It's Not the Heat, It's the Humidity.

Anyone who grew up in southern Ontario before the era of central air conditioning is all too familiar with the line, "it's not the heat, it's the humidity." As we sweltered and panted, regularly rehydrating and sometimes popping salt pills, we might be told of just how much better it was to be in a hot, dry desert. Right.

Turns out that is right.

It's becoming less of a rarity in recent years to see hotspots around the Earth hitting 50C on some days.  Iran and Iraq are a couple of examples.  Marble Bar, Australia, has also hit that mark.

But, when it comes to our newly minted Oven Earth, it's still the humidity that'll get you. Why? Because on days of extreme heat, high humidity can block the body's ability to cool itself through the evaporation of perspiration. You'll still sweat like a stuck pig, it just won't do you enough good to save your hide.

In January 2015, thermometers in Marble Bar, Western Australia, touched 50 °C – a single degree shy of the national record. But it’s extreme humidity records we should be taking more notice of, a wave of new research suggests.

As the climate changes, deadly heatwaves that combine high temperatures with humidity so severe that the human body can no longer cool itself, could start to affect regions of the world currently home to hundreds of millions of people. That’s the conclusion reached by Columbia University’s Ethan Coffel, reported at an American Geophysical Union meeting in San Francisco in December.

Coffel’s study used the latest IPCC climate projections for 2060 and found regional, relatively near-future effects from modest heating.

This extreme humidity is less likely to occur in arid spots like Marble Bar. Coffel’s climate models suggest that there is more risk in India, West Africa, Iran, Saudi Arabia and other countries along the Arabian Gulf – environments where hot air meets very warm coastal waters.

To model these events, Coffel looked at a number called the wet-bulb temperature, which combines heat and humidity into a single metric.

Wet-bulb temperature is taken by placing a damp cloth over the thermometer’s bulb. Evaporation cools the bulb, the same way perspiring cools the body. As humidity increases, the cooling effect slows. For many mammals, including humans, 35 °C wet-bulb temperature is critical.

“In theory, a 35 °C wet-bulb temperature is the point at which your sweat will not evaporate,” Coffel says.

At that point, even the fittest young adult is unlikely to survive more than a few hours before fatally overheating. But lower wet-bulb temperatures can still claim the lives of the elderly or infirm. Deadly heat waves in India and Pakistan that killed 5,000 people in 2015 only produced wet-bulb temperatures in the range of 29-31 °C, he says.

Of course the wealthier nations affected can cope provided they have a suitable electrical grid and plenty of air conditioning. The problem becomes what befalls their people if the electrical system malfunctions or, worse, is taken down in a cyber-attack? In the scheme of things this presents an enormous potential vulnerability.