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Wind Chill BlowsIt's time to get rid of a meaningless number.

By Daniel Engber
Posted Friday, Feb. 9, 2007, at 5:55 PM ET
Illustration by Rob Donnelly. Click image to expand.

If the weather makes headlines only when it's horrendous out, wind chill is its PR agent. This week, when temperatures in New York City dropped to single digits, newspapers and TV meteorologists breathlessly reported that the wind chill had hit minus 11. In Ohio, they told us, the thermometers read close to zero, but gusts of cold air made it feel like 25 below. Banner stories proclaimed a wind chill of 35 below in Chicago.

The weathermen trot out these arctic, pumped-down numbers to put an exclamation point on the banality of winter. Wind chill readings make excitement out of mere inconvenience; they imbue a miserable day with the air of epic calamity. A temperature of 5 degrees is unpleasant. A wind chill of 20 below—well, that's something to talk about.

The gaudy negative numbers do more than describe the weather; they try to tell us how we experience it. The reporting of wind chill carries with it a paternalistic impulse to explain not just how cold it is, but how cold we'll feel. Well, I've been out in the cold every day this week, and I know exactly what it's like. If wind chill can tell me only what I've already experienced—my cell phone hand too numb to dial a number, my moustache freezing on my face—then we should just get rid of it altogether.

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.

As the use of equivalent temperatures spread, people started to notice inconsistencies between real temperatures and their wind chill counterparts. For some reason, a day spent in a minus-40 wind chill was a lot easier to handle than a minus-40-degree day with no wind. Around 2000, two researchers—Randall Osczevski in Canada and Maurice Bluestein in the United States—began looking closely at this problem. Before long, they discovered that the adapted Siple-Passel equations grossly overestimated rates of heat loss.

The two nations' weather services formed a committee to address the problem. By 2001, the Joint Action Group on Temperature Indices had created a new system that toned down wind chill readings across the board. After the recalibration, conditions that were once said to feel like minus 40 now "felt like" minus 19. (Click here for a sidebar that explains how Osczevski and Bluestein came up with their new wind chill table.)

The updated model patches over the worst flaws of the old wind chill system, but it's not anything close to perfect. Osczevski and Bluestein made a set of new assumptions to determine wind-chill-equivalent temperatures. Namely, they 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.

Even the variables that Osczevski and Bluestein did include might be wildly off base. Air temperatures tend to remain fairly stable throughout the day, but wind speeds fluctuate a great deal. (It's much less breezy in the morning and at night, for example.) Wind speed also varies depending on where you are. Obstacles on a city street—like buildings, cars, and kiosks—can block the flow of air and reduce its average speed. But wind-chill-equivalent temperatures use a single number to represent all this variability.

Other meteorologists have tried to work out more-involved schemes to account for these flaws. One group is even trying to combine every possible variable—temperature, wind, humidity, sunlight, and so forth—to create a universal weather index. (This exhaustive model even takes into consideration an individual's height, weight, and style of dress.)

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Daniel Engber is an associate editor at Slate. He can be reached at .
Illustration by Rob Donnelly.
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