Degrees of guesswork

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:

Wind chill chart circa 2001

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.


  1. McGehee »

    9 January 2014 · 8:57 am

    There may actually be a simple scientific basis for the heat index, though — given that water vapor in the air amounts to latent heat. The amount of additional heat therefore needed to evaporate perspiration would be higher, and since evaporation of sweat is how our bodies regulate temperature in torrid conditions…

    Though, the fact it too is presented in the form of “feels like” kind of undermines it.

  2. canadienne »

    9 January 2014 · 1:21 pm

    -40 on a clear, calm sunny day (which is pretty normal. In those parts of this country that get -40, in the winter it means that a very chilly high pressure area is sitting right on top of you, which means very little wind) is much less miserable than a feels-like -40 with a nasty wind that blows cold air in through any crevices in your clothing, especially if you are walking right into it, plus blowing loose snow into your face.

    It’s all pretty subjective. How could it be anything else? The windchill rating just lets you know that it’s going to feel colder than the thermometer indicates.

    We have been know to say, “Hey, it’s a nice sunny day” and go hiking or skating on those -40 days. Hey, there’s hardly anyone else at the rink then.

  3. CGHill »

    9 January 2014 · 1:36 pm

    I have no experience with temperatures down that low, but the last time we plunged below zero Fahrenheit — in February ’11, when it reached -5°F (-21°C) — the wind was all but nonexistent. (The day before, when we got half a foot of snow, it was nominally warmer, but the stuff was blowing up one’s sleeves.)

  4. Roy »

    9 January 2014 · 9:50 pm

    I still say that “Wind Chill Factor” and “Heat Index” are both terms invented by newsies in order to make the weather sound worse than it really is.

  5. canadienne »

    10 January 2014 · 11:42 am

    I dunno, Roy. 30C (that’s 86F) in Saskatoon or Calgary with a pretty low relative humidity in July is pretty different from the same thermometer reading in Toronto next to that lake with a relative humidity of what feels like 98%. Same for -25C with and without windchill, and it’s possible to experience both of those in the same city.

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