ⓘ Humidex is defined in Celsius and applies above approximately 20°C. Input temperatures below 20°C will show a result but with reduced physical meaning.
| Parameter | Value | Notes |
|---|
Humidex Reference Table: Temperature vs. Dew Point (°C)
| T (°C) | Tdp 14°C | Tdp 16°C | Tdp 18°C | Tdp 20°C | Tdp 22°C | Tdp 24°C |
|---|
Bold = ECCC Great Discomfort level (humidex ≥ 40). Values rounded to nearest integer. ECCC formula: Humidex = T + 5/9 × (Pw(Tdp) in mbar − 10).
Canadian Provincial Heat Warning Thresholds
| Province | Heat Warning | Extreme Heat Warning | Basis |
|---|---|---|---|
| Ontario | T‑max ≥31°C + T‑min ≥20°C (2+ days) OR humidex ≥40 | T‑max ≥40°C or humidex ≥45 | Humidex + temperature |
| Quebec | T‑max ≥33°C (urban) for 3+ days or humidex ≥40 | T‑max ≥40°C | Humidex + temperature |
| British Columbia | T‑max ≥29°C (coast), ≥35°C (interior) for 2+ days | T‑max ≥39°C (interior) | Temperature primary |
| Manitoba | Humidex ≥40 or T‑max ≥36°C | Humidex ≥45 | Humidex + temperature |
| Alberta | T‑max ≥36°C for 2+ days | T‑max ≥40°C | Temperature primary |
| Atlantic Canada | Humidex ≥38 or T‑max ≥31°C for 2+ days | Humidex ≥43 | Humidex + temperature |
Thresholds are approximate and subject to Environment and Climate Change Canada policy updates. Always confirm with the applicable provincial alert authority.
The Canadian humidex: Canada's official apparent temperature index
The humidex (humidity index) was developed by J.M. Masterton and F.A. Richardson at the Atmospheric Environment Service of Canada and first operationally used in 1965. It represents how hot the combined effect of temperature and humidity feels to the average person. Unlike the US National Weather Service heat index, which uses temperature and relative humidity, the humidex uses dry-bulb temperature and the dew point temperature. The dew point is the temperature at which air becomes saturated; it's a direct measure of the actual amount of moisture in the air, independent of temperature. This makes the humidex formula more physically direct than one based on relative humidity, because relative humidity depends on both moisture content and temperature simultaneously.
The ECCC formula: Humidex = TC + (5/9) × (Pw(Tdp) in mbar − 10), where Pw(Tdp) is the saturation vapour pressure at the dew point temperature in millibars. This can be computed using the ASHRAE saturation pressure formula, built on the same water thermophysical reference data NIST maintains, to find Psat(Tdp) in kPa, then multiplying by 10 to convert to mbar. For dew points below 10°C, the formula gives a result less than the dry-bulb temperature (indicating humidity isn't adding to discomfort), so humidex is only reported above that threshold. Use the dew point calculator to find Tdp from T and RH before entering it here for maximum accuracy.
ECCC discomfort ratings and what they mean for building operators
Environment and Climate Change Canada's official discomfort scale has four core bands, with the top band commonly split further in workplace safety guidance. Below 30: little or no discomfort, no action needed. 30 to 39: some discomfort, monitor vulnerable occupants. 40 to 45: great discomfort, avoid exertion; HVAC systems in arenas, shelters, and community centres should operate at full cooling capacity and outdoor air intakes should be minimized during peak hours to reduce latent load. 46 to 54: dangerous, activate cooling centre protocols and building emergency plans. Above 54: heat stroke is imminent, emergency conditions that require evacuation of spaces without effective cooling. For HVAC designers, humidex values above 40 at design outdoor conditions signal that the latent cooling load will be significant and that coil selection must account for both sensible and latent duty simultaneously. Use the coil load calculator with actual outdoor design state points to quantify the cooling requirement.
Humidex vs. heat index: choosing the right tool
The Canadian humidex and the US NWS heat index measure the same phenomenon but use different formulas and produce somewhat different results. At moderate humidity levels, the humidex typically reads 2 to 5°C higher than the heat index for identical conditions. Near saturation they converge. Canadian weather services, public health advisories, and provincial heat alert systems all use humidex, so for any Canadian public communication or comparison against ECCC alerts, always reference humidex. For HVAC cooling load calculations against ASHRAE design conditions from the design temperature lookup, and international engineering work, the heat index calculator is more appropriate since it aligns with the NWS and ASHRAE references used in most North American mechanical engineering standards. Both are shown side-by-side here for direct comparison.
Frequently Asked Questions
The ECCC humidex formula is: Humidex = TC + (5/9) × (Pw(Tdp) in mbar − 10). Pw(Tdp) is the saturation vapour pressure at the dew point temperature in millibars. Since 1 kPa = 10 mbar, you can use the ASHRAE Psat formula to find Psat(Tdp) in kPa and multiply by 10. For example, at T = 30°C and Tdp = 20°C: Psat(20°C) ≈ 2.339 kPa = 23.39 mbar. Humidex = 30 + (5/9) × (23.39 − 10) = 30 + (5/9) × 13.39 = 30 + 7.4 = 37.4 ≈ 37. If you know RH instead of dew point, use the dew point calculator to convert, or enter RH directly in this tool and it computes Tdp internally.
Provincial thresholds vary. In Ontario, a Heat Warning is issued when humidex reaches 40 or when maximum temperature stays at or above 31°C and minimum temperature stays at or above 20°C for two or more consecutive days. Manitoba triggers warnings at humidex 40. Quebec uses humidex 40 or three or more consecutive days above 33°C in urban centres. British Columbia focuses more on absolute temperature and uses lower thresholds because the population is less heat-acclimatized. ECCC's own discomfort scale rates 40 to 45 as "Great discomfort, avoid exertion" and 46 and above as "Dangerous; possible heat stroke." For HVAC system operators, a humidex above 40 at outdoor design conditions indicates significant latent load that must be verified against cooling coil capacity; check the outdoor air calculator to confirm minimum ventilation rates before sizing, then use the SHR calculator and coil load calculator with actual design state points.