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The New Formula for Cold
It won’t have any effect on the actual outdoor air temperature this winter, or how quickly our bodies lose heat in frigid blustery weather, but the new and improved wind-chill index launched this fall will give Canadians a more accurate idea of how cold it really feels outside.
The wind-chill formula, which was developed over the past two years by a team of scientists and medical experts from Canada and the United States, is based on new research using human test subjects and advanced computer technology, combined with recent advances in the understanding of how the body loses heat when exposed to cold.
The resulting wind-chill index is expressed in "temperature-like" units so as not to be confused with the actual temperature. Although many Canadians are already familiar with wind-chill information being expressed in these terms, the new numbers are different from what they have grown accustomed to. Scientists have found that wind chill was slightly over-estimated in the past, so that fewer warnings of extreme conditions will be issued under the new system.
The reason for the difference is that the old wind-chill index, which was created 60 years ago, was estimated by measuring heat loss from cylinders of water in the Antarctic. Unlike people, however, plastic cylinders do not have any internal heat source, nor do they respond physiologically to cold like humans do when the blood vessels at the surface of our skin constrict to conserve core body heat. Wind speeds used in the previous formula were also measured in towers located more than 10 metres above the ground, rather than at an average human height of 1.5 metres, where friction reduces the wind speed to about two thirds.
Environment Canada led the development of the new formula by hosting the world’s first Internet workshop on wind chill in the spring of 2000. One of the first challenges facing scientists was to decide the best way to determine wind chill—a difficult task, because it expresses how our exposed skin feels as a result of the combined effect of temperature and wind, rather than measuring an objective value. To determine the temperature at which still air would cause the equivalent rate of heat loss from your skin, scientists focused on the most exposed and frostbite-susceptible area of the human body: the face.
Environment Canada and the Department of National Defence (DND) worked closely together to develop an accurate facial-cooling model, beginning with tests on a mannequin head with a skin made of special thermoconducting material. After measuring temperature changes and heat loss from the mannequin in different wind conditions, they devised a new mathematical model for estimating wind chill. To ensure the accuracy of the model, which factored in body and skin temperatures and skin resistance, the tests were then conducted on human subjects.
Human trials were carried out on six men and six women aged 22 to 42 at DND’s Defence and Civil Institute of Environmental Medicine in Toronto this spring. Each subject participated in four 90-minute tests at different temperatures and wind speeds inside a refrigerated wind tunnel. Dressed in weather-appropriate clothing, but with their faces exposed, they walked on a treadmill at a rate of 4.8 km/h into an artificially generated wind of 10, 20 and 30 km/h at three air temperatures: 10°C, 0°C and -10°C. In each test, the wind speed was initially set up at the low end and stepped up to the other two values at 30-minute intervals. In addition, a "wet" test was carried out at 10°C whereby each subject’s face was sprayed with water at 15-second intervals, to determine the impact of water on facial cooling.
Heat flow sensors and thermocouples attached to the subjects’ faces measured heat loss and changes in skin temperature at the chin, nose, cheeks and forehead. Since the cheeks were among the coldest of these areas, they were used to calculate worst-case skin conditions. At the same time, participants were asked at regular intervals for their perceptions of the temperature, and how cold they felt. Thermometers showed that participants’ core temperatures remained consistent despite changes in the external temperature and wind speed. However, significant differences were noted in facial skin temperature and heat loss—not only under different environmental conditions, but also among subjects.
The wet tests confirmed that wind makes you feel colder by evaporating any moisture on your skin—a process that draws more heat away from your body than when it is dry. Data collected showed the wind chill to be 5-10 degrees colder in wet conditions than in dry ones carried out at the same temperature. As a result of these findings, Environment Canada is considering the development of a marine wind-chill chart that will show different values for wet and dry conditions to help protect mariners exposed to heavy spray from frostbite.
Physiological differences among individuals are responsible for some people feeling the cold faster than others, even when exposed to the same combination of wind and temperature. One such difference is surface area—the less surface area you have in relation to your mass, the less heat you will lose. That means that people who are tall and thin tend to feel the cold sooner than those who are short and stocky. Over thousands of years, the facial features of people living in the Arctic have also developed specially to retain heat.
Graphs illustrating the differences in facial cooling between a volunteer whose heat loss during
the trials was similar to that predicted by the model (left), and one who exhibited a strong cold-induced vaso dilation response (right).
Another reason is that people with greater skin insulation (generally those who are heavier and better "padded") lose core body heat more slowly, and therefore have colder skin temperatures than people who have less insulation. As a result, these better-insulated people are more susceptible to frostbite—but less susceptible to hypothermia. People with less body insulation lose core body heat more rapidly, so their skin is warmer and, therefore, better protected from frostbite.
Another interesting difference demonstrated at the trials is that certain people who are well-adapted to cold exhibit a physiological response known as cold-induced vaso dilation—a mechanism that acts like a thermostat on a furnace. When the surface temperature of their skin falls to a certain level, the blood vessels that were constricted to conserve core body heat suddenly open up and send a flood of warm blood to the area to keep it from freezing. The sensors then kick in and close the blood vessels again until the temperature dips down to the critical level again, when the whole process is repeated. Seven of the 12 subjects in the trials exhibited this response.
In order to devise a wind-chill formula that protects people who are most susceptible to frostbite, researchers based their model on the five per cent of the population who experience the greatest facial cooling. The model was run more than 800 times with different combinations of wind speed and air temperature, to ensure consistency and accuracy. So as not to confuse it with actual temperature—which does not change regardless of how hard the wind blows—wind chill is now expressed in temperature-like units, without the use of the degree symbol. These temperature-like units liken how cold it feels under certain wind and temperature conditions to the temperature on a calm day.
In the future, scientists will study other factors that have an effect on the perception of cold, and incorporate them into the model to make it even more accurate. Warm sunlight on a winter day, for example, has a major impact on how cold exposed skin feels, but this impact is influenced by many factors—including latitude, longitude, elevation, cloud cover, time of day and date. Humidity will also be studied more closely, because a dry cold is generally perceived as warmer than a humid cold. As a first stage in this process, the new index gives Canadians a more realistic basis for protecting themselves against the potentially hazardous cooling effects of wind and temperature.
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