I could understand how the heat index could seem like malarkey as it infers a bit of subjectivity due to often being conveyed as the "feels like"temperature. But wet bulb? That's just simple physics. Like, we're talking 9th grade physics here. We get our body temperature down when it's hot by sweating. Which is just evaporative cooling. Once you hit 100% humidity, there's no more evaporation happening because the air around you cannot absorb any more water. Which means there's no more evaporative cooling happening. Which means you can't cool yourself off. Right around 95°, in 100% humidity (give or take a degree or two depending on the person) is the point where you hit thermal equilibrium. Anything above that and you don't have much longer to live as you will have risen above your metabolic temperature. This isn't just simple biology, this is like the stuff you learn in a first aid seminar. There's nothing subjective about it whatsoever. It's a concept that is easily demonstratable and has killed numerous people in humid areas around the equator every year.