Basically, it's when the wet-bulb temperature exceeds 35C. From the Wikipedia entry:
"It has been thought that a sustained wet-bulb temperature exceeding 35 °C (95 °F)—given the body's requirement to maintain a core temperature of about 37°C—is likely to be fatal even to fit and healthy people, unclothed in the shade next to a fan; at this temperature human bodies switch from shedding heat to the environment, to gaining heat from it"
Temperature and humidity are high enough that you can’t cool off naturally via sweat. I would suspect most people don’t realize that’s even a thing. I’m from Texas, and everyone thinks they’re heat adapted and tries to act macho. A lot of people would just stay outside and keep working, ignoring any warnings they received.
In part, I'm sure culture in this particular instance is partly to blame here.
That said, in more general terms, this is a relatively new concern for many people, even in Texas or Arizona, where these wet bulb events were rare. All told, I can understand how people really don't grok it because 10+ years ago, they could keep working in the heat taking relatively simple precautions.
However since 2021 web bulb events have been increasing in regularity and many models suggest that starting this year and going forward, these extreme heat events are going to become the norm, with increasing intensity, making this a permanent concern now. This makes a lot of simple previous precautions folks take less effective or even worthless.
I expect Texas to be the first state to experience a wet bulb event.
Its only a matter of time before we get a combination of extreme heat and a massive failure of their power grid.
Hundreds of deaths in a single day will lead to... nothing.
Same politicians will be voted in and the deaths will just be counted as the price of freedom
We had hundreds of deaths in the span of 2-3 days during the winter storm in 2021. It didn’t catalyze any political change then, so unfortunately I think you’re right about the effect the next time it happens.
I have been surprised by the resilience of the grid this summer. Thank god for solar.
A wet bulb event occurs when the combined temperature and humidity (the so called "wet bulb temperature") exceeds the limits of human physiology, causing humans to overheat and perish even in the absence of any activity beyond basal metabolism.
I'm not sure it'd look like the "bodies in the street" image GP likes to call up, but yes, a prolonged wet bulb event would result in mass casualties.
All that's needed is an intersection of high-humidity heat wave + densely populated area + extended power outage.
And/or too few buildings that have A/C or other places to escape the heat like swimming pools, metro tunnels or whatever.
Probably more likely to occur in less-developed areas like India or Africa. But regardless, could happen in many places. Just a few degrees might be the difference between 1000s or 100k+ deaths.
The wet-bulb temperature is the minimum temperature a wet body can reach via evaporation--effectively the minimum temperature you can reach via sweating. When the wet-bulb temperature reaches body temperature, you can only maintain body temperature via active A/C.
Even if you are the fittest, healthiest human being alive, standing in the shade in a thorough breeze, your core body temperature can only go up. Let it go up, and you get heatstroke and eventually death. And if you're not the fittest, healthiest being alive, or not standing in the shade? It's only going to go up faster, and heatstroke and death will beckon much sooner.
"Wet bulb events" are, in short, when "no A/C means outdoors = death"
Ambient temperature is always at or above the wet-bulb temperature, by definition. A wet-bulb temperature measures the temperature that can be achieved after energy loss due to evaporation of water, so as long as evaporation is possible (humidity < 100%), it is lower than the actual temperature.
"It has been thought that a sustained wet-bulb temperature exceeding 35 °C (95 °F)—given the body's requirement to maintain a core temperature of about 37°C—is likely to be fatal even to fit and healthy people, unclothed in the shade next to a fan; at this temperature human bodies switch from shedding heat to the environment, to gaining heat from it"