The next few days here in Toronto are supposed to be quite hot and humid—the perfect time for a blog post about humidex.

You may or may not have heard the term “humidex” before, and if you have, you’ve probably heard about it during a weather forecast. The term itself is a portmanteau of the two words “humidity” and “index”. It is a quantitative way for Environment Canada to describe the effect of humidity on how hot a particular day will feel to a person. The number used for humidex is actually unit-less, however, you can directly equate it to a temperature in degrees Celsius that indicates how hot the day will feel. You’ll typically hear a “feels like” temperature during a weather forecast, which refers to the humidex value.

Now, the humidex value is not pulled out of thin-air. It is calculated by Environment Canada using the dew point. If you know the dew point and the dry air temperature, you can calculate the humidex yourself using this handy formula below provided by Environment Canada:

where $$H$$ represents the humidex value, $$T$$ represents the dry air temperature in degrees Celsius, $$\mathrm{e}$$ is Euler’s number and $$D$$ represents the dew point in Kelvin.

Of course, the humidex is usually calculated for you in Canada. But you never know, maybe one day you will be abroad and will want to determine the humidex. This formula will come in handy.

Try it out for yourself! You can find weather data, including the humidex and dew point, on Environment Canada’s website.

Stay cool during the summer!