The term — a Canadian innovation — is one used most often by southern Ontarians and Quebeckers, though Canada's weather guru says that's changing.
"I get more calls in Saskatchewan and Manitoba now: 'What is this thing called humidex? Is it something that Toronto manufactured?'" says David Phillips, Environment Canada's senior climatologist.
Those calls are because the typically dry Prairie provinces are experiencing higher humidity levels due to increased air flows from the south these days, and these are adding to the high moisture levels caused by the "feverishly" growing crops, says Phillips.
But he also notes that, for most Westerners, the higher humidity "is not nearly as debilitating because they cool down at night. So there's not nearly as much tossing and turning."
For years, the southern Ontario city of Windsor earned the reputation as the humidex capital of Canada. It held the record of a humidex of 52.1 from June 20, 1953 for more than half a century.
"That has been surpassed by an unlikely place," said Phillips. Carmen, a rural farming town in southern Manitoba, hit a humidex level of 53 on July 25, 2007.
Body a 'great air conditioner'
The humidex — short for humidity index — is a Canadian innovation first used in 1965, according to Environment Canada.
If the forecast cites a humidex of 40 C, for example, it means that the temperature might be 35 C but, with the humidity, the discomfort feels like it would at a dry temperature of 40.
The index is based on a calculation of heat and humidity by using current air temperature and the dew point (the temperature and barometric pressure at which water vapour condenses into liquid). It matters because humidity can wreak havoc on a body's internal cooling systems.
"The body is a great air conditioner. It really truly is," says Phillips. "The way it air conditions is when we perspire, that moisture in our skin evaporates into the air and that takes away some of the heat from our body."
Bodies try to maintain a temperature of 37 C. In summer, sweat helps cool you down.
But when the humidity is high and the air is already nearly saturated with moisture, sweat evaporation stops. That's when a person's body temperature rises and they can suffer a slew of heat-related problems, from a minor heat rash to a potentially deadly heatstroke.
"The issue really is a health issue," says Peter Taylor, a professor of atmospheric science at Toronto's York University. The humidex "gives you a measure of how effectively your body can cool down,"
Providing the public with a way to assess the danger of the sweltering outdoors is the reason for the development of the humidiex in the first place. But it really is kind of an imperfect index by science's standards, and it is one of several ways of trying to sound the misery alarm about summertime heat.
When the humidex reaches 30 to 39, Canada's weather agency suggests those outside may feel "some discomfort."
That rises to "great discomfort" from 40 to 45, when residents are told to avoid exertion. Above 45 is considered dangerous since it brings the possibility of heatstroke.
In the past century, there have been numerous attempts in North America to try to calculate how uncomfortable people feel in the sticky heat.
Among them are the discomfort index, humiture index and even the aptly named humisery index.
The winter equivalent to the humidex is wind chill — an index ironically invented by Americans instead of by someone north of the 49th parallel.
Phillips says that the wind chill measurement endures even more criticism that its summertime counterpart, but both are imperfect.
"It's kind of a fudged quantity. But it's almost like it's got a hold, it's become popular."
"It may be impure from a scientific point of view, but it is actually proven to work. There is more stress on the body in a humid situation. You can't be as productive on a humid, moist day as you would be on a hot, dry day."
The humidex also doesn't take into account certain factors that can change how hot it feels, such as strong winds — which help wick away perspiration — and whether the person is walking in the sunshine, which significantly increases how hot it feels.
The climatologist also notes that no two people react the same way to weather. Age and health, including respiratory issues and fitness levels, affect how hot it feels.
"The same kind of conditions of heat to a child might cause prickly heat and to a teenager it might be something like heat cramps, while to a senior it could be heatstroke," he said.
While it's helpful to pay attention to the humidex, ultimately, people must also stay tuned to their own personal limits.
"We all know how weather affects ourselves," says Phillips. "We all have our little sort of threshold that jerks us into a certain action that we take to save ourselves from the weather."