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When air flows over a wet surface, some of the water will evaporate into water vapor. This change from liquid water to water vapor absorbs energy (cools). This cooling from the evaporation of water is called evaporative cooling.
There is a limit to how much water vapor the air can hold. Hot air can hold more water vapor than cold air. This is why winter air is so dry – cold air can hold little or no water vapor.
The wet bulb temperature is the temperature which would be achieved if the maximum possible amount of evaporative cooling occurred. In other words the temperature after the cooling caused by evaporation if water evaporated until the air became saturated (I.e., 100% humidity).
This is important because humans use evaporative cooling to maintain body temperature. Humans sweat when hot, this sweat evaporates (it is especially effective if air is blowing over the sweaty body parts), and the evaporation of sweat cools the skins surface.
The wet bulb temperature represents a limit on human body cooling by sweating. As the wet bulb temperature approaches skin temp of a human body (97F/35C), we lose the ability to cool down by sweating. We lose the ability to regulate our body temperature. This makes the location unsurvivable without external cooling.
With global warming, we’re seeing locations approach this point where the wet bulb temperature approaches the human body temperature, making places unsuitable for human life.
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