ELI5: What is "wet bulb temperature" and why does it matter?

Kent - July 6, 2023


I’m going to start off with why it matters because the definition of what it is makes a little more sense with the background.

Like a car engine, our bodies can overheat and break. If it’s hot outside, we need something to cool us off. Luckily for us, evolution gave us a solution: sweat. Sweat is mostly water and has a high thermal conductivity, which means that heat transfers to/from it faster than other materials. When we sweat, it absorbs some of our body heat then evaporates into the air, taking the heat with it.

Now, this isn’t perfect. There are situations where sweat will do nothing. Air can only hold so much water. When you see humidity measurements, it’s always in %. Well, that % is how much water is in the air compared to how much it can hold. At 100% humidity, the air is holding a much water as it can and water can no longer evaporate.

When this happens, sweat can no longer do anything to cool us off so we have to rely on the air temperature, which most of the time is also enough to prevent us from overheating.

However, in recent years, we’ve been having weather events where not only is it very humid but also very hot. It’s humid enough where sweat can’t cool us off and hot enough where the ambient temperature doesn’t do it either, so we overheat. This is a “Wet Bulb Event”

So then, what exactly is “Wet Bulb Temperature”? What we do to get it is take a thermometer and wrap the bulb with a wet rag. The rag acts like sweat soaked skin, so it cools off the thermometer. It’s effectively a measurement of how effective our natural cooling will work. To add to this, while our bodies operate at 98.6 °F, it actually needs to be cooler than that to prevent overheating. 94 °F is around the temperature we begin to overheat. If the Wet Bulb Temperature is 94°F or higher, being outside is incredibly dangerous as you WILL begin to overheat, and as such when the wet bulb temperature is 94 or greater, that’s a wet bulb event.

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