It was developed initially by the US military in the 1950s to identify levels of heat stress, restrict activity and monitor required rest intervals and water needs for soldiers.
Known as "the wet bulb globe temperature", it takes into account temperature, humidity, wind speed, and exposure to sunlight. When you combine all that information, you get a better indicator of how comfortable the weather really is.
The wet bulb globe temperature formula produces an adjusted temperature, one that is never as high as the number you get from measuring outside air temperature.
But although it may look like a lower temperature, even a few degrees of heat can make all the difference to how our bodies function.
If your core body temperature gets too hot, your organs will start to fail - and that kind of heat stress can be deadly.
To keep our bodies working properly, we eat. Those calories give our body the energy it needs for muscles, organs and the nervous system to work properly. The biochemical reactions that turn the calories from the food into energy produce heat.
When outside conditions are temperate, this heat keeps us at a comfortable core body temperature of around 37C.
As the external temperature increases, that heat has to be lost from our bodies, which is done by dissipating it through our skin.
That's why when you are hot, you sweat, take off layers of clothes, stand near a fan, go into the shade, do less exercise, and drink water - all very good ways to quickly get the heat out of your core through your skin.
But when this temperature gradient is reduced, for example by an increase in temperature and humidity, your body has an increasingly difficult time getting rid of this heat quickly enough.
And like paper, our vital organs have a self-combustion point - that is, a temperature at which they stop working.
Once your core body temperature reaches 40C, your organs begin to fail, and unless you get into cooler conditions immediately, you will die.
So how hot does it have to be to make your core body temperature rise to lethal levels?
Depending on a range of conditions, as measured by the wet bulb globe temperature, it's commonly thought that your skin temperature has to be below 35C for your body to effectively dissipate the heat that it is producing.
This means the wet bulb globe temperature must be well below 35C, which, fortunately, it is in the vast majority of the inhabited places on the Earth.
Complicating this daytime heat extreme is a longer term factor. If the higher temperatures do not relent at night, resulting in a run of hot days and hot nights, people will die.
That's what has happened during heatwaves in many parts of the world over the past decade, including in Europe, Russia and the US.
In many of these places, deaths have occurred where the temperatures are considered only "moderate". That is because people have not been able to acclimatise to extremes in temperature, either physiologically or by taking precautions such as by resting inside during the heat of the day.
Why would a climate scientist be thinking about what is ostensibly a health problem?
As the Intergovernmental Panel on Climate Change's forthcoming climate impacts and adaptation report is likely to suggest, Australia and much of the rest of the world are projected to have an increase in extreme weather.
That is expected to include rising average temperatures in many parts of the world, and more frequent heatwaves in Australia.
What does that mean for us today? The climate change projections indicate that in less time than it will take for a toddler today to reach retirement age, that many areas of the world will begin to experience conditions inhospitable to humans.
These projections indicate we must not only reduce global greenhouse gas emissions, but also be prepared to live in a more extreme world.
• Donna Green is a senior lecturer and researcher at the Climate Change Research Centre, and associate investigator for the ARC Centre of Excellence for Climate Systems Science at the University of New South Wales.
