Ever heard of the humidex? Many Canadians have, especially if they live in the south eastern parts of the country.
The term — short for humidity index — is a Canadian innovation. It was first used in 1965, according to Environment Canada.
The index is based on a calculation of heat and humidity by using current air temperature and the dew point (the temperature and barometric pressure at which water vapour condenses into liquid).
For example, a humidex of 40 C could mean that the temperature is around 35 C, but because of humidity, it feels more like 40 C.
Most often used by weather forecasters in southern Ontario and Quebec, the humidex is becoming more frequent in the typically dry Prairie Provinces of Manitoba and Saskatchewan. High moisture levels caused by growing crops, combined with higher humidity are responsible for the change, says David Phillips, Environment Canada’s senior climatologist.
The term was invented to help Canadians asses the danger of the heat outdoors.
“The issue really is a health issue,” says Peter Taylor, a professor of atmospheric science at Toronto’s York University. The humidex “gives you a measure of how effectively your body can cool down,”
The humidex doesn’t take into account certain factors that can change how hot it feels, such as strong winds or the amount of sunshine.
While it’s helpful to pay attention to the humidex, people must also take into account their own personal limits, for example, their age, health conditions or how fit they are.
Facts about the humidex:
- A humidex of 30 to 39 means people who are outside may feel “some discomfort”. If it’s from 40 to 45, it means “great discomfort”. People are told to avoid exertion. Above 45 is considered dangerous since it brings the possibility of heatstroke.
- The winter equivalent to the humidex is the wind chill, an index invented by Americans.
- Other indexes also try to take into account the effect of the humidity on the human body, such as the discomfort index, the humitur index or the humisery index.
- For years, the southern Ontario city of Windsor was known as the humidex capital of Canada. It held the record of a humidex of 52.1 from June 20, 1953 for more than half a century.On July 25, 2007, that record was surpassed by Carman, a rural farming town in southern Manitoba, where the humidex hit a level of 53.
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