It’s not the U.S. heat, it’s the Canadian humidex

Here’s an offbeat solution to Ottawa’s heat wave: Invite the U.S. National Weather Service to run things and cool us all down.

Somehow or other, American cities with weather just like ours — same heat and humidity — always end up with “feels like” temperatures that are a lot cooler than our sky-high humidex.

Is it really cooler across the border? Let’s do the math and find out.

First, the Citizen looked at the curious case of Detroit and Windsor, which face each other across the Detroit River.

At noon Wednesday, they had virtually identical temperatures (27.7 in Windsor, 28.0 in Detroit.) The humidity was around 70 per cent in both.

Yet the Windsor humidex was 37, and the Detroit “feels like” factor was only 30.

Thursday morning, Toronto and nearby Buffalo were both at 24 C. The humidity was 85 per cent in Toronto, 86 per cent in Buffalo. Grey and drizzly in both cities.

Buffalo had a “feels like” figure of 24. It felt like the actual temperature. But in Toronto the humidex was a steamy 32.

It raises the question of whether the humidex is exaggerated. After all, this is an index that sometimes has values in the low 50s in Canadian cities — nearly as high as the hottest temperature ever recorded on Earth (56.2 C).

So, we asked David Phillips, why is our humidex so much higher than indexes in the United States that attempt to capture the same feel of hot weather?

Phillips, Environment Canada’s senior climatologist, likes the Canadian humidex best.

“It’s so interesting. I’ve talked to some Americans (and) they actually like the humidex. They think it’s more accurate,” he said.

Phillips argues the U.S. system underestimates the true feeling of humidity.

But more than that, he likes the idea of a figure that emphasizes a major difference between dry and moist heat. For him it’s a safety measure.

“You want people to say, ‘Hey, pay attention, there are some things out there that are injurious to you, in particular some groups of the population like infants and seniors and homeless and people who work outdoors.’ ”

He also argues that temperature readings alone can mislead some people because they are always taken in the shade, and people then have to go into the sunshine, where it’s warmer.

“It works … I’m a fan of the humidex.”

He acknowledges that some of the upper values are pushing the limits and may be unrealistically high. For instance, Ottawa’s humidex repeatedly hit 40 this week. “Nobody in Ottawa could ever say they lived through that sort of temperature unless they lived in Libya, or Death Valley, California.”

He has worked out the number of days in an average year that different cities reach a humidex of 35 or higher. They include: Windsor: 30 Toronto: 18 Ottawa: 16 Montreal: 15 Winnipeg: 12 Medicine Hat: 4 Calgary: less than one a year Vancouver: less than one a year Whitehorse: 0 Phillips believes the highest humidex reading Ottawa has ever reached was 46 on July 1, 1955.

Here are the major indexes in use:

The humidex, a Canadian invention. It combines heat and humidity to get a number that is commonly seven or eight points above the actual temperature. These numbers are NOT degrees, even though your radio station may say so. They are approximations of what the weather feels like.

The U.S. heat index, run by the government weather service. You can use the official website here and punch in Canadian conditions, as it accepts Celsius. It asks for temperature and humidity, and gives an index that is often just two or three points above the (Celsius) temperature. But it will not accept temperatures below 26.7 C (80 F).

“Feels like” numbers come from the Weather Network. They are similar to the heat index.

Accuweather, also a private U.S. company, promotes its “real feel” index which factors in wind as well.