What Does Wind Chill Mean, Exactly?

What Does Wind Chill Mean, Exactly?

2021-02-19 21:46:00

You know the weather forecast is bleak when two sets of numbers appear on your local TV weather report during the winter months. A series of numbers is the daily temperature. The other, colder numbers indicate the wind chill – something the meteorologist could describe with the phrase feels like

While the phrase is an easy way to talk about wind chill, it's not quite accurate shorthand. The concept of wind chill – and the formula meteorologists use to calculate it – measures how wind speed and outside temperature combine to create more challenging conditions than the latter could achieve on its own.

The formula has been (and will) evolve over time as researchers refine their understanding of the interaction between wind and our bodies. "Wind chill, I think, is probably a good representation of the scientific process in plain sight," said Neil Laird, a meteorologist at Hobart and William Smith Colleges.

The wind effect

Windchill calculations assess something we instinctively know: wind makes a cold day worse. Our body generates heat that is released into the atmosphere. When air lingers around our skin – a phenomenon that happens in the airy layers of a down jacket – our body can heat it up and we stay relatively warm. Breezes create the opposite situation. Air, heated by our bodies, is carried away and replaced by colder temperatures, Laird says. The stronger the wind, the faster we lose the heated air. Constant heat loss to the atmosphere forces our bodies to work even harder so that we stay warm enough.

The most brutal examples of how wind makes the cold more unbearable can be found in Antarctica, where the idea was born to measure wind chill. "Perhaps there is no place on Earth where people are as acutely aware of the need for a suitable scale to express reasonable temperatures as the polar regions," Paul Siple and Charles Passel, two members of the United States Antarctic Service, wrote at the end. the tirthies. and early 1940s. While at the Little America III reconnaissance base, the duo conducted the first experiments to measure what they the wind chill indexThe team hung a jerry can of water outside, measured how long it took to freeze at different wind speeds and temperatures, and then devised a formula to measure the "total cooling capacity of the atmosphere."

Granted, a jar of water isn't a human body, so these initial readings of the wind chill were pretty crude. “It didn't start out as representative, but it was information that was not available at the time,” says Laird. "It was more of a way of thinking about (how) these extreme conditions can really make an impact in a very short time." Since then, other researchers have used measurements of wind chill outside Antarctica and adjusted the calculation to reflect people's actual experiences. The original formula assumed people would be standing still outside, so later versions took average human walking speed into account when calculating the effects of the wind. The standard protocol for measuring wind speed is nearly 10 meters above the ground – but since no one is that tall, the wind chill calculators eventually explained what a breeze can feel like at human body heights.

Doing good

In 2001, US and Canadian weather and climate agencies worked together to revise the protocols for calculating wind chill. The latest equation they've achieved is the standard used in both countries' weather services today, Laird says. And relatively recently (in the history of wind chill calculations), forecasters started using the wind chill index like feels like temperatures – as in, a 30 degrees Fahrenheit day "feels like" 19 degrees Fahrenheit.

But the feels like Not everything has been recorded approximately that should convey the perceived temperature. Combined wind and cold put more stress on bodies, Laird says. "It can have more consequences than just being cold." And the formula does not take into account how everyone deals with cold and wind. How quickly someone outside loses body heat depends on other climatic conditions and personal factors, such as their size and what they wear – details that the original developers knew about the wind chill made a difference, and those other, independent wind chill calculators like the Universal Thermal Climate Index try to take into account

Despite these imperfections, the descriptor can still provide useful information. When the message gets across that the conditions outside are less pleasant (and potentially more dangerous) than what the temperature alone will tell you – and whether that's enough to convince people to spend less time outside that day, or to grab warmer gloves before heading out. door – that's valuable, says Laird. In some cases, wind chill ratings can provide better indicators of weather-related damage than temperature alone. A 2018 study found that in the winter months, Wind chill temperatures were better predictors of visits to emergency rooms for cardiovascular disease in New York than air temperatures.

At the same time, the National Weather Service is still evaluate how risk communicates – from the wording that meteorologists use to how people perceive warningsThe process has led those involved to dive into other professions for help, Laird says. "Physical scientists are now working with social scientists to figure out the best way to communicate (this information) with people." While those conversations continue, who knows? Maybe what you see on your local weather forecast is going to look a little different.


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