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What exactly is humidex? How come sometimes you mention it, and sometimes you don't?

Question submitted by Phyllis Gee
(Burnaby, BC)

The humidex is an index (a computed value as opposed to something measured) devised to describe how hot or humid weather feels to the average person. The humidex combines the temperature and humidity into one number to reflect the perceived temperature. It takes into account these two important factors that affect summer comfort. It is therefore a better measure of how stifling the air feels than either temperature or humidity alone. A humidex of 40 with, for example, a temperature of 30 degrees means that the sensation of heat when it is 30 degrees and the air is humid is more or less the same as when it is 40 degrees and the air is dry. We must be careful not to depend on this interpretation alone: it is a mere indication of physiological reactions, not an absolute measure.

The humidex is particularly significant when its value is greater than 30. We only display humidex values of 25 or higher for a location which reports a dew point temperature above zero (0°C) and an air temperature of 20°C or more. Below this value, the humidex is too close to the air temperature to be considered significant.

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