What Is the ‘Wind Chill Index,’ Anyway?

When Natalie Hasell, a warning preparedness meteorologist for the Canadian government, lived in Montreal, she used her own informal system for measuring cold that went beyond the thermometer. “I used to describe how cold it was by how long it took my nostril hairs to freeze walking from home to the bus stop,” she said.

These days, Ms. Hasell uses a different method for capturing the feeling of cold that takes into account both temperature and wind: the wind chill index.

Here’s how the index works.

Even if the ambient temperature stays the same, you might feel colder when you are hit by a gust of wind. Why? If you are standing still, out of the wind, your body’s natural heat will warm the air in the space around you — your “personal boundary layer” — by a small amount, Ms. Hasell said.

That boundary layer is dissipated by the moving air, increasing the pace at which your skin cools. “You turn on a fan or you go outside; now, any body or thing that is hotter than its environment will lose heat to that environment,” she said.

The wind chill index we use today expresses how cold the ambient temperature would have to be for you to lose heat at the same rate.

What’s the history?

There have been several iterations of the wind chill index dating to at least 1939, according to a historical review published in the journal Wilderness & Environmental Medicine.

In a doctoral dissertation that year, Paul Siple, an Antarctic explorer and geographer, observed that “it is not always the windiest days or the coldest ones that affect man’s sensation of chill most.”

To put this into numbers, Dr. Siple created a formula that multiplied the temperature below freezing in degrees Celsius by the wind speed in meters per second. In this context, a value of 50 meant good conditions for travel; between 50 and 150 meant fair traveling conditions; and over 300 meant “conditions become dangerous for travel or temporary shelter, and over 8,000 calories of food are required.”

During one of his trips to Antarctica, Dr. Siple conducted experiments with a geologist, Charles Passel, measuring the rate at which water froze, to inform a new iteration of the index that actually measured heat loss. At a cooling rate of 100 kilocalories per square meter per hour, they hypothesized, nude sunbathing would be possible. A rate of 600 would be agreeable “when dressed in wool underwear, socks, mitts, ski boots, ski headband, and thin cotton wind-breaker suits.” Around 1,350, they found, the “freezing of human flesh begins.”

In 2001, after more versions came and went, the formula currently used to calculate the wind chill was created by the Joint Action Group for Temperature Indices, which convened experts from American and Canadian meteorological services and academic institutions to develop a standardized and more precise formula.

“There were weaknesses of the original Siple-Passel algorithm,” said Mark Tew, who co-chaired the group and is now chief of the analysis division of the National Weather Service. “Some of the criticisms were that the body heat production and transfer should have been considered,” he added.

The Siple-Passel technique for calculating wind chill was also based on experiments the researchers conducted in the Antarctic, where they measured the wind speed 33 feet off the ground, meaning their findings would not map accurately onto the human body, he said.

In the human clinical trials that aided in the development of the latest index, six men and women were placed in a chilled wind tunnel. A National Weather Service publication laid out what happened next: “Thermal transducers were stuck to their faces to measure heat flow from the cheeks, forehead, nose and chin while walking 3 m.p.h. on a treadmill. Each volunteer took part in four trials of 90 minutes each and was exposed to varying wind speeds and temperatures.”

The end result was this formula, which renders the wind chill value in an equivalent temperature. This is what allows us to say it “feels like” a certain temperature outside because of the wind, even if the ambient temperature is higher.

The caveats

The formula currently in use is conservative, Mr. Tew said. Its creators wanted to avoid overburdening the Weather Service’s computing system, so it does not include as many variables as a private company’s proprietary one might, he said.

“Say you’re outside at peak sun, 1 p.m., on a winter day,” he said. “The sun’s not warming up much, but say you’re in the wind and the sun’s hitting you in the face. You are absorbing some solar radiation, which probably offsets the actual wind chill time.” This absorbed heat from the sun is not reflected in the formula used by the Weather Service.

Still, the formula works, and is a crucial tool — especially in situations of dangerous cold. “We’re in the business of life-saving, so we would probably be giving the worst case scenario,” Mr. Tew said.

What should I do with this information?

The Weather Service advises that the best way to prevent frostbite and hypothermia, which can be accelerated by the wind, is to stay warm indoors and out. Besides wearing a hat and covering your mouth to protect your lungs, “when you must go outside, wear several layers of loose-fitting, lightweight, warm clothing,” the Weather Service says. Wear a hat and mittens, and try, of course, to stay out of the wind.