmless - so long as you know that's not necessarily the outside air temperature (OAT) that is often a bit lower than that of the air in your intake thanks to the proximity of the engine block. That, as noted, can be dangerous in near-freezing conditions. I've experienced an indicated air temperature stuck at 114° (that probably maxed out the indicator's range) on blacktop secondary roads in the late morning in North Texas. OK, it's hot . . . but not THAT hot.
I've experienced an indicated air temperature in the 114° range on blacktop secondary roads in the late morning in North Texas. OK, it's hot . . . but not THAT hot.
When I was in Austin as a young man, I can remember the (official) air temperature at Robert Mueller Airport or Bergstrom AFB being above 100 degrees, and the road temperature hot enough to partially melt the tar-strips on Burnet Road. It was easily 114 degrees or more on the road itself. And Austin, in general, due to prevailing southeast winds off the Gulf 160 miles away, doesn't get as hot as more inland and drier places further west. Del Rio, Uvalde, Presidio, El Paso, etc..... can get like an oven, though El Paso's 4000-foot altitude helps mitigate it somewhat.
Even here in the Mid-Atlantic, inland areas east of the Appalachians sometimes see 100 degrees or more in the summer (and with more humidity than Central or Western Texas)...but not as many days, or every year, like in Texas.
Last edited by mmarshall; Jun 14, 2017 at 07:24 PM.
I've always thought car thermometers ran hot on the basis that the car is a metal box, that traps and adsorbs the solar rays, I mean get into your car on a hot day after being parked in the sun, no way that thermometer is correctly displaying the OUTSIDE air temperature. Its displaying the air temperature in some external part of your car, or maybe the air temperature under your car. Also it wouldn't surprise me if black car thermometers ran a few degrees warmer than white car thermometers in the deep south or in the desert west. I know for a fact that on a couple of cars I've owned with outside thermometers, once you get moving, IE from parked in that hot lot at work for 8 hours, then on the interstate home, that outside temp reading can drop from anywhere from 3 to 10 degrees.
Although I do have an interesting question for you, Lincolns, Cadillacs, and maybe some other luxury cars back in the 70's/early 80's had a thermometer mounted on the driver's side door mirror. How do these buggers work, and are they subjected to the same variances/inaccuracy as modern car thermometers???
Personally this is in like the top 5 of old Cadillac options that they don't offer anymore.
Those old thermometers used a bi-metallic strip wound into a coil. As temperature increased, the differential expansion rates of the two metals "uncoiled" the scroll, resulting in the indicator moving over the numbers. As I recall, the problem wasn't in the bi-metal strip (that is still popularly used in dial-type thermometers and thermostats today), but that clear plastic housing that would often discolor or warp after years of summertime sunshine.
They were pretty accurate, given the short scale dictated by the overall size of the instrument. They were handy, very cheap to manufacture, and immensely profitable for the dealership, as I recall. Sorta like "rustproofing". Anyone remember "Rusty Jones" or "Ziebart" treatments?
^ I could see that outside plastic housing cracking/yellowing if the car was stored outside. The only car I ever saw that option on was a MINT 1979 Coupe Deville, bright, blind you silver paint, burgundy 1/3rd roof, burgundy pin stripes, burgundy leather interior,, split bench seat, damn sharp car. Owner told me the silver was a factory color, but he had it kicked up a notch on the repaint, a bit brighter, a bit more metallic.
My LS430 said 106F the other day as I was leaving the office. Common sense said there's no way that is correct as it was only 91F. I decided to take a pic of it, and by the time I did, it said 102F. In this interval, the ambient temp did not drop 4F.....from observation, once a person gets moving, it would then be accurate....because down the road it said 91F....
Why your car's outside-temperature reading may be wrong.
