Toronto residents are in store for yet another round of soaring temperatures from Sunday to Tuesday, Environment Canada says.

For example, when the air temperature is 100 degrees Fahrenheit, and the relative humidity is 55 percent, the heat index can shoot up to a sweltering 124 degrees Fahrenheit. And that’s in the shade.

CREDIT: National Weather Service / NOAA. Feels-like: heat index While the old adage “it’s not the heat, it’s the humidity,” sounds a bit clichè, it is true.

A new heat dome is forming across the United States, locking in sweltering conditions from the Gulf Coast to the Midwest and ...

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.

The heat index and humidex are the most common values. Heat index readings are common in the U.S. and around the world, while the humidex is a uniquely Canadian metric.

The wet-bulb globe temperature is one way meteorologists quantify heat. It includes not just the air’s temperature, but also factors including humidity, wind, and sunlight.