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Old 10-04-2012, 07:22 PM
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Quote:
Originally Posted by Zooks527 View Post
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 a the 10,000 degree ball of fusing hydrogen in the sky is raised significantly above freezing.

FWIW,
George
George,
Interesting post...but I've heard that NITROGEN in your tires will protect you from interstellar freezing on bridges. I have at least three good sources.
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