Canada's New Wind Chill Index

A brisk wind on a winter's day can make it feel much colder outside than the temperature shown on the thermometer. That colder sensation is wind chill – the combined effect of temperature and wind.

Just in time for winter, Canada's wind chill factor is now easier to understand, thanks to a new formula developed by an international research effort. The formula gives Canadians a more accurate idea of how cold it really feels outside, so they can enjoy winter activities and take the necessary precautions.

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 as a temperature-like index, without the degree symbol. The index likens how cold it feels on a calm day.

Human Scale

The index was developed using human test subjects, advanced technology and recent medical advances in understanding how the body loses heat when exposed to the cold. The human guinea pigs were subjected to cold, in combination with varying wind speeds and the occasional spray of water on their faces, while the temperature of their cheeks, forehead and chin was monitored. They also had to rate their degree of comfort on a scale ranging from very comfortable to highly uncomfortable. Researchers used the data collected to create the new index.

Scientists determined that wind chill was previously over-estimated, particularly at high wind speeds. Under the new system, Environment Canada expects to issue fewer warnings of extreme cold conditions to Canadians.

The multi-year study was carried out by scientists and medical experts in Canada and the United States, and included an international workshop conducted over the Internet. The workshop was the first significant coordinated effort to better define wind chill since the development of the original Siple-Passel formula in the early 1940s. The original formula was based on how fast water in plastic containers freezes under different wind and temperature conditions.

A Chill in the Air

The new index will also be used in the United States, although it will be provided according to the Fahrenheit scale.

Wind chill is now included in Environment Canada's forecasts, when weather conditions warrant. A wind chill warning is issued when conditions become hazardous. Weather forecasts are available through radio and TV broadcasts, as well as on Environment Canada's Weatheradio, recorded telephone messages, and web sites.