Humidex

Computation formula

When the temperature is 30 °C (86 °F) and the dew point is 15 °C (59 °F), the humidex is 34. If the temperature remains 30 °C (86 °F) and the dew point rises to 25 °C (77 °F), the humidex rises to 42. The humidex is higher than the U.S. heat index at equal temperature and relative humidity.

The humidex formula is as follows: $${\displaystyle \text{Humidex}=T_{\text{air}}+0.5555(e-10.0),}$$ where ${\displaystyle e}$ is the vapour pressure in millibars given by: $${\displaystyle e=6.11\exp \left[5417.7530\left(\frac{1}{273.16}-\frac{1}{T_{\text{dewpoint}}}\right)\right].}$$

Also note:

• ${\displaystyle T_{\text{air}}}$ is the air temperature in degrees Celsius
• ${\displaystyle T_{\text{dewpoint}}}$ is the dewpoint in kelvins, which can be calculated from dewpoint in degrees Celsius using ${\displaystyle \{T{\}}_{\text{K}}=\{T{\}}_{{}^{\circ }\text{C}}+273.15}$

5417.7530 is a rounded constant based on the molecular weight of water, latent heat of evaporation, and the universal gas constant. The humidity adjustment approximately amounts to one Fahrenheit degree for every millibar by which the partial pressure of water in the atmosphere exceeds 10 millibars (10 hPa).

At the time the humidex was originally developed in 1965, Canada was still on the Fahrenheit scale, and thus the humidex was originally based on that. The 1979 reformulation added the 0.5555 factor (from the relation 1 °F = ⁠5/9⁠ °C), to address metrication in Canada as the country switched to the Celsius scale.

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