Case 1
- Air temperature
- 30°C
- Dew point
- 20°C
37.6
Open with these valuesHumidex Calculator
37.6
Result: 37.6The humidex turns temperature and dew point into one comfort number: 30 °C with a 20 °C dew point comes out at 37.6. It reads like a Celsius value but is an index, not a temperature. Above 40 Environment Canada advises against exertion; above 45 it calls the conditions dangerous.
Worked examples
Case 2
- Air temperature
- 35°C
- Dew point
- 24°C
46.3
Open with these valuesCase 3
- Air temperature
- 28°C
- Dew point
- 18°C
34.0
Open with these valuesHow it's calculated
humidex = T + 0.5555 × (e − 10), e = 6.11 × exp(5417.753 × (1/273.16 − 1/(Td + 273.15)))
- StepEnter the shade air temperature in degrees Celsius.
- StepEnter the dew point in degrees Celsius — it may be negative.
- StepThe dew point becomes a vapour pressure e in hectopascals.
- ResultEverything above 10 hPa is scaled by 0.5555 and added to the temperature.
Reference table
| Temperature, dew point | Comfort scale | Humidex |
|---|---|---|
| -5, -8 | Below 20 — not reported | -8.7 |
| 25, 15 | Little to no discomfort | 29.0 |
| 28, 18 | Some discomfort | 34.0 |
| 30, 20 | Some discomfort | 37.6 |
| 32, 21 | Great discomfort; avoid exertion | 40.4 |
| 35, 24 | Dangerous; heat stroke quite possible | 46.3 |
| 40, 28 | Dangerous; heat stroke quite possible | 55.9 |
Questions
What is the humidex?
The humidex is Canada's feels-like index — a single number describing how hot a humid day actually feels. It combines the air temperature with the moisture in the air, measured by the dew point. At 30 °C with a 20 °C dew point the humidex is about 38.
How is the humidex calculated?
The dew point is first turned into the air's water-vapour pressure e using an exponential formula anchored at the triple point of water. The humidex is then the air temperature plus 0.5555 times the amount by which e exceeds 10 hPa. The scaling factor is 0.5555 as published, not exactly five ninths.
Why is the humidex higher than the temperature?
Humid air slows the evaporation of sweat, so your body cools less and the same temperature feels hotter. The humidex adds that extra felt heat on top of the air temperature, which is why a muggy 30 °C day can come out at 38 or more.
When is a humidex reported at all?
Environment Canada reports a humidex only when it exceeds the air temperature, roughly from 20 upwards. In cool or dry air the moisture correction is negligible or negative, so the number stops meaning anything — the first row of the table is exactly that case.
How does the humidex differ from the heat index?
Both describe a feels-like temperature, but from different inputs and different formulas. The humidex (Canada) is built from temperature and dew point and is unitless; the heat index (United States) uses temperature and relative humidity and is reported in degrees. They give similar but not identical values for the same weather.
Sources and last check
Information, not professional advice.