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re: wind chill factor and windshields freezing
21 dec 2000
steve offiler wrote:
>here's a question. actual experience from a few days ago. ambient temp,
>read off a well-calibrated thermometer, is 35f, yet i'm scraping frost off
>my windshield. why?
too impatient to wait for spring? :-)
if it's 35 f (275 k) and the dew point is say, 28 f (-2 c, the average for
pa in december), at 6 am, duffie and beckman ("solar engineering of thermal
processes") say the sky temp is 275(0.711+0.0056(-2)+0.000073(-2)^2)^0.25
= 251.5 k (6.8 f or 453 r.) they also say your windshield at temperature
t (k) is radiating about 0.1714x10^-8(t^4-453^4) btu/h-ft^2 to the sky.
say it's also gaining (460+35-t)1.5 btu/h-ft^2 from the warmer still air.
if these 2 heatflows are equal (why not?), with a linearized radiation
heat transfer coefficient h = 0.1714x10^-8(495^2+453^2)(495+453) = 0.732
btu/h-f-ft^2, then (495-t)1.5 = 0.732(t-453), so t = 481 r (21 f.)
a v mph "wind warm factor" can supply (495-t)(2+v/2) btu/h-ft^2 of heat
to the windshield, which won't freeze until the windspeed drops to make
(495-492)(2+v/2) = 0.732(492-453), below v = 15 mph.
happy solstice.
nick
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