Application of the Universal Thermal Climate Index for Operational Forecasting in Canada

The Universal Thermal Climate Index (UTCI) has been explored as an evidence-based tool for alerting Canadians of thermal stress due to extreme outdoor temperatures. UTCI, an equivalent temperature, incorporates both physiological and meteorological factors of heat and cold stress including air temperature, humidity, wind speed and solar radiation. These features allow the UTCI to be used as a health impact-based forecast index for the various climates found in Canada. Communicating forecast values of UTCI presents several challenges. The UTCI's sensitivity to wind speed causes it to be unreliable in very high wind conditions. It is not recommended that UTCI be used when wind speeds exceed 17ms-1, speeds that are not unheard of in Canada. Winter values can be much lower than the currently used Canadian Wind Chill Index. In tests conducted at Canadian sites in summer, the UTCI demonstrated less sensitivity to humidity than the Canadian Humidex and, in high heat, exhibited lower daily maximum values than the Humidex. When considering forecasting the UTCI, questions remain on the capacity to reliably forecast the mean radian temperature, a key element during summer. Other challenges are related to the best method for communicating a forecast UTCI value, either through an equivalent temperature or thermal stress category. UTCI's sensitivity to wind speed and solar radiation could make it difficult to communicate to the public and may cause confusion amidst other thermal indices currently used by the public and private media on both sides of the Canada-US border. A forecast UTCI could consider a daily worst case scenario, such as maximum solar radiation and sheltered wind conditions in the warm season, and full shade and strongest forecast winds in the cold season. Assumptions regarding exposure time and to simplify the calculation of the UTCI may be necessary in order to forecast and communicate the risks associated with thermal stress so that it is understandable and useful to Canadians while retaining the integrity of the UTCI. Validation of any assumptions will be necessary before application of the UTCI in Canada.