[jcranmer]

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"