Wind Chill Program

Wind chill and Environment Canada's new program

Why a new program in 2001?

Confusion among the public

For a couple of weeks around New Year's Day in 1999, unusually cold air moved over southern Ontario. As a result, wind chill values started appearing in weather forecasts as it was significant enough to create concern. As was normal practice then, wind chill was calculated as a cooling rate, using the Siple-Passel equation with the wind speed measured at the standard 10-metre height. It was expressed by the "wind chill factor", which was a number in the thousands (e.g. 1800) based on the cooling rate in watts per square metre (W/m²).

At the time, although Environment Canada was using the wind chill factor in its forecasts, most Canadian media were using equivalent temperatures to express wind chill. Environment Canada was also using equivalent temperatures in observations, but had long been hesitant to use them in its forecasts because people living in cold regions like the Arctic had often reported that the so-called temperature equivalence was not right. In other words, although the wind chill equivalent temperature was supposed to be the temperature that would have caused the same cooling with a very light wind, such was not the case (e.g. an equivalent temperature of -40 and a real temperature of -40 were not giving the same feeling of cold). Consequently, equivalent temperatures were found to be misleading as they could give people a false sense of security by making them believe they could tolerate lower temperatures than what they really could. Overall, these considerations led to the adoption in the early 1980s of the wind chill factor based on W/m² as the official way to express wind chill numerically, though other methods persisted, including equivalent temperatures or some descriptive terms.

The 1997-98 winter had been abnormally mild in southern Ontario. Therefore, by January 1999, it had been a long time since wind chill had appeared in a forecast for that region. Moreover, that winter, the department had decided to use numerical values (the wind chill factor) to express wind chill - a radical change for a lot of people. As a result, we received many enquiries as to the meaning of the factor, as well as plenty of comments on its use. This made us realize that people's understanding of the wind chill factor was problematic. To know more, we decided to conduct a public opinion study, which consisted of focus group sessions followed by a survey of adult Canadians. Questions on wind chill were also asked in other surveys. This research showed that there was indeed confusion regarding what wind chill is and is not, as well as a marked preference for a temperature-like index to express wind chill. More details on this research can be found on this page.

Need to review the science of wind chill

At the same time, we examined the science of wind chill. We discovered that significant scientific advances had occurred since the 1940s when the Siple-Passel equation was developed, and that several scientists were questioning that equation. To determine the best course of action, we held in April 2000 an Internet Workshop on Wind Chill, which led to further developments in research.

We also discovered that the National Weather Service (NWS) of the United States was also looking at reviewing its wind chill index. This led to collaboration between the two countries, as well as with research organizations, particularly universities and the Defence and Civil Institute of Environmental Medicine (DCIEM - now Defence Research and Developement Canada - Toronto) of the Department of National Defence where some research on human reactions to cold was conducted.

Towards a new equation and a new program

During the fall of 2000, a special group, called the Joint Action Group on Temperature Indices (JAG/TI), was formed to evaluate the existing wind chill formula and make necessary changes to improve it. This group consists of Environment Canada's Meteorological Service of Canada, several American government agencies, the university community and the International Society of Biometeorology. The JAG/TI's goal is to upgrade and standardize indices, like the wind chill index, that are used for temperature extremes.

In February 2001 the JAG/TI agreed on a new wind chill formula, and discussed a process for its scientific verification and implementation. There was also agreement to use only a temperature-like index to report and forecast wind chill. The new wind chill equation, by Randall Osczevski of DCIEM and Maurice Bluestein of Purdue University in Indiana, was presented at the JAG/TI meeting held in Toronto in August 2001. The new equation makes use of advances in science, technology and computer modelling to provide a more accurate, understandable and useful formula for estimating the dangers arising from winter winds and freezing temperatures. Clinical trials have been conducted at the DCIEM and the results of those trials have been used to improve the new formula's accuracy.

Twelve volunteers (six men and six women) participated in the clinical trials. These consisted in four walks, at 4.8 km/h, on a treadmill in a refrigerated wind tunnel at the DCIEM facilities in Toronto: one walk at each of -10°, 0° and +10°C, plus a "wet trial" at +10° during which participants received, every 15 seconds, a light one-second splash of water in their faces. During each 90-minute walk, the volunteers were walking while facing a wind of 2 metres per second (m/s) for 30 minutes, followed by 30 minutes at 5 m/s, and 30 minutes at 8 m/s (or about 8, 18 and 29 km/h, respectively). Sensors were fixed to participants' forehead, cheeks, chin and nose, as well as to the inside of one cheek, to measure skin temperature and heat loss. The results from these trials were used to determine the various thresholds for frostbite, as seen on the new wind chill charts. You may also read the personal account from a volunteer in those trials.

The new wind chill equation is now in use in both Canada and the United States. Therefore, there is now a consistent wind chill formula across North America.

The Green Lane^TM,
Environment Canada's World Wide Web Site.