I’ve looked at dozens of wind chill charts, both the old formula and the new one, and they always leave me scratching my head: “What does this really mean?” The answer, it appears, is “Not a whole hell of a lot”:
The weatherman’s favorite alarmist statistic has been around for more than 60 years. Its ignoble history began with a pair of Antarctic explorers named Paul Siple and Charles Passel. In 1945, the two men left plastic bottles of water outside in the wind and observed the rate at which they froze. The equation they worked out used the wind speed and air temperature to describe the rate at which the bottles gave off heat, expressed in watts per square meter.
In the 1970s, the Canadian weather service started reporting numbers based on Siple and Passel’s work. These three- and four-digit values meant little to the average person, however the “wind chill factor” might have been 1,200 one day and 1,800 the next. American weathermen took a more pragmatic approach, converting the output from the Siple-Passel equation into the familiar language of temperature statements like “it’s 5 degrees outside, but it feels like 40 below.” What exactly did these phrases mean? The meteorologists would figure the rate of heat loss in watts per square meter and then try to match it up to an equivalent rate produced in low-wind conditions. For example, the rate of heat loss in 5-degree weather and 30 mph wind matched up with the one for minus-40-degree weather and very little wind. So, 5 degrees “felt like” 40 below.
This might make sense, maybe, if we all felt the same way. But we don’t, and frankly, I am uncomfortable with substituting “To me, it feels like …” in the place of actual data. A corrected version was conjured up. Now just imagine why this might not apply to you:
[T]hey geared their calculations toward people who are 5 feet tall, somewhat portly, and walk at an even clip directly into the wind. They also left out crucial variables that have an important effect on how we experience the weather, like solar radiation. Direct sunlight can make us feel 10 to 15 degrees warmer, even on a frigid winter day. The wind chill equivalent temperature, though, assumes that we’re taking a stroll in the dead of night.
This is the current chart:
Note the formula, which very nearly defies comprehension.
My own quick-and-dirty routine, which I’ve used for at least a decade, seems, if Wikipedia is to be believed, to have an official name: the McMillan Coefficient. Take the temperature in degrees Fahrenheit, subtract the wind speed in miles per hour. If it’s 25° out and you have a 10-mph wind, it’s gonna feel like fifteen.
Coming this summer, maybe: why the heat index also sucks.