Work out wet bulb temperature instantly with clear inputs, formula shown and shareable results.
Wet bulb temperature is the lowest temperature reachable by evaporative cooling, computed here with Stull's empirical fit. At 32 C and 70% humidity it is about 27.5 C. It matters because human cooling depends on sweat evaporation: a wet bulb of 35 C is the theoretical survivability limit, since the body can no longer shed metabolic heat at all.
Stull approximation
Tw = T atan(0.151977 sqrt(RH + 8.313659)) + atan(T + RH) - atan(RH - 1.676331) + 0.00391838 RH^1.5 atan(0.023101 RH) - 4.686035
Wet bulb depression
depression = dry bulb - wet bulb
Because skin sits near 35 C, so no evaporative gradient remains. Metabolic heat then accumulates regardless of shade, water or fanning.
Within about 0.3 C for humidity between 5 and 99% at normal pressures - ample for heat stress assessment.