Canada’s humidex: confusing, flawed, but life-saving

After days of sweltering, oppressive weather in Ontario and Quebec, we've seen 'humidex' values come up again and again in the news and weather reports, but as we suffer through the heat and humidity, do these values really help us?

Although the humidex — or humidity index — isn't perfect, it's meant to be a way of combining temperature and humidity into something a bit more useful for us. Go onto the Environment Canada weather page for your city (or any other) and you'll see values for temperature, dewpoint and humidity, which are all related to each other, but unless you've sat down in a few atmospheric science or meteorology classes, exactly how they're related to each other isn't so clear.

[ Related: Hot, sticky weather settles in over Ontario, Quebec ]

We're all familiar with temperature. 'Dewpoint' is short for 'dew point temperature', which is the temperature the air would have to be at for water to condense out, ie: for dew to form. 'Humidity' is short for 'relative humidity', which is just a percentage value of how close the temperature is to the dew point temperature.

However, do those values — as they are — help you if you're trying to figure out what it feels like outside?

We generally know that if it's 30 degrees out, it's hot, and if it's 40 degrees out, it's HOT. Those temperature have a certain gut-feeling attached to them. You might be fine with spending most of the day outdoors when it's 30°C, but you'd probably hesitate to do so if it was up to 40 degrees.

When you're inundated with "it's 30 degrees out, with a dewpoint of 19 degrees and a relative humidity of 52%", you might say "Whoa, whoa... that doesn't really mean anything to me. Can't you just tell me what it feels like?"

That's what the humidex value tries to do.

According to Environment Canada's scale for the index, humidex levels in the 30s are considered 'uncomfortable', values in the low 40s are labeled as 'great discomfort' and they caution people to limit their exertion, and anything above 45 is 'dangerous', with the possibility of suffering from heat stroke.

[ Related: Eastern Canada continues to sweat as humidex tops 40 ]

There's a confusing part to humidex, though: it's just a number. A humidex of 40 is just that. It's not a humidex of '40 degrees'. The reason for that is due to some of the rules of science and the somewhat complicated calculation used to find the extra bit we're adding to the temperature. However, the humidex value is meant to be a proxy for temperature. So, if the humidex is 40, regardless of what the temperature actually is, you can expect that if you go out into that hot, sticky weather, it's going to have roughly the same effect on you as if you were out on a dry, 40°C day.

The reason for that is from the way our bodies cool down.

"The body is a great air conditioner. It really truly is," Environment Canada senior climatologist David Phillips said in a CBC News interview. "The way it air conditions is when we perspire, that moisture in our skin evaporates into the air and that takes away some of the heat from our body."

The humidex value isn't perfect though. One flaw in the index is that it doesn't take into account how a breeze can help remove sweat from our skin and thus make it seem cooler. Another is that it won't tell you the difference between what it feels like standing in the shade vs standing in the sun. Still, since we can't depend on the breeze on days like this, and we may be in and out of the shade as we make our way around, it gives us an idea of what to expect, so we can plan accordingly.

It saves lives too. The simpler we can make this kind of information, so that it can be absorbed in a moment, without having to ponder over it, the better. It makes it easier for us to make decisions about our day, it's easier for us to know when we should be checking on our family and friends and pets, and it's easier for officials to know when they issue alerts and open up cooling stations, like the cities of Toronto, Hamilton, Ottawa and Montreal have already done this week.

[ Related: Heat wave prompts health alerts in Ontario, Quebec ]

Humidex values were in the high 30s across Ontario and topping 40 in Quebec earlier this week, but as this stagnant weather has stuck around, it's been ramping those numbers up.

Yesterday, the humidex reached 44 in Toronto, it got up to 43 in Montreal, but the 'winners' were places like Windsor, London and Ottawa. The temperatures in those cities didn't get up to Toronto's high of 34°C, but they all reached 45 on the humidex.

More of the same sweltering, oppressive weather is in the works for today and tomorrow. Temperatures in the mid-30s expected across southern Ontario, and slightly cooler in eastern Ontario and southern Quebec where they're likely to stay below 30 degrees for the rest of the week. The humidity will still make it feel about 10 degrees hotter, though, with humidex values reaching the mid-40s in southern Ontario and the mid-to-high 30s.

Relief is still on the way, though, as the the weekend is expected to bring cooler, drier weather along with it, giving us a break for at least a few days.

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