So if it's 35ºF on a windy day, that can be enough to cause a puddle to freeze? That is *not* my understanding of thermodynamics (but again, I could easily be mistaken).

~Neal

 

I'm simply saying the road could be colder than the air at the particular time, or, conversely, the road can have ice on it even if the air is above freezing. I am saying wind chill will not affect the road temp, because it's a measure of perception.

 

With this, I completely agree! I think lotsie's wind chill explanation for why this may come about, however, is nothing but hot air.

I know that overpasses, say, freeze first because the effect of intermal ground heat isn't there to help keep the road surface warmer than that of the ambient air about it.

[edit:]And to conclude my little side conjecture, I've found two links which validate my guess. The movement of above-freezing ambient air cannot, alone, cause an above-freezing substance to freeze. This is true in the case of frostbite on an animal's skin and for water on a road surface.

~Neal

 

The bit with ice forming at 37 -- with radiational cooling (roads are generally black), they can be cooler than the ambient air temperature.

 

this discussion happens every year about this time.

 

The bing is merely there to freak you out for a split second.

*bing* "AH SH*T WHAT WHAT WHAT?!?!?!"

... oh... 37 degrees.

 

According to the owner's manual, it is a warning that "There is an increased risk of black ice."

Check this for more info on black ice: http://en.wikipedia.org/wiki/Black_ice

 

I also found out that as it warns you about 37 F, if you have the nav display and the temp is still dropping, the warning message at the bottom of the nav drops with the temp. The message started out at 37 but changed to 36 and 35 while it was still up. Nice warning for the unexpected ice conditions.

 

Mark

 

driving from Turlock To Patterson last night at 6pm - got the "ding" right at 37 degrees - I am really wondering just how accurate this thing really is... I am thinking it is reading somewhat Low - I really question it being 37 last night - Ya, I know dips in the road can hold cold air... on the way to Patterson, it actually got down to 32 on the dash but then went right back up to 40?????????? Anyone else question their numbers ?

 

BINGO!!!! WE HAVE A WINNER!!!

According to James Spann, Birmingham Metorologist; ICE can and more often than not forms on bridges before on roads due to the wind chill factor. If the outside temp dips to at least 37, you can bet if there is any moisture at all due to dew fall or even light rain, you will have ice on bridges.

MINI is so smart
and so thoughtful the way they watch out for all of us

 

Ice can form on bridges before roads because there is no ground underneath to "keep them warm." The wind (if cold enough) will speed up the process. But the T of the water on the bridge still needs to be 32 or below. It will not freeze at 37 degrees.

Meterologists have a standard distance above ground from which temperatures are measured and reported (I forget exactly what it is though). Hence, the above ground (air) T can be above freezing (37, say), but the ground may be at freezing point or below (i.e., 32 or below). Water will not freeze at 37 even with wind (unless of course the wind is 32 or less thus reducing the T of the water below 32).