What is “wet bulb temperature” and why does it matter?
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You take a wet cloth and cover the end of the thermometer, then pass the air over it. The water will evaporate and cool the thermometer so the temp will drop. This is the wet bulb temp. Lick your finger and blow on it, you’ll feel the same effect.
It is important because people sweat and cool themselves as the sweat evaporates. If the humidity is too high, water won’t evaporate readily and you won’t be able to cool down. You’ll over heat and die. This is a bad thing.
Evaporation cools you down. So a wet thermometer bulb is colder then a dry thermometer bulb. How much of a difference this makes depend on how much evaporation there is which depend on the temperature and humidity.
The wet bulb temperature matters a lot because us humans are better approximated by a wet thermometer bulb then a dry one. When it is hot we sweat which makes us wet. The evaporation of our sweat cause us to cool down. So the wet bulb temperature is a better approximation of how hot it feels then the dry bulb temperature. It describes why Florida feels so much hotter then Arizona even when the dry bulb temperature is the same in both places.
What makes it important is that it marks a point where heat and humidity get high enough that humans can’t survive without a climate controlled indoor environment, because it’s too humid for our sweat to work and too hot to survive without it. This point is becoming more common in countries worldwide.
When you take the temperature of the air, you use a thermometer. Traditionally a bulb thermometer 🌡️. Wet bulb means you have wet the bulb. Typically this is done by wrapping it in a cloth and wetting the cloth.
Why is this done? Because water evaporates. And when it evaporates it pulls heat from its surroundings, the thermometer, to do so, thus cooling the thermometer down. This lets us measure how hot it feels a little better, but more importantly, it lets us estimate how dangerous it is.
Why would we do that? Humans have a body temperature we regulate through various means, and the biggest one by far for staying cool is sweating. Sweat works the same way as that wet bulb thermometer, water evaporates taking heat away from us cooling is down. So what happens when the sweat isn’t enough? That’s when you start hearing wet bulb danger and such. This is saying that the bulb we use to determine temperature is so hot even when wet that it’s dangerous. This means we as humans have to take shelter or else we will overheat and die. It’s actually that serious, death is the consequence.
It’s the limit temperature that evaporative cooling can go. Below that temperature, it’s just not possible to cool by evaporation. It matters because sweating is how we thermoregulate. If wet bulm temperature is too high, getting close to our normal body temperature, we simply start overheating and in time die.
You can’t stay in a sauna indefinitely to bring one example. And a pet or a child can easily die in a car left in the sun, adult too if they dont get out of the car in time. But such temperatures can also rarely occur outdoors, in which case, air conditioning becomes a matter of survival.
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.
The “wet bulb temperature” is simply a temperature scale that takes in to account the relative humidity.
Relative humidity and temperature are basically the main two factors that determine whether or not someone will die to prolonged heat exposure. As relative humidity increases, the body is less able to cool off through sweat since evaporation occurs at a slower rate.
Wet bulb temperature matters in the sense that its the easiest way to compare if a certain area is approaching a lethal temperature, because various areas in the globe differ drastically in their average relative humidities.
**A more technical explanation will follow beneath this line**
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When water goes from liquid form to vapor (evaporation), this will cool the surface that the water was resting on. Therefore, if you take a thermometer and cover the bulb of it with a damp cloth, the thermometer will read a lower temperature since water is evaporating on the thermometer surface.
This temperature is the wet bulb temperature.
For example, say that you are in a completely dry desert at 110 degrees F. If you then covered the thermometer bulb with a wet cloth, the temperature would decrease pretty significantly as the water on the cloth would evaporate quickly. This lower temperature (lets just say its 90 F) would then be the “wet bulb temperature”.
The evaporation rate of water in an environment is determined by numerous factors, but probably the most powerful factor is the relative humidity. At 100% relative humidity, there will be no net evaporation that occurs and therefore the thermometer will not be cooled by wrapping the bulb in a wet cloth. At 100% relative humidity, the real temperature and the wet bulb temperature are equal.
So in other words, the higher the wet bulb temperature is, one can deduce that some combination of either the real temperature or the relative humidity is very high.
A wet bulb temperature of 95 F is lethal with prolonged exposure to the average human.
When you sweat, the water goes into the air and takes some heat with it.
If you cover a thermometer with a wet towel, it acts like how a person does when they sweat.
We call this wet bulb because scientists used to cover the bulbs of thermometers with water.
Humidity is how much water is in the air. If there’s too much water in the air, sweat can’t go into the air and cool you off and can’t make the temperature of a wetbulb thermometer go down.
This means that when it’s too humid and too hot, the thermometer and you can’t cool down.
To measure humidity, you can use either a psychrometer or a hygrometer. The wet and dry bulb refers to a psychrometer.
The dry bulb is just a normal thermometer. The wet bulb is a thermometer that lets water evaporate off of it. That process cools the wet bulb down until it is too cool for water to evaporate. This gives us the wet bulb temperature, also known as the dew point.
With both the temperature and the dewpoint, you can calculate the relative humidity, but the dewpoint alone can tell us a lot. A high dewpoint means the air is very humid and could indicate rain is on the way. A low dewpoint tells us the air is very dry, and able to absorb a lot of water.
The wet bulb temperature is the coolest you can make the air by evaporation of water. So think of the wet bulb as the lowest temp you can cool your body down just by sweating.
Do not confuse this with the wet bulb globe temperature (WBGT), which is a measure of heat stress outdoors.
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