| Parameter | Metric (°C) | Imperial (°F) |
|---|
Heat Index Reference Table: Selected Temperatures (°C)
| T (°C) | RH 30% | RH 40% | RH 50% | RH 60% | RH 70% | RH 80% | RH 90% |
|---|
NWS Rothfusz regression. Values in °C. Bold = Extreme Caution or higher (≥32°C HI). Formula most accurate for T ≥27°C and RH ≥40%.
Heat index: apparent temperature from temperature and humidity
The heat index (sometimes called "feels like" or apparent temperature) combines air temperature and relative humidity into a single value representing how hot conditions feel to the average person. It was developed by R.G. Steadman (1979) and refined into the Rothfusz regression formula published by the US National Weather Service (NWS). The regression takes dry-bulb temperature and relative humidity as inputs and applies a nine-term polynomial in degrees Fahrenheit. This tool converts results to Celsius for Canadian applications. The formula is most reliable above 27°C (80°F) and 40% RH; below those thresholds, humidity doesn't meaningfully amplify perceived heat, and the formula can produce values below the actual air temperature.
High humidity increases perceived heat because human cooling depends on sweat evaporation. When RH is high, sweat evaporates more slowly, reducing the body's ability to cool itself. The result is that 35°C at 70% RH produces a heat index of about 50°C, the same apparent temperature that roughly 47°C at 20% RH would produce, a striking illustration of how much humidity amplifies perceived heat even though the actual air temperature is 12°C lower. For HVAC engineers, this difference matters when sizing outdoor air systems with the outdoor air calculator, assessing cooling load during extreme heat events, and evaluating building envelope performance for summer comfort. The humidex calculator provides the Canadian equivalent used by Environment and Climate Change Canada.
Heat index danger levels and Canadian public health thresholds
The NWS categorizes heat index into five danger levels. In Canada, provincial health authorities and Environment and Climate Change Canada use slightly different thresholds based on humidex and multi-day temperature forecasts. Ontario's Heat Warning threshold is a daytime high at or above 31°C and an overnight low at or above 25°C for two or more consecutive days, or a heat index at or above 40°C on any single day. Quebec uses humidex thresholds rather than heat index. British Columbia, which has seen extreme heat events in 2021 with fatalities, issues heat warnings at lower absolute temperatures because the population is less acclimatized. For building owners and operators in hot-summer Canadian cities like Windsor, Toronto, Ottawa, and Montreal, a heat index above 38°C (100°F) should trigger emergency cooling plans and HVAC system checks.
Heat index vs. humidex: choosing the right tool for Canadian applications
The NWS heat index and Canadian humidex both measure perceived heat discomfort but use different formulas. The heat index uses dry-bulb temperature and relative humidity. The humidex uses dry-bulb temperature and dew point temperature. For the same meteorological conditions, the two often differ by 2 to 5°C, with the humidex typically higher at moderate RH levels and the two converging near saturation. Canadian weather reporting uses humidex exclusively, so for public communication and comparison against Environment Canada alerts, use the humidex calculator. For HVAC cooling load analysis and comparison against ASHRAE or international standards, the heat index is more appropriate since it aligns with the NWS and ASHRAE frameworks used in most North American engineering references.
Frequently Asked Questions
The heat index uses the NWS Rothfusz regression with dry-bulb temperature and relative humidity as inputs. The Canadian humidex, developed by Environment and Climate Change Canada, uses dry-bulb temperature and dew point temperature. Both measure perceived heat discomfort but give slightly different values. At the same conditions, the humidex is typically 2 to 5°C higher than the heat index at moderate RH, and they converge near saturation. Canadian weather reports use humidex; US and international HVAC references use heat index. The humidex calculator provides the Canadian metric, and both are shown side-by-side in this tool for direct comparison. For HVAC cooling system design against ASHRAE design conditions, use heat index; for comparison against Environment Canada weather alerts, use humidex.
HVAC cooling systems are typically sized to the ASHRAE 0.4% or 1% design dry-bulb and wet-bulb temperature for the location (available from the design temperature lookup), which implicitly captures the heat index at those conditions. However, for high-occupancy, high-outdoor-air spaces such as arenas, transit hubs, community centres, and emergency shelters, check the heat index at the design outdoor condition. In southern Ontario and Quebec, design conditions can produce heat index values above 40°C (104°F, Danger level), which may require additional latent cooling capacity beyond what a standard design day calculation produces. Use the coil load calculator with the design state points to verify both sensible and latent cooling meet the demand during extreme heat events.