We both pointed out that there could be ice from the bridge and the air being colder overnight, and the bridge not having warmed up to ambient air temperature yet.

The water on the bridge isn't freezing in that case, it is melting. It just hasn't melted fully yet. And the wind has no effect.

Correct. I agree completely.

As I said, the "early" temperature warning is because the road temperature may be different from the air temperature flowing past the temp sensor in the vehicle.

But I would note that if the wind speed is high, the bridge might also warm-up faster than the other road surface, e.g., a 30 mph wind at 34 degrees would likely warm-up the bridge faster than the road surface on land.

So you never know which way it could go, hence the "safety margin" in the temperature warning threshold.

Minor point, but the wind can speed-up the heating or cooling of any object, just not as much as the human wind chill factor would suggest, e.g., take two 70 degree steel rods and put them in 32 degree air, one in still air and the other in a 30 mph air flow, and the one in the 30 mph air flow will cool down faster due to more convective heat transfer. (Remember the old threesome, Convection, Conduction, and Radiation?)

The key thing is that in either case, neither rod would go below 32 degrees, one would just get there faster, while the human wind chill factor is always lower than the air temperature when the wind speed is above zero.

As an aside, Heat Transfer is important enough and complex enough that when I got my Mechanical Engineering degree many years ago, there was a five-day a week course called "Heat Transfer" everyone had to take, and it was considered to be one of the hardest courses, exceeded in difficulty only by Fluid Dynamics and Thermodynamics.

Point taken. The wind has no effect on the end temperature, only the speed at which something gets there. It can't cool below the ambient, as you said. Wind chill is often quoted in posts about battery problems or low temperature oil flow. It just doesn't come into play, unless you are walking home without a hat after your X5 wouldn't start.

I enjoyed the heat transfer courses at university. I remember an assignment to figure out the heat balance of an automotive disk brake using finite element analysis. Came in handy many years later for discussions about drilled rotors. I never minded thermodynamics. Fluid mechanics, on the other hand, was brutal. I also recall a fourth year controls course that was a little abstract.

I've only seenthis alert go off once in the 3 BMW's I've owned :nanana:

You are a better man than I... I strongly disliked mine.

My understanding is that bridges and overpasses do not have the insulating "solid earth" underneath them that would otherwise retain some heat. Thus, a warning that air temps are near freezing is prudent, since overpasses can have slick spots. These surfaces are likely to freeze first and are often pre-treated the most by road crews.

Yup. And for the same reason the bridges can warm-up sooner as well.

So basically we all agree on the same thing and have said the same point about 20 times. Awesome!

At least we aren't arguing...

...Sorry, I had to.

^ shut the hell up! You're wrong!


:D