It’s May, the coldest month of the year, so it’s the perfect time to talk about a cold-weather car phenomenon I bet we’ve all wondered about! Well, that’s not entirely true, I suppose. It’s hot, and I can’t remember when I last saw anything resembling ice on the roads, but that didn’t stop Autopian reader Geir from asking the important question: why the hell do cars that have icy road warnings ding their little warning chimes at 37° to 40° F, when we all know damn well that water freezes at 32°? What’s going on here? Let’s find out.
First, you know what I’m talking about, right? On most cars made since, oh, 2000 or so, there’s a little light on the dashboard that will come on, usually accompanied by some sort of audio cue like a ding or beep or chime or, if you’re lucky, gong, and usually the light or icon will be in the shape of a little snowflake. The temperature that this light may come on tends to vary somewhere in the high 30° to 40° zone, with the most common temperature seeming to be 37°. As you may have guessed, people have been wondering about why this particular temperature was picked for quite a while. Here’s a nice lady who works for Mini talking about just this thing:
I like the sassy way she ends the video by saying “and that’s icy warning,” too. This video suggests that 37° was chosen because that’s the temperature that ice can form on bridges and other road surfaces. This explanation sort of makes sense, because it is absolutely true that bridges do ice before roads, as everyone’s favorite Joint Toll Bridge Commission, the Delaware one, reminds us:
Why do bridges freeze before a road? The reason bridges freeze before other surfaces is basic physics. Four factors contribute to rapid icing conditions on bridges:
• Exposure of the structure to air from below and above
• The absence of soil that provides an insulating effect on non-bridge road surfaces
• The tendency of bridges to be situated over cold spots like rivers and deep ravines
• The use of construction materials like steel and concrete that do not retain heat When freezing winds pass over and below a bridge, the structure loses heat from every side.
Bridges inherently lack the ability to trap any heat, so they will freeze shortly after atmospheric temperatures hit the freezing point. In contrast, most roads are made of asphalt, a material less-prone to heat loss. Roadways also can take advantage of the insulating and warming effects from the soil below them.
Now, this doesn’t mean that water somehow, magically knows it’s on a bridge and decides to ice up five degrees sooner for shits and, where applicable, giggles. Water doesn’t work that way. But, if the car’s temperature sensor hits a number like 37°, then it’s likely that the temperature could be less in different places, or surfaces may have lower temperatures from wind chill or evaporative cooling, and a buffer of five degrees is just a prudent precaution.
I reached out to Kia, who chimes their snowflake warning on the upper side of the spectrum, at 40°, and the buffer zone/abundance of caution explanation was confirmed:
Spoke to our Product Planning department, and “Bridge freezes before roadway” is a perfect example. Basically, it’s exercising an abundance of caution for varying / dynamic weather factors and roadway situations.
So, really, there’s nothing magical about 37° or 40° or anything like that–the reason the warning is not right at the actual freezing temperature of water is because the world is an imprecise place, and it makes sense to give a little wiggle room and prepare for freezing temperatures and icy roads before they actually are encountered. That’s the whole point of a warning after, all, isn’t it? Better to be made aware that ice is likely to happen before it actually does than given no warning at all.
This all makes a lot of sense, really. In hindsight, I wonder why I was ever so baffled! Well, me and Geir.
The 2005 VUE my family had didn’t have any kind of info center, but it did have the factory auto-dimming rearview mirror with compass and temp display. Those microwave oven digits would blink between that info and “ICE” at the appropriate temp.
My mum’s 2009 Volkswagen Polo (German version) has the same warning when the temperature drops down to 4°C. Really annoying feature when the roads are as dry as Sahara desert all day and night long.
Am I the only one who is really tired of the phrase “abundance of caution” ?
It’s right up there with “rate of speed” and “years of age”, eh?
Now that we discussed notifications on the low end are there ones for the high end of the temp scale?
Let me tell you a story about high temps. A few years back on a family trip I decided “Hey family lets quickly stop by Death Valley” in the middle of the summer. We were driving close by so sure lets do it. Well as we got closer, told everyone look at the outside temp display and as we got closer it started going up 95,100, 110, 115.. Then I said look its only 122 degrees F, shouldn’t be that bad and after reaching that temp it just stayed there. Well we pull into the visitor center and it’s showing 127, so yeah the Toyota outside temp gauge does have a limit. The other thing I wanted to mention was that I never heard cars scream in agony as we walked through the visitor center parking lot, every cars radiator fans were spinning at 100% and it was loud.
Pretty useless. Idiots asking this question arent in the snow belt. If you are in say PA. Up on the hill open field 37 degrees fine. Quarter mile up the road you go down hill with treetops and snowmelt at the bottom of the hill black ice. Tbis is why this warning is useless. It doesnt really know but the driver does.
Mini Driver is right, bridges can freeze before 32 degrees. Best to err on the side of caution.
Lots of people park in garages also. If it’s 37 in your garage there’s a decent chance it’s below freezing outside.
Air temperature means nothing. All that matters in the road icing equation is the pavement temperature, which can be quite a bit higher or lower than the air temperature.
Or road temp 38 over a bridge falls to frozen. Signs say Bridge freezes before road.
There is also a bit of a difference between the actual temperature outside and what your car’s exterior thermometer is detecting. And, as experience has taught me, black ice can stay around for quite a while after the temperature has warmed up in the morning. A curve that stays in the shade can have a nice slick patch when the rest of the road is already dry.
I was under the impression that concrete has a high thermal mass and thus retains heat very well.
It can also retain cold, air temp of 40 does not mean a ground temp of 40.
Excellent on top of a mtn wind chill can take 38 degrees to 0 degrees an freeze the road. Mini Driver would be a great srx companion after slip sliding away but freeze warning is useless.
“Wind chill” doesn’t make water freeze if the temp is above freezing. It just makes objects (including the road, and flesh) get to air temp faster.
Right, so the temp was below freezing overnight, but by the time you get going the sun has just come up and the air has struggled it’s way up to 37F. Guess what the temperature of the ground/concrete (with high thermal mass) is? It’s still below freezing.