Edit: Thank you all for the detailed answers! You guys are awesome.
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Earth science teacher here. Wet bulb temperature kinda represents how thirsty the air is. If the WBT is close to the air temperature, the air isn’t thirsty, meaning it’s already got a lot of water molecules in it…so your sweat will stay on you, not evaporating. Now, mind you, this isn’t really a problem if the air temperature is reasonable. It only becomes a problem if you NEED sweat to evaporate to cool you.
To understand this, it’s important to remember that for water to evaporate, it needs to take a little heat from somewhere in order to make the jump from a liquid to a gas. In the case of sweating, the sweat takes the heat from YOU, cooling you down.
It’s called wet bulb temperature because it literally comes from a wet bulb. If you wrap the end of a classic glass thermometer in a wet cloth, then let it evaporate, the evaporation cools the thermometer by taking some energy from it (like sweat would cool you.) A bigger drop in temperature means there was more evaporation, which means the air was thirstier.
If air temp is near WBT, the air is wet, so sweating doesn’t help.
If air temp very different from WBT = the air is thirsty, so sweating cools you off.
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