Glasses by Chado Nihi, Pixabay / Wynn Pointaux thermometers, Pixabay
Every winter, meteorologists chatter hyperbolically before vast arrays of charts, maps, and satellite images. They’re particularly fond of explaining not only how cold it is outside – any thermometer can do that – but also how cold it feels. For this they use a grid known as the ‘windchill’ index, and anyone who ventures out in the cold – from Antarctica to the northern temperate zone – knows how critical that windchill factor can be, both for comfort and for survival.
Modern science having been reared where cold temps were common, the analogous indices for heat are less well known. But as the world warms, heat waves are becoming more widespread, frequent, unseasonal, and severe, and we’d all do well to become savvier about the Heat Index (HI).
The Heat Index – the tool meteorologists use to measure how hot it feels, and the danger heat represents – is increasing in importance even as the windchill factor lessens. Indeed, to every EMT, physician, athlete, event planner, and lifeguard, an awareness of the heat index must now be considered a standard safety precaution.
Heat Index Chart. Source: National Weather Service /
National Oceanic and Atmospheric Administration (NOAA)
There are actually two indices used to express the impact of heat: the Heat Index and the WetBulb Globe Temperature (WBGT). The first uses a complex formula factoring temperature in the shade with relative humidity to express how hot it will feel in given conditions.
The second, more comprehensive formula, WBGT, combines temps in direct sunlight with humidity, wind speed, sun angle, and cloud cover, to render a single value for heat stress. It gives a more localized and detailed reading of the actual physical impacts of heat in a specific place and time. And, like windchill, both these metrics are more than academic exercises: they can represent the difference between life and death.
Under normal conditions, the human body’s thermoregulatory strategies keep us comfortable and safe. For cold, we radiate insulating heat from within. When wind whisks that insulation away – the windchill factor – we feel colder and are more easily subject to frostbite and hypothermia than when the air is still.
Heat, on the other hand, promotes sweat, through which evaporation – an inherently cooling process – keeps us comfortable. But higher temps and humidity can stifle that evaporative process and the cooling it provides. Fatigue, cramping, heat exhaustion, even life-threatening organ damage and heat stroke, can result.
Source: U.S. National Weather Service
Now, all this should have remained standard meteorological fare, trotted out only on rare occasions when the thermometer broke 90˚F for a few days. But there has been a decades-long, unambiguous upward trend with respect to heat waves, and that trend is accelerating as the world’s ambient temperature rises.
Indeed, according to the IPCC, “with every additional increment of global warming, changes in extremes continue to become larger… every additional 0.5°C of global warming causes clearly discernible increases in the intensity and frequency of hot extremes, including heat waves (very likely), and heavy precipitation (high confidence).”
Hmm. More intense and frequent heat extremes, coupled with higher humidity: what could go wrong? As the HI and WBGT make clear, plenty.
Heat Wave Characteristics in the United States by Decade, 1961–2019.
Source: U.S. Environmental Protection Agency (EPA),
Climate Change Indicators: Heat Waves, Feb. 2022
Lest anyone think said trend is some garden-variety geological shift, scientists say the months-long spring heat wave that hit south Asia in early 2022, and caused “an estimated 10-35 percent reduction in crop yields in Haryana, Uttar Pradesh, and Punjab,” was about 30 times more likely and 1˚C (1.8˚F) hotter than it would have been before the industrial era. Of the massive heat wave that struck the West Coast of North America in 2021, scientists predict that, “looking into the future, in a world with 2°C of global warming” (which is expected as early as the 2040s), “an event like this – estimated to occur only once every 1,000 years, would occur roughly every 5 to 10 years.”
And, while climate researchers have long talked in the abstract of the need to hold warming to <1.5˚C, there’s nothing hypothetical about the human-health consequences of failure. As the IPCC asserted in their most recent report, “limiting global warming to 1.5°C instead of 2°C could result in around 420 million fewer people being frequently exposed to extreme heat waves…” That’s approximately the population of the United States and France combined. And keep in mind that +2˚C is looking ever-more like warming’s floor rather than ceiling. People, hundreds of thousands more than in the past, are going to be exposed to, and die from, heat.
Photo by Logga Wiggler / Pixabay
The future estimates are sobering. Currently, in the U.S. alone, there are some 12,000 heat-related deaths annually. Under the IPCC’s moderate Representative Concentration Pathway (RCP4.5 scenario), between 2080 and 2100, that number will quadruple, meaning an additional 36,000 people will succumb. The high-warming scenario (RCP 8.5) is truly frightening. Under it, during the same period, the U.S. would see a truly staggering additional 97,000 annual heat-related deaths. Africa, South Asia, Central and South America, the pacific islands, the Mediterranean region, Australia… millions would die of heat on virtually every continent save Antarctica.
It should be noted that this discussion focuses on the impacts of heat on the human animal directly. However, there are more far-reaching secondary and tertiary impacts of the heat waves to come: lost productivity, health-care challenges, drought, famine, thirst, and of course the wildfires that appear in the wake of intense heat. According to the International Labor Organization (ILO), “an increase in heat stress resulting from global warming is projected to lead to a global productivity loss equivalent to 80 million full-time jobs in the year 2030.” Eighty million jobs! Only 8 years hence?!
Meanwhile, in late June of 2021, a heat wave struck the Pacific Northwest, peaking at 49.6˚C (121.3˚F), a new Canadian record, in the small town of Lytton, British Columbia. A day after the record was set, “Lytton was largely destroyed in a wildfire,” according to World Weather Attribution. And for those with very short memories, the brutal and record-breaking heat that descended on almost the entire United States, the U.K., and most of Western Europe this very July, is a prime example. Many of the highs were more than 20˚F (11.1˚C) above average local temperatures for that date. The record-setting heat in Siberia in 2020, also found to be 600 times more likely than in the pre-industrial period – and virtually impossible without human intervention – suggests these heat waves are already upon us.
A wildfire burns in Lytton, British Columbia. Sentinel 2-A Satellite, 1 July 2021.
Photo by Antti Lipponen. Source: Wikipedia
The heat indices noted above are no less than survival equipment, to be consulted before venturing out on land, sea, forest, or plain, all around the world. The 21st century will be – and will feel like – a crucible for the race.
We don’t know with certainty exactly how hot the heat waves will be; just how often they will arise; precisely how much longer they will last, or for how long their seasons will be extended. But we do know this: the heat index should be in every pocket, wallet, backpack, vehicle, and event plan, and on every athletic director’s desk. It should be referenced as casually, and as naturally, as checking to see if it’s raining. Winter still has its chill, but heat – coming in waves – will prove the more important factor.
 The International Meteorological Association’s definition of a heat wave requires sustained temperatures at least 9˚F (5˚C) above average for a given location.