What is Humidex?

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  • Written By: Mary McMahon
  • Edited By: Bronwyn Harris
  • Last Modified Date: 13 October 2019
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Humidex is a value which is used to express the perceived temperature by combining temperature and humidity data in one number. The Humidex reflects temperatures in Celsius, because it was developed in Canada. Canadian meteorologists developed the Humidex in response to a desire to come up with a formula which could more accurately express the way the temperature felt, combining the factors of heat and humidity.

As you may have noted, when the weather is humid, it feels much warmer. This is because the humidity makes it difficult for sweat to evaporate from the body, which makes it harder to cool down. The accumulated sweat may also leave people feeling slick or sticky, increasing their discomfort. Residents of notoriously humid areas often have to take steps in warm weather to ensure that they stay healthy and comfortable, such as using dehumidifies and fans in their homes.

The formula used to derive the Humidex is a bit complex, but essentially it combines the dew point and measured temperature. A Humidex of 40, which would be equivalent to 104 degrees Fahrenheit, is considered “uncomfortable.” When the Humidex hits 45 (113 Fahrenheit), it starts to become dangerous, and at 54 (almost 130 degrees Fahrenheit), heat stroke is imminent. Recordings in the 50s are relatively rare.


As a general rule, people ignore the Humidex when it is below 30 (equivalent to 86 degrees Fahrenheit), because this temperature is viewed as safe for most people. It is only when the Humidex starts to creep towards 40 that people may begin to be concerned. Numbers above 45 may result in high temperature warnings, alerting people to the danger so that they can adjust their lives accordingly until the temperatures cool down.

The Humidex debuted in 1965 in Canada, and has since spread to other regions of the world. This useful measure can be a useful rule of thumb for quickly assessing weather conditions to determine whether or not they are potentially hazardous. Looking at temperature predictions alone, it is sometimes possible to miss potentially dangerous weather, which could result in a variety of health problems associated with higher temperatures.


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