Thursday, February 2, 2012

How long does it take to get Frostbite?


How long does it take to get frostbite? Sounds like an easy question but I find many different answers on the Internet (including one tough old New Englander who answers, "never", with proper clothing). Part of the confusion lies in the definition of "Wind Chill". See http://perpetualpreparedness.blogspot.com/2010/01/wind-chill.html for details. There have been two changes.

1st:  In Nov. 2001 the wind chill values were changed based upon a new equations, see http://www.crh.noaa.gov/ddc/?n=windchill. Whereas 15°F used to be extreme cold in 20mph wind, now it's "safe" at all wind speeds and never drops below -11 wind chill.

2nd: Some revised charts appear to use the new wind chill values but kept the color bands the same for exposure times. See this NOAA chart from 2010 which still has 15°F freezing within 5 minutes with strong wind. http://www.srh.noaa.gov/jetstream//global/chill.htm

A new NOAA chart from 2011 (pictured above) updates the color zones. http://antarcticfudgesicles.wordpress.com/afsa-student-page/how-long-does-it-take-to-get-frostbite/ The exposure time colors don't exactly match the wind chill numbers anymore (for instance -40 wind chill is in two different color zones).
Wind chill of -20°F to -35°F (approx) Frostbite in 30 minutes for exposed face, at night.
Wind chill of -35°F to -50°F (approx) Frostbite in 10 minutes.

There used to be a -30-30-30 rule that -30°F with a 30 mph wind could freeze skin solid in 30 seconds. Under the new chart it's -30 & 30mph & 5 minutes for frostbite.

Bottom Line
So what happened to the NOAA charts? During the fall of 2000, the Office of the Federal Coordinator for Meteorological Services and Supporting Research (OFCM) organized a joint U.S.–Canadian government-sponsored action group to develop, test and implement a new Wind Chill Temperature Index. Human trials were done in Toronto, Canada with 12 volunteers in chilled wind tunnels with thermal transducers stuck to their face while walking on a 3mph treadmill. Brrrrr.

So the new chart is based on solid human evidence of facial skin instead of mathematical modes and time to freeze bottles of water.  See http://www.nws.noaa.gov/om/windchill/images/wind-chill-brochure.pdf for more information.

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