What is the Humidex?

In Canada, weather forecasters and analysts report a number called the Humidex along with temperature and humidity reports. Like the heat index, which is used in the US, the Humidex gives an indication of the felt temperature when it is especially hot and muggy outside. The Humidex number is based on the current temperature and the dew point.

The dew point is the temperature to which humid air must be cooled in order for the water vapor to condense into water. (The condensed water is called dew.) The dew point is related to the relative humidity level. At high humidity levels, the dew point is close to the current air temperature because the air is nearly saturated with water vapor. At low humidity levels, the dew point is much lower than the current temperature. For the Humidex calculator at left, you can use either the dew point or relative humidity level.

The Humidex is often interpreted according to the following scale:

Less than 29°C, no discomfort
30°C-39°C, some discomfort
40°C-45°C, great discomfort
45°C-54°C, dangerous
Above 54°C, heat stroke imminent

Humidex Equation

If the outside temperature is T degrees Celsius and the dew point is D degrees Celsius, then the felt temperature is given by the equation

T + (3.39556)e^[19.8336 - 5417.75/(D+273.15)] - 5.5556

Example: Suppose the current temperature is 30°C and the dew point is 21°C. Then the Humidex temperature is

30 + (3.39556)e^[19.8336 - 5417.75/(21+273.15)] - 5.5556
= 38.4°C

Even though the outside temperature is 30°C, the relative humidity and associated dew point will make you feel as though the temperature is near 38°C.

© Had2Know 2010