Not new. I heard that in Basic Training (1970) .

Athletic Trainers still do this. As others have said it's nothing new. U.I.L. has a policy in place. For those who can navigate links here is some good info. For those who hate links and are more visual below is the visual.

U.I.L. WBGT Info

Yes, that is where I learned about the sling psychrometer. The jet engine test cell folks had one. I worked electrical systems and several times worked on the test cell.

…it seems like the entire US military has used it since 1956

All I can tell ya boy's is I pounded the "flat rock" (flightline) for 20 plus years in Arizona, Nevada and Saudi Arabia with temps of 115 degrees plus and never heard the term wet bulb mentioned one time.

Last time I ever heard it was the mid 80's when I was stationed at Ft. Hood.

Also used in HVAC systems & engineering.

Were there not “heat flag conditions” back then?

Not a one! TAC /ACC just wants jets in the air.

Dang that sucks! Speaking of ACC, it’s a mess right now…glad I’m out! I did 5 in AFSOC, 16 in ACC (fat girls, not fighters)

Each of those stations are desert regions with little to no humidity.

Humidity and dew point are the differentiators/indicators. No or low humidity and the wet bulb readings are below normal air temp.

I just use the same old term. HOT AS F.

It works & very accurate.