Suzanne Briggs

Extreme heat events are forecast to increase in frequency, intensity, and duration throughout the 21st century. The public health consequences of heat are felt most intensely in metropolitan areas where population density is high and the presence of the urban heat island (UHI) phenomenon exacerbates the potential for heat exposure. In this study we use geostatistics (ordinary kriging) with meteorological data to interpolate hourly prediction surfaces describing apparent temperature (humidex index) across the Greater Toronto Area (GTA), Canada. Meteorological data were obtained from 77 locations for six days in 2008 when extreme heat alerts were issued for the City of Toronto. Temperature and humidity were combined to produce a humidex index for each hour and then amalgamated into a composite hourly dataset, with 24 maps representative of a 'typical' extreme heat alert day. Results show a significant difference in humidex values between urban and rural locations, this discrepancy is greatest at 3 AM (3.8°C), t(42) = 6.362, p<0.000. A cumulative heat exposure map, showing humidex degree hours (HDH) ≥ 30°C, identifies the downtown core of the City of Toronto and much of Mississauga (west of Toronto) as likely to experience dangerous levels of prolonged heat and humidity (HDH ≤ 65) during a heat alert. We recommend that public health officials use apparent temperature and exposure duration when developing spatially-explicit heat vulnerability assessment tools; HDH is one approach that unites these risk factors into a single metric.