Humidex? That’s not what 40 feels like

a measurement. It’s a fairly arbitrary calculation where someone estimated that a certain amount of humidity makes heat more oppressive because the sweat just lies on your skin instead of evaporating and cooling you. And the underlying biology is true. Sticky air really is unpleasant.

But Canadians are routinely told in summer’s hottest days that the humidex is 40 or more, even on days when people are cycling to work and playing tennis. Forty? Really? I asked my daughter, who has been living in dry, southern New Mexico since 2015, what days of 40 C and up are really like. The e-mail answer came in a flash: • bought gummy bears and by time got home 10 mins later all melted • can’t touch steering wheel for first 5 mins in car bc burn fingers • get out of pool and by time pick up towel you’re already dry • ppl literally hang out at mall where best store is dollarama bc home air cons aren’t strong enough • spf 60? Good for about 5 mins • lip gloss in purse turns to liquid and leaks all over your purses • you think the breeze will relieve things but just thrusts more hot air at you • people literally fight over shade • runners go at 6am bc it’s over 30 degrees by 7am • you’re relieved when it’s the low 40s.

Not, then, quite what we see here.

Which brings us back to the vast difference between Canadian and U.S. indexes. If you combine temperature and humidity and multiply them by some factor to get a perceived heat, there is clearly some room for variation. Canadians and Americans don’t use the same formula.

Canada’s humidex has led to some values that even David Phillips of Environment Canada finds dubious. In July 2007, the temperature in Carman, Man., was 34 C. But the humidex hit 53.

“Well, that’s almost a 20-degree difference,” he said. That 53 in Carman is one degree from the generally accepted hottest temperature on the planet, which was in Death Valley (54 C). “It just seems a little unreal.” His native Windsor had a similar day in 1953: Temperature 35 C, humidex 52.

And when the Toronto humidex hit 42 last summer, Phillips shook his head again. The only time Toronto passed 40 actual degrees, during three July days in 1936, 800 people across Ontario died.

And there’s a further glitch. When it’s 25 C but there’s a humidex of 32, are we saying it feels like a 32 C day with no humidity? Not much of a comparison, is it? Because (a) we never get 32 C and dry, so it’s a comparison with conditions that never happen here; and (b) hot and sticky will never feel anything like hot and dry. They are different. I’ve tramped around enough of the New Mexican desert to know.

Maybe some day a forecaster will say it’s 30 degrees, and this is what 30 degrees feels like in Ottawa.

That would be nice. In the meantime summer will end all too soon, and with it the humidex. And eventually December will be cold — but colder with the wind chill.