Posted at 12:53 PM on November 6, 2007
by Craig Edwards
From out of nowhere the National Weather Service rolled out an updated wind chill temperature (WCT) in November 2001. As Chief meteorologist for NOAA in the Twin Cities I was stunned that I was excluded from the discussion on this significant change. I believed the old index did exactly what it was designed to do, significantly exaggerate the dangerous impact of the combined cold temperatures and wind.
You can read the scientific background on this change in the story linked below.
Essentially two things were considered beyond the initial study which established the index in 1945. One, the reported winds are taken from instruments extending about 30 feet above the ground. Our exposed skin is usually limited to the first six to seven feet above ground level. Accounting for friction, the formula reduces the impact of the wind by a considerable factor. Secondly, the WCT applies laboratory tests on human skin tissue.
To demonstrate the magnitude of the change, take this example: under the old formula a temperature of zero degrees Fahrenheit and a reported 20 mph wind resulted in a wind chill temperature of minus 39 degrees. The new index, applying the same reported conditions determines the wind chill to be minus 22 degrees.
Until the index comes under further scientific scrutiny we will enjoy a gentler and relatively warmer wind chill index. Seldom will you see an index of minus 45. Keep this in mind; a WCT of minus 20 is dangerous.