Humidity is the amount of water vapor in the air. High humidity makes people feel hotter than they would on a drier day. This is because the perspiration that cools us down cannot evaporate as quickly in moist, saturated air. To better describe how hot it feels in such circumstances, Canadian meteorologists developed the Humidex, a parameter that combines temperature and humidity in order to reflect the perceived temperature.

The Humidex combines the temperature and humidity into one number to reflect the perceived temperature. Because it takes into account the two most important factors that affect summer comfort, it can be a better measure of how stifling the air feels than either temperature or humidity alone.