How hot and humid will your city be in the future?

The humidex is a uniquely Canadian way of calculating how hot it feels when the air temperature is combined with humidity.

A humidex above 35 is considered to be high for the average healthy adult, according to Environment and Climate Change Canada. That’s when it’s generally recommended to tone down outdoor physical activity to prevent heatstroke.

Humidity makes heat especially dangerous at a point called the wet-bulb temperature. Humans cool down by sweating, releasing heat through evaporation. When the air is saturated with moisture, that process doesn’t work as well.

In some major cities, like Toronto and Montreal, checking the humidex in the summer is as normal as checking the wind chill in the winter.

But in other regions, especially parts of Western Canada, hot and humid days are rare or virtually non-existent.

That could change.

Without deep and immediate cuts to greenhouse gas emissions, climate change could significantly increase the average humidex. To put it simply: climate change means more days with high temperatures, and as air warms, it can hold more moisture, making it more humid.

In all of Canada, southwestern Ontario is likely to see the most number of days a year with a humidex above 35.

Take Windsor, Ont., for example: Even in a low-emissions scenario, the number of days that feel warmer than 35 C could climb from around 26 a year historically (1981-2010) to around 57 a year by the end of the century. In a high-emissions scenario, that could jump to around 102 days a year.

Edmonton, on the other hand, could see days that feel warmer than 35 C climb from around one a year historically (1981-2010) to around four days a year by the end of the century in a low-emissions scenario. In a high-emissions scenario, that could jump to as many as 31 days a year.

While some parts of the country, like southwestern Ontario, are likely to be better adapted to high humidex because people there are already used to it, regions less familiar with humid heat might not be ready for what’s to come.

“Regions not used to experiencing high humidex values may be even more vulnerable,” said Kenneth Chow, a climate scientist with Environment and Climate Change Canada.

Most homes in Toronto or Montreal, for instance, have some form of air conditioning, but most homes in Edmonton and Calgary do not.

Chow said hot and humid weather is so rare in parts of Western Canada that some people there have never even heard the term “humidex” before.

One of Canada’s deadliest heat events — the 2021 Western heat dome — saw temperatures soar to nearly 50 C in some places, leading to 619 deaths in British Columbia and an estimated 66 deaths in Alberta. But that incident happened in hot and dry conditions.

When humidity joins in, heat can be lethal at much lower temperatures.

According to Environment and Change Canada, a humidex in the mid to high 30s is when the average healthy person should be more careful. Over 40 is considered extremely high and all unnecessary physical activity should be avoided.

Glen Kenny, the director of Ottawa University’s Human and Environmental Physiology Research Unit, adds a caveat to those general guidelines. He said what’s a safe range for one person could be over the limit for someone else.

“The humidex, essentially, is one side of the equation,” he said. Other components of the heat equation include environment, physical activity and clothing.

For instance, a young, healthy adult living in Texas who is used to working in hot and humid conditions would tolerate a high humidex differently than someone from Victoria, B.C., or Iqaluit.

Kenny said seniors typically have a harder time tolerating heat, and female bodies in general don’t cool off as easily as male bodies. Other factors like sleep deprivation, several days of exposure to high heat and health conditions such as diabetes can also lower someone’s tolerance.

Kenny said projections can help people understand how they might need to prepare for the changing climate — looking at everything from how to keep cool at home to expanding the number of parks and providing community spaces to help beat the heat.

Chow said even in places that are used to high humidity, the sheer increase in potential high humidex days in coming decades is a concern, especially for the most vulnerable populations.

“There definitely needs to be continued efforts in building resilience,” Chow said.

Methodology

This humidex data comes from the Power Analytics and Visualization for Climate Science (PAVICS) platform, which contains projections from 30-year averages from the CMIP6, the most current global climate model data available. The historical data goes from 1950 to 2014. The projections start in 2015 and end in 2100.

There are three emissions scenarios in this project: high emissions (CMIP6 SSP 5-8.5), medium emissions (CMIP6 SSP 2-4.5) and low emissions (CMIP6 SSP 1-2.6).

Available locations are Canada’s largest population centres, capitals and locations of CBC/Radio-Canada stations.

The formula for calculating humidex comes from the Canadian Centre for Climate Services.