Humidex Calculator
Calculate the Canadian humidex from air temperature and relative humidity or dew point — the Environment Canada formula, the comfort level from little discomfort to dangerous, and the vapour pressure.
How it works
Humidex = T + 0.5555 × (e − 10) · e = 6.11 × exp[5417.7530 × (1/273.16 − 1/(273.15 + T_d))] hPa · T = air temperature (°C), T_d = dew point (°C); from relative humidity the dew point comes from the Magnus formula
The humidex was introduced by Environment Canada in 1965 to describe how hot and humid weather feels to the average person. It adds to the air temperature a term for the water-vapour pressure, worked out from the dew point: every hectopascal of vapour above 10 hPa adds about 0.56 degrees. It is reported as a number without a unit, on the same scale as °C. Environment Canada issues heat warnings in many regions when the humidex reaches about 40, and above 45 heat stroke becomes possible.
Worked example
At 30 °C with 60% relative humidity, the dew point is about 21.4 °C and the vapour pressure 25.8 hPa. Humidex = 30 + 0.5555 × (25.8 − 10) = 38.8, reported as 39 — some discomfort, feeling about 8.8 °C hotter than the thermometer reading. With a dew point of 15 °C instead, the humidex would be 34.
Sources: Environment and Climate Change Canada — Glossary: humidex · Wikipedia — Humidex
Frequently asked questions
What is the humidex?
A Canadian index that combines temperature and humidity into one number describing how hot it feels. A humidex of 40 means the air feels roughly as uncomfortable as dry air at 40 °C.
What is the difference between humidex and heat index?
Both measure how humidity makes heat feel worse. The humidex (Canada) uses the dew point and a simple vapour-pressure formula; the US heat index (NWS) uses a regression on temperature and relative humidity in °F. They give similar but not identical numbers.
What humidex level is dangerous?
Environment Canada describes 40–45 as great discomfort where exertion should be avoided, and above 45 as dangerous, with heat stroke possible. Many heat warnings are issued from about 40.
Why is humidex shown without a unit?
It is an index, not a measured temperature, so it is written without °C, although its scale matches degrees Celsius.
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This calculator is for educational purposes. Double-check important results before acting on them.