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7 things you probably didn't know about the Humidex | The Star

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7 things you probably didn't know about the Humidex

By Jackie HongStaff Reporter Mon., Aug. 17, 2015
Sun seekers hit Sugar Beach Monday despite the high humidex rating. Toronto may get a break from the high readings by the end of the week.  (David Bateman/Toronto Star)

Ah, the humidex. The thing that can make a summer day in Toronto go from a hot 31C to feeling like a stifling 39C. The city, and most of Ontario, has been hit with heat warning after heat warning this summer, in part thanks to a value that can make what could be a pleasantly warm day feel like the Sahara instead.

But what exactly is the humidex, anyway?

A Canadian creation

The humidex is a Canadian creation first used in 1965, according to Environment Canada’s website. It combines the temperature and humidity levels into one value that shows the perceived temperature, or how the heat and humidity feels “to the average person.”

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“The key thing for humidex is really related to how the human body cools itself,” Environment Canada warning preparedness meteorologist Geoff Coulson explained. “So when we exert ourselves on a warm day, we sweat. That sweat stays on our skin as water droplets and as wind flows around our skin, if the air is dry enough, then a fair amount of that sweat will be evaporated off our skin, and it’s that evaporative process that cools the body. We start to have problems when we’re exerting ourselves to a fair amount on a day where there’s already a lot of moisture in the air. So as the air blows over our skin, it doesn’t really evaporate all that much of the sweat that’s on our skin, it sort of stays there. And so that doesn’t let us get the relief we would normally get.”

How does it work?

The humidex is broken down into degrees of comfort:

20 to 29: No discomfort

30 to 39: Some discomfort

40 to 45: Great discomfort; avoid exertion

46 and over: Dangerous; possible heat stroke

Who does it apply to?

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The humidex is based on how comfortable an adult in good health would feel in a mix of temperature and humidity.

“This is somebody who doesn’t have any underlying medical conditions that may compromise them or makes them suffering more under these high heat-humidity situations,” Coulson said. “And it also wouldn’t include the elderly or the very young.”

In the case of someone with a health problem or the very young and elderly, they may feel more discomfort at a lower humidex value – at 35, for example, instead of 40.

It’s an eastern thing

Higher humidex values are most often seen in Manitoba, Ontario and Quebec, with the west being spared for the most part from the humidity.

“The Maritimes can get some occasions when the humidex can get into the uncomfortable range, but in terms of when it happens most frequently, that tends to be sort of southern Manitoba, Ontario and into southern Quebec,” Coulson said.

Windsor holds the dubious honour of the “humidex capital of Canada” because it gets the most days where the heat and humidity combine to give values higher than anywhere else in the country. In general, southern Ontario gets it bad: “I’d say sort of Windsor, Sarnia, London, Toronto, eastwards towards the Ottawa Valley, any of those locations can experience those stretches of high heat and humidity that could result in humidex readings of 40 or higher,” Coulson said.

So, what is it not?

“I think where a lot of the confusion comes about is, people tend to think of it as the actual temperature. But what humidex is meant to be used for is a sort of comfort index,” Coulson said. “It really is this idea of people learning what their own tolerances are, what their own preferences are and behaving accordingly.”

Does anyone else use it?

Not really. Accuweather has a “RealFeel” value which is basically the same thing but also incorporates things like wind and cloud coverage. In the U.S., there’s something called the heat index or heat factor.

What else you should know:

Again, the humidex doesn’t have to hit 40 for things to get uncomfortable – it varies from person to person. And even if the humidex is below 40, children and pets should not be left in cars and people who work outside should stay hydrated, get into shade whenever possible, use sunscreen and take frequent breaks.

Toronto may get a break from the humidex by the end of the week. According to Coulson, the agency is predicting a warm and humid Tuesday and Wednesday, but a front should be moving in Thursday that could bring showers, thunderstorms and cooler temperatures.

“It’s really Friday and the weekend we get out of the humidity once and for all…We are forecasting temperatures around seasonal values or even a little warmer than seasonal values at 26C to 27C, but the humidity value should be much more comfortable,” Coulson said.

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