It’s not the heat, it’s the humidex that gets you
Meteorologist explains facts behind the measurement of perceived temperature
Ah, the humidex. The factor that can make a summer day in Toronto go from a hot 31C to feeling like a stifling 39 C. The city, and most of Ontario, has been hit with heat warning after heat warning this summer, in part, thanks to a mix of conditions that can make what could be a pleasantly warm day feel like the Amazon jungle instead.
But what exactly is the humidex, anyway? ACanadian invention 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 combination of heat and humidity feels “to the average person.”
“The key thing for humidex is really related to how the human body cools itself,” explains Geoff Coulson, Environment Canada’s warning preparedness meteorologist. “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? The humidex is based on how comfortable an adult in good health would feel in a particular mix of temperature and humidity.
“This is somebody who doesn’t have any underlying medical conditions that may compromise them or make them suffer more under these high heat-humidity situations,” Coulson said. “It also wouldn’t include the elderly or the very young.”
People with health problems, or those who are very young or elderly, 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 high humidity. “The Maritimes can get some occasions when the humidex can get into the uncomfortable range, but in terms of when it happens most fre- quently, 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 the humidex 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 that is similar but also incorporates factors such as 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; comfort levels vary from person to person. 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. Some good news 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 Wednesday, but a front should be moving in Thursday that could bring showers, thunderstorms and cooler temperatures.
“It’s really Friday and the weekend (that) 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 26 C to 27 C, but the humidity value should be much more comfortable,” Coulson said.