Humidex Explained: Canada's Version of the Heat Index
Canada measures oppressive heat with its own index, invented in 1979. Here's how the humidex works, what the numbers mean, and why it isn't the US heat index.
The humidex is Canada's home-grown measure of how hot the air actually feels once humidity is factored in. Developed by Canadian meteorologists in 1979, it combines the air temperature with the amount of moisture in the air (via the dew point) into a single number, expressed like a temperature in degrees Celsius. A humidex of 40 means the muggy air feels roughly like a dry 40 °C.
What is the humidex, exactly?
The humidex (a portmanteau of "humidity" and "index") was devised in 1979 by Canadian meteorologists J.M. Masterton and F.A. Richardson at what was then the Atmospheric Environment Service. It answers a simple question: when the air is sticky, how hot does it feel? Sweat is the body's air-conditioning, but it only cools you as it evaporates. When the air is already saturated with moisture, evaporation slows, less heat leaves your skin, and the same 30 °C feels far more oppressive than it would on a dry day. The humidex puts a number on that penalty.
Crucially, ECCC treats the humidex as an index, not a temperature. It is written without a degree symbol — "the humidex is 40," not "40 °C" — precisely because it is a comfort scale, not a reading a thermometer would ever show.
How is the humidex calculated?
The humidex is derived from the air temperature and the dew point (the temperature at which the air would become saturated). ECCC's formula is:
Humidex = Tair + 0.5555 × (e − 10)
where Tair is the air temperature in °C and e is the water-vapour pressure in hectopascals, itself calculated from the dew point. The higher the dew point, the more moisture in the air, the bigger the added "e" term, and the higher the humidex climbs above the true temperature. ECCC generally computes a humidex once the air temperature reaches about 20 °C, and reports it in forecasts when it climbs to 25 or higher.
How do I read the humidex scale?
ECCC publishes a widely used comfort scale. Because it is a scale of human discomfort rather than a strict physiological threshold, treat the bands as guidance, not guarantees.
| Humidex range | Degree of comfort |
|---|---|
| 20–29 | Comfortable |
| 30–39 | Some discomfort |
| 40–45 | Great discomfort; avoid exertion |
| Above 45 | Dangerous; heat stroke possible |
| 54 and over | Heat stroke imminent |
Once the humidex passes 40, ECCC advises limiting strenuous activity; above 45, outdoor exertion becomes genuinely risky. These are the same numbers that feed into ECCC's heat warnings, which in much of the country trigger around a humidex of 40 (criteria vary by region and are often stated as a combination of daytime highs and warm overnight lows).
Humidex versus the American heat index — are they the same?
No, and this catches out plenty of cross-border travellers. Both answer "how hot does it feel?", but they use different maths and different inputs, so they rarely land on the same number for the same weather.
| Humidex (Canada) | Heat index (United States) | |
|---|---|---|
| Used by | Environment and Climate Change Canada | US National Weather Service |
| Moisture input | Dew point / vapour pressure | Relative humidity |
| Basis | Physics of vapour pressure (1979) | Steadman human-comfort model |
| Expressed as | An index (no degree symbol) | An "apparent temperature" in °F/°C |
Because the humidex is anchored to dew point and the heat index to relative humidity, the humidex will often read a few degrees higher than the equivalent heat index for the same conditions. Neither is "wrong" — they are simply two national systems built on different assumptions. When you compare a Canadian and an American forecast for the same heatwave, expect the feels-like numbers to differ.
Humidex and its winter twin, wind chill
The humidex has a cold-season counterpart: the wind chill index, which ECCC and the US jointly revised in 2001. Where the humidex adds degrees to reflect oppressive heat, wind chill subtracts them to reflect how much faster wind strips heat from exposed skin. Both are "apparent temperature" tools designed to translate raw physics into a health-relevant number — humidex for summer, wind chill for winter.
Why does the feels-like number vary between apps?
Because a humidex is only as good as the temperature and dew-point forecast it is built on, and those come from weather models that do not always agree. ECCC bases its humidex on the GEM model; a third-party app might use ECMWF, the American GFS or Germany's ICON, each with a slightly different dew-point forecast — and a degree or two of dew-point disagreement can swing the humidex by several points.
This is where a model-comparison tool earns its keep. checkweather.io lines up ECCC's GEM against ECMWF, GFS and ICON, and converts their level of agreement into a forecast-confidence score, with an accuracy scoreboard grading each model against what actually happened. On a borderline day — humidex hovering near the 40 warning line — knowing whether the models agree tells you whether to trust the "great discomfort" call or treat it as a coin-flip.
Staying safe when the humidex climbs
When the humidex is 40 or higher, ECCC and provincial health authorities advise drinking water before you feel thirsty, seeking shade or air conditioning, rescheduling hard physical work to the cooler morning, and checking on older neighbours and anyone without cooling. Watch for heat-related illness — dizziness, nausea, a rapid pulse and confusion — and treat the humidex as a prompt to change plans, not just a talking point.