Three different numbers get called "wet bulb" in team group chats, and mixing them up leads to bad practice decisions. Here is what each one actually measures.
A weighted composite built for people exerting themselves outdoors: 70% natural wet bulb (evaporative cooling power), 20% black globe (solar radiation load), 10% dry air temperature. Wind, sun, humidity and heat all move it. Sports heat policies (UIL, GHSA, NCAA, the military) are written against WBGT.
The plain (psychrometric) wet bulb only captures evaporation — no sun load. On a sunny field, WBGT runs well above plain wet bulb. If your weather app shows "wet bulb 78°" on a cloudless afternoon, the WBGT on the turf can already be in flag territory.
Heat index models how hot it feels to a person at rest in the shade, in normal clothes. It has little relevance to a football practice. The scales do not line up: per GHSA's own FAQ, a WBGT of 92 is roughly comparable to a heat index of 104-105.
Some states (e.g. New York) write their policies against heat index instead — that is a policy choice, not an equivalence. Never read a WBGT threshold with a heat index number, or vice versa.