Wet-Bulb temperature

Wet-bulb temperature

The pacific west coast of the North-American continent has just seen an unprecedented heat-wave. One Twitter, I ran into mention of the so-called Wet-Bulb temperature, and what that meant in relation to such a heat wave. And given the impending climate-catastrophe, it seems worthwhile to explore.

Wet-bulb what-now?

You’re probably familiar with the definition of the Celsius temperature scale — unless you live in the US, of course. 0 degrees Celsius is the temperature of melting ice made from pure water. 100 degrees Celsius is the temperature of pure boiling water.

One way to measure temperature is with a thermometer. A thermometer is a transparent air-tight column with a bulb attached to the bottom, filled with a fluid. The total amount of fluid expands and contracts when the temperature changes. Fluid contracts and expands with temperature, see Boyle’s law. Because there is a lot of fluid, the height of the fluid column changes measurably with the temperature, and because the thing is airtight, air pressure doesn’t affect it. Add a scale to the column, and voila. The earliest examples used mercury as that fluid, because that expands and contracts well.

Now, then, wet-bulb. Wrap the bulb of the thermometer in a water-soaked cloth — with room-temperature water. Then run a current of air over the cloth. Now, you’ve got yourself a wet-bulb thermometer.

Why would you do that?

The water-soaked-cloth-with-air-current thing has a very good reason. Let’s look at what such a setup does. The water-soaked cloth will have the temperature of the air, initially. However, the air current causes evaporation. And that causes the temperature of the cloth to go down. The water-soaked cloth effectively shares it’s temperature with the bulb. So, the wet-bulb thermometer measures how cool you can make something by using water-evaporation.

So, this thermometer gives you an indication how fast your tea will cool off when you blow on it. More importantly, it says something about us humans. Because human bodies cool themselves with, you guessed it, water evaporation — a.k.a. sweating. Wet-bulb temperature also tells us humans how well we can keep cool.

A wet-bulb thermometer effectively takes the difference between a dry-heat and a wet-heat into account. In a desert, air humidity is very low, meaning a wet cloth will provide excellent cooling. As we humans also experience. I was once in an air conditioned museum in Egypt, thinking it was pretty cold, then seeing 28 degrees on the thermometer by the door. In a wet heat you’d be sweating like crazy.

Wet-bulb in relation to heat waves

Back to the pacific. Or rather, back to heat waves in general. As written, wet-bulb temperature says something about what temperatures you can reach with water-evaporation cooling. Because our bodies operate that way, the wet-bulb temperature also gives an upper limit for human survival.

A human body can carry out normal tasks up to a wet-bulb temperature of about 32 degrees Celsius. At 35 degrees, humans will die in a few hours, regardless of staying in the shade and how much water we drink. Let that sink in for a moment. We cannot naturally survive in an environment with a wet-bulb temperature of 35 degrees or more.

The pacific reached 50 degrees Celcius as a regular temperature this past period. That’s insanely hot for areas above the arctic circle, but the wet-bulb temperature was not above the threshold.

Not yet.

Climate change hell

For most of human history, the wet-bulb temperature rarely reached 35 degrees anywhere. And only briefly. Our changing the climate is increasing that frequency, and lengthening occurrences. In the coming decades, deadly wet-bulb highs will increase. Certain areas of the world may see such temperatures for days or even weeks on end. And that is pretty frightening.

It means that air conditioning is not just a luxury at these times, but a necessity. If your air conditioning breaks during a wet-bulb 35+ event, you could die.

Climate scientists point to a 1.5 degree average temperature rise as the boundary of safety for climate change. If we go above that, we are very, very screwed. And one of the ways in which we will be screwed, is that large sections of the world will face wet-bulb 35+ periods.

If you think refugees are a problem now, imagine what happens when large parts of the world become lethal to humans for periods of the year. Especially when large numbers of those people are poor and have no air conditioning. Imagine the sheer number of deaths. This is not a happy thought.


When I first read about this, I was horrified. I still am. However, there is a small nuance to this. The fact that humans can only survive somewhere with the aid of machines like air conditioning is not as scary as I first thought.

You see, in large parts of the world, you cannot survive the winters without machines either. If your environment is in a period of -10 degrees Celcius or lower and has no heating, you will also die. If you have no shelter when a hurricane flies past, you die. And so on, and so forth.

Still, this added frightening scenario for our future is not a happy one.


Wet-bulb temperature is a useful measurement, and it shows us a scary future. A future that is quite deadly and, worse, it’s coming.

Maybe it’s time to get off our asses and actually save the environment.

Martin Stellinga Written by:

I'm a science fiction and fantasy writer from the Netherlands

One Comment

  1. Sander
    July 9, 2021

    Read the first chapter of Kim Stanley Robinson’s “The Ministry for the Future” for a far too realistic scenario of what can happen if you combine high wet bulb temperatures with power grid failure. I had actual nightmares for the next few days after picking up the book, and none of his attempts at charting possible paths upward for the rest of the book managed to quite get rid of that horror. (That book, last year, was the first place where I encountered the wet bulb temperature concept. I hadn’t expected it to go ‘mainstream’ quite this swiftly.) :/

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