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How Cold Was It? Wind Chill Tells the Story


Even though things are relatively warmer at this point, last week’s cold spell will likely hang in memory for some time.  The answer to the question “How cold was it?” may be somewhat surprising: warmer, in terms of windchill, than it would have been reported under the same conditions some 13 years ago.

“Wind chill” was coined to describe “the rate of heat loss on the human body resulting from the combined effect of low temperature and wind,” according to the website of the National Oceanic and Atmospheric Administration, noaa.gov.

The rationale of calculating wind chill is to determine the possible damage to humans and animals – frostbite and hypothermia, most commonly – from exposure to very cold weather. According to the Mayo Clinic website (www.mayoclinic.org), frostbite occurs when the skin and the body tissue underneath freezes. The skin becomes very cold, then numb, hard and pale. Hypothermia occurs when the body loses heat faster than it can produce heat.

According to the National Oceanic and Atmospheric Administration website, in November, 2001, the National Weather Service, part of NOAA, implemented a new wind chill temperature index, which yields warmer temperature readings than the prior one. “In general, given the same actual air temperature and wind ... the new index will usually be warmer than what you would have expected with the old index,” according to NOAA.

NOAA also adds some wind chill lore: “The first wind chill formula and tables were developed by Paul Allman Siple and Charles Passel working in the Antarctic before the Second World War and were made available by the National Weather Service by the 1970s. [These were] based on the cooling rate of a small plastic bottle as its contents turned to ice while suspended in the wind on the expedition hut roof, at the same level as the anemometer. The so-called Wind Chill Index provided a pretty good indication of the severity of the weather.”

By the 1960s, people began to report wind chill as “wind-chill-equivalent temperature (WCET), which led,” according to NOAA, “to equivalent temperatures that were obviously exaggerations of the severity of the weather.” During that time  Charles Eagan “redefined the absence of wind to be an air speed of 1.8 meters per second (4.0 mph) which led to more realistic (warmer-sounding) values of equivalent temperature,” according to NOAA.

The new – that is, 2001 – wind chill temperature index is accepted by the United States, Canada and the United Kingdom. This National Weather Service index calculates the wind speed at an average height of 5 feet, the typical height of an adult human face, based on readings from the national standard height of 33 feet, typical height of an anemometer. It also incorporates heat-transfer theory (heat loss from the body to its surroundings, during cold and breezy/windy days) and assumes no impact from the sun.

The new wind chill formula is as follows:

New Wind Chill T(wc) =35.74 + 0.6215T - 35.75(V0.16) +0.4275T(V0.16) where T(wc) is the Wind Chill in degrees F, V is the Wind Speed in MPH, and T is the temperature in degrees F.

“Relatively warmer” can still be cold. The wind chill at the AFC championship game between the San Diego Chargers and the Cincinnati Bengals on Jan. 10, 1982 – said to be the coldest football game in history – was reportedly -59 degrees. Using the 2001 index, the wind chill would reportedly have been only -37. Similarly, a report compared the cold of the Jan. 5 Green Bay Packers game using both the old and the new indices: -47 with the old but only -36 with the new.




 

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