Title: Humidex? That's not what 40 feels like
Author: Tom Spears
Date: July 14th, 2017
Source: https://ottawasun.com/2017/07/14/humide ... feels-like
School’s out but the Science of Summer continues. Today, Postmedia’s Tom Spears looks at the wonky world of telling people that every summer day is hotter than the thermometer shows.
Thursday was a pleasant summer day in Windsor  27 C and mostly sunny with a west wind. You’d think it would be the same in Detroit, just across the river.
Detroit’s temperature and humidity were almost exactly the same. But then came the human need to interpret.
In Detroit, the Weather Network reported a “feels like†equivalent of 30.5 C. But in Windsor, Environment Canada had a humidex of 38. They were telling the public that the weather felt 11 degrees hotter than the real temperature.
Once again, Canadians were being fed numbers that soared far above the real temperature, and far above what Americans are told about their weather in identical conditions.
Summer is a season when every sunny day we are told it feels hotter than it really is. If it’s 25 C, it feels like the low 30s. If it’s 32 C, it edges into the 40s.
You won’t find a sunny day in July or August where the weather forecast tells you it’s 27 C, and feels like 27 C.
Now, as TV and radio weather forecasters increasingly tell people the humidex instead of the temperature alone, it’s time to ask: how solid is this number?
First, the humidex is not issued in Celsius degrees. In proper usage, one can’t say the word “degrees†when discussing humidex. The humidex isn’t 30 Celsius, or 30 degrees, it’s just 30 with no further description  a point on a scale that is not in fact temperature.
But that distinction doesn’t always get through to people. When you hear that the temperature is 27 but the humidex is 38, your brain doesn’t beep out a little warning that these are two different units of measurement. It just thinks “hot.â€Â
In fact, the humidex isn’t even 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 Ottawa residents 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 email 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 6 am bc it’s over 30 degrees by 7am
• you’re relieved when it’s the low 40s.â€Â
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.â€Â
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.
