What are ‘wet-bulb’ temperatures — and why can they be so deadly?

Humans lose about 80 per cent of heat through sweating, but when both the humidity and air temperatures are high, it's harder to shed that heat. 

A heat dome in the western U.S. continues to smash temperature records, the latest in cases of significant heat around the world so far this year.

In Canada, temperatures are climbing, with Ottawa soaring past 30 C on Wednesday. But with more heat on the way people should be aware there can be a temperature that’s too hot for humans.

When people check the weather they often look at the temperature, also known as the “dry-bulb temperature,” and the humidity to get a sense of how hot it will feel.

Some meteorologists say people should also be aware of the “wet-bulb temperature,” which can signal deadly heat.

It gets its name from the act of covering a thermometer with a water-soaked cloth. When water evaporates and lowers the temperature on the device, it mirrors how the human body cools down with sweat.

But the more humidity there is in the air, the harder it is for that moisture to evaporate and cool the thermometer — or you — down.

“If you’re at 100 per cent humidity then you can’t evaporate any water and so that’s the reason why humidity is such a big impact, because it does really increase our heat stress,” Kent Moore, an atmospheric physics professor at the University of Toronto Mississauga, told Global News.

“Although the temperature might be the same, we can’t cool ourselves off.”