Si vous avez regardé l'indice de chaleur pendant les vagues de chaleur collantes de cet été et que vous vous êtes dit :« C'est sûr qu'il fait plus chaud », vous avez peut-être raison.

Une analyse menée par des climatologues de l'Université de Californie à Berkeley a révélé que la température apparente, ou indice de chaleur, calculé par les météorologues et le National Weather Service (NWS) pour indiquer à quel point il fait chaud - en tenant compte de l'humidité - sous-estime la perception température pour les jours les plus étouffants que nous connaissons actuellement, parfois de plus de 20 degrés Fahrenheit.

La découverte a des implications pour ceux qui souffrent de ces vagues de chaleur, puisque l'indice de chaleur est une mesure de la façon dont le corps gère la chaleur lorsque l'humidité est élevée, et la transpiration devient moins efficace pour nous rafraîchir. La transpiration et les bouffées de chaleur (où le sang est dévié vers les capillaires proches de la peau pour dissiper la chaleur) et la perte de vêtements sont les principales façons dont les humains s'adaptent aux températures élevées.

Un indice de chaleur plus élevé signifie que le corps humain est plus stressé pendant ces vagues de chaleur que ne le pensent les responsables de la santé publique, selon les chercheurs. Le NWS considère actuellement un indice de chaleur supérieur à 103 comme dangereux et supérieur à 125 comme extrêmement dangereux.

"La plupart du temps, l'indice de chaleur que le National Weather Service vous donne est juste la bonne valeur. Ce n'est que dans ces cas extrêmes où ils obtiennent le mauvais numéro", a déclaré David Romps, professeur de terre et planétaire à l'UC Berkeley. la science. "Ce qui compte, c'est lorsque vous commencez à cartographier l'indice de chaleur sur les états physiologiques et que vous réalisez, oh, ces personnes sont stressées par une condition de flux sanguin cutané très élevé où le corps est sur le point de manquer d'astuces pour compenser pour ce genre de chaleur et d'humidité. Nous sommes donc plus proches de ce bord que nous ne le pensions auparavant."

Romps et l'étudiant diplômé Yi-Chuan Lu ont détaillé leur analyse dans un article accepté par la revue Environmental Research Letters et mis en ligne le 12 août.

L'indice de chaleur a été conçu en 1979 par un physicien du textile, Robert Steadman, qui a créé des équations simples pour calculer ce qu'il a appelé la "sensation" relative des conditions chaudes et humides, ainsi que chaudes et arides, pendant l'été. Il y voyait un complément au facteur de refroidissement éolien couramment utilisé en hiver pour estimer la sensation de froid.

Son modèle a pris en compte la façon dont les humains régulent leur température interne pour atteindre un confort thermique dans différentes conditions extérieures de température et d'humidité - en modifiant consciemment l'épaisseur des vêtements ou en ajustant inconsciemment la respiration, la transpiration et le flux sanguin du cœur du corps vers la peau.

Dans son modèle, la température apparente dans des conditions idéales - une personne de taille moyenne à l'ombre avec de l'eau à volonté - correspond à la chaleur ressentie par quelqu'un si l'humidité relative était à un niveau confortable, ce que Steadman a considéré comme une pression de vapeur de 1 600 pascals. .

For example, at 70% relative humidity and 68 F—which is often taken as average humidity and temperature—a person would feel like it's 68 F. But at the same humidity and 86 F, it would feel like 94 F.

The heat index has since been adopted widely in the United States, including by the NWS, as a useful indicator of people's comfort. But Steadman left the index undefined for many conditions that are now becoming increasingly common. For example, for a relative humidity of 80%, the heat index is not defined for temperatures above 88 F or below 59 F. Today, temperatures routinely rise above 90 F for weeks at a time in some areas, including the Midwest and Southeast.

To account for these gaps in Steadman's chart, meteorologists extrapolated into these areas to get numbers, Romps said, that are correct most of the time, but not based on any understanding of human physiology.

"There's no scientific basis for these numbers," Romps said.

He and Lu set out to extend Steadman's work so that the heat index is accurate at all temperatures and all humidities between zero and 100%.

"The original table had a very short range of temperature and humidity and then a blank region where Steadman said the human model failed," Lu said. "Steadman had the right physics. Our aim was to extend it to all temperatures so that we have a more accurate formula."

One condition under which Steadman's model breaks down is when people perspire so much that sweat pools on the skin. At that point, his model incorrectly had the relative humidity at the skin surface exceeding 100%, which is physically impossible.

"It was at that point where this model seems to break, but it's just the model telling him, hey, let sweat drip off the skin. That's all it was," Romps said. "Just let the sweat drop off the skin."

That and a few other tweaks to Steadman's equations yielded an extended heat index that agrees with the old heat index 99.99% of the time, Romps said, but also accurately represents the apparent temperature for regimes outside those Steadman originally calculated. When he originally published his apparent temperature scale, he considered these regimes too rare to worry about, but high temperatures and humidities are becoming increasingly common because of climate change.

Romps and Lu had published the revised heat index equation earlier this year. In the most recent paper, they apply the extended heat index to the top 100 heat waves that occurred between 1984 and 2020. The researchers find mostly minor disagreements with what the NWS reported at the time, but also some extreme situations where the NWS heat index was way off.

One surprise was that seven of the 10 most physiologically stressful heat waves over that time period were in the Midwest—mostly in Illinois, Iowa and Missouri—not the Southeast, as meteorologists assumed. The largest discrepancies between the NWS heat index and the extended heat index were seen in a wide swath, from the Great Lakes south to Louisiana.

During the July 1995 heat wave in Chicago, for example, which killed at least 465 people, the maximum heat index reported by the NWS was 135 F, when it actually felt like 154 F. The revised heat index at Midway Airport, 141 F, implies that people in the shade would have experienced blood flow to the skin that was 170% above normal. The heat index reported at the time, 124 F, implied only a 90% increase in skin blood flow. At some places during the heat wave, the extended heat index implies that people would have experienced an increase of 820% above normal skin blood flow.

"I'm no physiologist, but a lot of things happen to the body when it gets really hot," Romps said. "Diverting blood to the skin stresses the system because you're pulling blood that would otherwise be sent to internal organs and sending it to the skin to try to bring up the skin's temperature. The approximate calculation used by the NWS, and widely adopted, inadvertently downplays the health risks of severe heat waves."

Physiologically, the body starts going haywire when the skin temperature rises to equal the body's core temperature, typically taken as 98.6 F. After that, the core temperature begins to increase. The maximum sustainable core temperature is thought to be 107 F—the threshold for heat death. For the healthiest of individuals, that threshold is reached at a heat index of 200 F.

Luckily, humidity tends to decrease as temperature increases, so Earth is unlikely to reach those conditions in the next few decades. Less extreme—though still deadly—conditions are nevertheless becoming common around the globe.

"A 200 F heat index is an upper bound of what is survivable," Romps said. "But now that we've got this model of human thermoregulation that works out at these conditions, what does it actually mean for the future habitability of the United States and the planet as a whole? There are some frightening things we are looking at." + Explorer plus loin

Hot and getting hotter:Five essential reads on high temps and human bodies