Screen capture of The Weather Channel broadcast, featuring Charlie Welsh, showing wind chill temperatures valid at 8 a.m. EST Jan. 20, 1985. Nashville's wind chill of -56 would be -37 using the post-2001 wind chill formula, while St. Louis would be -48 instead of the -69 calculated in 1985.

Does it seem like wind chills aren’t what they used to be when you grew up? There’s a reason.

It’s not global warming – well, mostly not global warming. True, it’s getting harder to achieve extremely low temperatures nowadays against the backdrop of a warming climate. But as we’re noticing now, sometimes extremely cold and windy weather still happens.

The real reason you don’t get those 60- and 70-below wind chills you may have had in your younger years is because the formula changed.

Original Formula: Made in Antarctica

The original wind chill formula was developed by Paul Siple and Charles Passel as part of an Antarctic expedition from 1939 to 1941.

Siple and Passel’s experiment was quite simple: From a post on the roof of the expedition building, they hung a small plastic bottle containing water and measured how long it took to freeze. From those observations, they calculated the rate of heat loss per unit surface area. If you’re Canadian, you may remember when Canada’s wind chill was given in “watts per square metre per hour” instead of a temperature in degrees; same concept.

Screen capture of The Weather Channel broadcast, featuring Charlie Welsh, showing wind chill temperatures valid at 8 a.m. EST Jan. 20, 1985. Nashville's wind chill of -56 would be -37 using the post-2001 wind chill formula, while St. Louis would be -48 instead of the -69 calculated in 1985.

Eventually, in the 1960s and 1970s, formulas were developed to express wind chill as an equivalent temperature, to give people an idea of what kind of calm-air temperature would feel the same as a given temperature-wind combination.

The early formulas used by the military yielded seriously overblown results; for example, a temperature of 30ºF and a wind of 25 mph yielded a wind-chill temperature of 40 below zero – obviously unrealistic.

Even the modified version of that wind chill formula still exaggerated the chilling effect of the wind, at least within the range of everyday wind speeds. But as wind speeds increased beyond the range we normally experience, the mathematical properties of the old formula caused wind chills to reverse and warm up.

In fact, at 30 degrees and a 200-mph wind, the old wind chill would have been 50 degrees – totally counterintuitive!

New Formula: Tested on Real People

It turns out that basing the wind chill on a limited set of measurements involving a water bottle was a well-intentioned but less-than-perfect way of estimating the wind’s effect on the human body.

In the year 2000, U.S. and Canadian scientists organized an effort to revamp the wind chill formula. The new formula, deployed in the U.S. in November 2001, uses a more sophisticated set of assumptions.

Importantly, the formula reduces the anemometer-based wind speed, measured 33 feet above ground in standard weather observations, to a lower wind speed typically found at the level of a human face (about 5 feet). Researchers also studied the thermal properties of actual human faces to determine the value of a key constant in the equation.

As a result, modern wind chills do not “sound” as extreme as they used to. Combinations of wind and brutal cold that used to result in wind chills of 70 to 80 below zero back when Generation X grew up are more likely to produce figures closer to 50 below zero now.

Randall Osczevski and Maurice Bluestein, two of the primary researchers involved in the wind chill revision, admitted in the October 2005 Bulletin of the American Meteorological Society that

For one, there is much diversity in the thermal properties of people’s skin. Some people’s skin simply gets colder faster than others’. For another, the current index doesn’t factor in the warming rays of the sun, moisture in the air, or a person’s level of physical exertion.

Osczevski and Bluestein wrote, “It seems unlikely that another half century will go by before the wind chill is again upgraded.”

If you're curious how the new and old wind chills compare, the National Weather Service has a

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