What is Wet-Bulb Temperature and how does it work?


What is Wet-Bulb Temperature and how does it work?

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It’s the temperature when the thermometer bulb is wet. That means it’s impacted by humidity. It more accurately represents the way people perceive temperature than just looking at the dry bulb temperature.

Wet bulb temp includes the effect of evaporation, so it is a closer approximation of how it feels to a regular person.

So what is important about Wet-Bulb temperature is that if that temp is above ~35 sweating no longer works to cool you off. This is the temp you stop working outside and just hunker down in the shade before you die of heat stroke and dehydration.

If you wrap a thermometer in wet cloth and have air passed over it (By waving it around, or pointing a fan at it) it will show a cooler temperature than if the thermometer is just sitting there dry. It shows what the temperature will feel like for a person that is wet (say, from sweat) and trying to cool off by evaporation. (So if it’s humid out, web bulb temperature will be higher than if it were dry out, because humidity prevents evaporation.)

If the wet bulb temperature is much above about 90 F, people are going to start dying without air conditioners. Even someone who is used to the heat can’t survive that for long.

Wet-Bulb isn’t just a temp, it is a humidity ratio of 100%. Relative humidity is the amount of water in the air compared to the amount of water that air can physically hold. Your body generates heat through metabolism and the surrounding air evaporates it away. When the surrounding air matches your body temp the only thing that makes it keep evaporating is the dryness of the air. A relative humidity of 100% means there is already as much moisture in the air that it can hold. This prevents evaporation so you overheat and go into heat exhaustion and then heatstroke.