Why are we still reporting on the "wind chill"?

Previously published Slate articles made new.
Jan. 7 2010 12:17 PM

Wind Chill Blows

It's time to get rid of a meaningless number.

(Continued from Page 1)

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.

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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. (A researcher in Australia devised a model that takes into consideration an individual's height, weight, and style of dress.)

But no amount of tweaking will make wind chill more comprehensible. The language of "equivalent temperatures" creates a fundamental misconception about what wind chill really means. It doesn't tell you how cold your skin will get; that's determined by air temperature alone. Wind chill just tells you the rate at which your skin will reach the air temperature. If it were 35 degrees outside with a wind chill of 25, you might think you're in danger of getting frostbite. But your skin can freeze only if the air temperature is below freezing. At a real temperature of 35 degrees, you'll never get frostbite no matter how long you stand outside. And despite a popular misconception, a below-32 wind chill can't freeze our pipes or car radiators by itself, either.

The recent fiddling with wind chill has only made the numbers less useful. The old system might have overstated the numbers when it said that 5 degrees could feel like minus 40. But after three decades of practice, we all got pretty good at translating from the outrageous numbers in the weather reports to our own experience. When the weather service recalibrated the system in 2001, we had to start all over and rebuild our frame of reference from scratch.

Rather than trying to patch up wind chill's inconsistencies, we should just dump it altogether. The best algorithm we'll ever have for determining how cold it feels comes from our own experience. A look out the window gives us most of the variables we need to compute our own, personal weather index. The sight of a few leafy trees will tell us how windy it is on our corner and whether the breeze is swirling or gusting. We'll see if the sun is shining or if the sky is overcast. We'll also know how we're dressed, how tall we are, how much we weigh, and how quickly we walk down the street. We can even stick our hand outside for a moment, to get a sample of the ambient air temperature.

That's more than enough data to know how it might feel to step outside our front door. After all, our brains have been tallying up these variables for our entire lives. Weather reports can give us more specific information than we can get on our own, like predictions on what the wind and temperature will be in the future. But there's something absurd in the notion that the weatherman can tell us how we feel. Even the most rigorous meteorological model just mimics the one we build for ourselves.

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