The wet bulb temperature is the temperature of a bulb of mercury inside a damp cloth, in the shade. Basically, what's the temperature taking evaporative cooling into account? When the wet bulb temperature is too high, that's dangerous for human beings, since we depend on evaporative cooling - we're essentially wet bulbs. If the wet bulb temperature and the dry bulb temperature are the same, you're not getting any benefit from evaporative cooling.

Wet bulb takes a minute to grok because it's not about *heat,* per se. It's about the absorptive capacity of air. A wet bulb temperature in the mid-80s F can, and does, kill humans. Heat waves in the EU & Russia in 2003 and 2010 killed over a hundred thousand people at ~ 82 F.

— (((Matthew Lewis))) has some Shoup for you (@mateosfo) June 29, 2021