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Old 10-04-2012, 01:59 PM
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Zooks527 Zooks527 is offline
Winding up the M235xi
Location: Mansfield, MA
Join Date: Mar 2009
Posts: 5,628
Mein Auto: 2015 M235xi / 2005 Tacoma
Originally Posted by pointandgo View Post
I'm wondering if the BMW "asterisk" calculates that the temperature is declining when the symbol appears and chime goes off. Last time that I checked, ice doesn't form at 38 deg. F.
Ice doesn't form at a temperature of 38 degrees, but it can certainly form on a road surface that drops below the air temperature.

Posit a situation where it is a 38 degree night, crystal clear, with no wind. The road is radiating IR in all directions. In most directions, there is an object of similar temperature radiating IR back at it. But, we've said it's a clear night. The road is radiating IR towards the clear sky, but is receiving no IR from the (approx) -450 degree section of interstellar space in the opposing direction. The road loses temperature continually, dropping below the freezing point even though the air around it is several degrees above freezing. The effect is especially pronounced on bridges and overpasses. It's also why you first see frost warnings in the fall on clear nights, but not on cloudy ones.

You can do a demonstration with your car's front and rear glass to show the effect. Park the car so that there are branches with leaves above the rear glass, but nothing above the windshield. You will often find that frost or dew will form on the glass exposed to the sky, but not on the glass shaded by the leaves, as they are radiating back at the glass, while the exposed surface has dropped below the dew point / freezing point.

The reverse is the case during the day. The air temperature may be below freezing, but the portion of the road looking at the 10,000 degree ball of fusing hydrogen in the sky is raised significantly above freezing.

2015 M235xi coupe, Black Sapphire Metallic, Black Leather, Fineline Stream trim, Steptronic, xDrive, ZPP, ZTP, ZCW, ZDA, ZDB, 5DP, hk w/ BimmerTech amp, Enh. BT
Ordered 3/12/15, Off the line 4/7/15, Delivered 5/14/15

Prior 40 years of cars: 1967 BelAir wagon / 1968 LeMans Tempest / 1970 Mustang Mach 1 / 1972 El Dorado / 1978 Corvette / 1981 Subaru GL wagon 4WD / 1983 s10 Blazer 4x4 / 1985 Bronco 4x4 / 1996 Bronco 4x4 / 2004 Passat 4motion / 2009 BMW 335i xDrive E92

Last edited by Zooks527; 10-05-2012 at 05:20 AM.
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