When Natalie Hasell, a warning preparedness meteorologist for the Canadian government, resided in Montreal, she had her own unconventional method of gauging cold temperatures that went beyond relying on a 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 shared. However, nowadays, Ms. Hasell employs a different approach to capture the sensation of cold that takes into account both temperature and wind: the wind chill index.
The wind chill index is designed to address the fact that even if the ambient temperature remains constant, the impact of cold can feel more intense when accompanied by strong winds. Ms. Hasell explains that when you are standing still in a windless area, your body’s natural heat warms up the air around you, creating a small pocket of warmth called the “personal boundary layer.” However, when wind is present, it dissipates this boundary layer, causing your skin to cool at a faster rate. She further illustrates, “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.”
The wind chill index we employ today measures how cold the ambient temperature would need to be for you to lose heat at the same rate. It takes into consideration both the temperature below freezing in degrees Celsius and the wind speed in meters per second.
The history of the wind chill index dates back to at least 1939, with several iterations developed over the years. Paul Siple, an Antarctic explorer and geographer, was one of the early pioneers in this field. In his doctoral dissertation that year, he observed that the sensation of chill is not solely determined by the coldest or windiest days.
To quantify his observations, Dr. Siple devised a formula that multiplied the below-freezing temperature by the wind speed. This formula provided different ranges of values, with 50 indicating favorable travel conditions, 50 to 150 representing fair traveling conditions, and over 300 indicating dangerous conditions for travel or temporary shelter.
During one of his Antarctic expeditions, Dr. Siple collaborated with geologist Charles Passel to conduct experiments measuring the rate at which water froze. Their findings informed a new version of the wind chill index that focused on actual heat loss. They discovered that at a cooling rate of 100 kilocalories per square meter per hour, nude sunbathing would be possible. A rate of 600 would be comfortable when dressed in wool underwear, socks, mitts, ski boots, a ski headband, and a thin cotton windbreaker suit. At around 1,350, the freezing of human flesh would begin.
In 2001, the Joint Action Group for Temperature Indices, composed of experts from American and Canadian meteorological services and academic institutions, developed a standardized and more precise formula for calculating wind chill. This formula took into account criticisms of the original Siple-Passel algorithm, including the consideration of body heat production and transfer.
The latest wind chill formula was developed through human clinical trials, where volunteers were placed in a chilled wind tunnel while thermal transducers measured heat flow from different parts of their faces. The formula converts the wind chill value into an equivalent temperature, allowing us to describe how it “feels like” a specific temperature due to the wind, even if the ambient temperature is higher.
It is important to note that the current formula used for wind chill is conservative. It was designed to prevent overburdening the computing system of the National Weather Service, so it does not incorporate as many variables as a private company’s proprietary formula might. For example, it does not account for the heat absorbed from the sun during peak sunlight hours.
Nonetheless, the wind chill formula remains a crucial tool, particularly in hazardous cold conditions. Mark Tew, co-chair of the Joint Action Group, emphasizes the importance of providing worst-case scenario information since their primary goal is to save lives.
To protect yourself from frostbite and hypothermia, the Weather Service advises staying warm and dry both indoors and outdoors. This includes wearing multiple layers of loose-fitting, lightweight, warm clothing, covering your mouth and wearing a hat and mittens. It is also recommended to avoid exposure to the wind as much as possible.
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