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.