Suffering is better when you can put a number on it. Hence the wind chill, that index of our winter discomfort. But putting a subjective number on cold—measuring not the actual temperature, but how people experience it—is a relatively recent invention.

None of these numbers are perfectly accurate. There's no way they can be

The wind chill was invented on Antarctica, where temperatures this time of year have been below -30 Fahrenheit, while wind chills have dipped into the -40s and 50s. In 1945, two Antarctic explorers named Paul Siple and Charles Passel measured how wind speed affects the rate of heat loss. Their experiment was simple: Fill a plastic container with water, hang it from a pole, and measure how quickly the water loses heat (in this case, how quickly the water turned to ice). They found that the faster the wind was blowing, the faster water turned to ice. For people, that means the windier it is, the more heat we lose, and the colder it feels.

Why? Usually, when an object loses heat through convection—like when water turns to ice in the cold, losing its heat to the colder air around it—there's a layer of heat between the warm object and its cold surroundings. But when it's windy, the moving air breaks up this insulating layer. It speeds up heat loss by whisking away the warmth.

Wind chill guide by NOAA

Siple and Passle measured the heat loss that leads to wind chill in watts per square meter. However, that unit didn't quite grab the public imagination the way we've come to expect from modern meteorological hype and winter weather portmanteaus. (Snowmaggedon! Snowpocalypse!). Canadian meteorologists and not many other people used wind chill until the 1970s, when they and their fellow meteorologists began converting it to the familiar temperature equivalents that allow forecasters to say, "It's 21 degrees this morning, but it feels like 5 below out there."

Volunteers walked on a treadmill in a cold wind tunnel with sensors on the outsides of their faces

That was a boon for TV meteorologists, but not for scientific accuracy. It turned out that the wind chills calculated using the original 1945 formula were—excuse the pun—overblown. So in 2001, the Joint Action Group on Temperature Indices (its real name!) introduced a new formula for the wind chill index.

The new numbers, still in use today, are based on a somewhat perverse experiment: Volunteers walked on a treadmill in a cold wind tunnel with sensors attached to the outsides of their faces and the insides of their cheeks to take temperature readings, which the experimenters used to calculate the rate of heat loss. Because of sensor placement in the experiment, our modern wind chill index is based on the wind speed at five feet in the air—average face height.

That still isn't the end of the story, because private weather services have now introduced proprietary wind chill formulas, like Accuweather's "RealFeel" and The Weather Channel's similar "Feels like." Both of those currently list colder equivalent temperatures for Chicago than does the National Weather Service.

Wind chill was a boon for TV meteorologists, but not for scientific accuracy

None of these numbers are perfectly accurate. There's no way they can be: What the temperature "feels like" is a subjective measure of how we experience the weather, which varies from person to person, kind of like the whole "what if your color blue isn't the same as my color blue?" argument.

The best relief from wind chill is counting down the days till summer, the months when you'll quantify your discomfort not with the wind chill but with the heat index. Whereas the wind chill takes into account the effect of wind on cooling, the heat index takes into account relative humidity; the higher the humidity, the slower sweat will evaporate from your glistening skin to cool you off, and the hotter you'll feel.

Until then, remember this. No matter how cold it feels, one thing is objectively true: If the actual temperature is above freezing, even if the wind chill is below freezing, water won't freeze (nor will your face).

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