I don’t typically talk about climate change on my blog. I mostly write about technology, entertainment, and my hobbies. But given the heat wave we are currently going through, I thought it was important to write a post explaining a concept I recently learned about — wet bulb temperature.

Since learning about it, I have been following news articles about how it has reached dangerous levels in several countries — and how this is expected to become more common as the Earth heats up. It is something everyone should be aware of. If not to motivate actions that attempt to mitigate climate change to some extent, then at least to make people aware of the issue so that they can remain safe.

So what is wet bulb temperature anyway?

When most of us talk about the temperature, we mean the temperature of the air around us measured using an exposed thermometer — this is sometimes called the dry bulb temperature.

The bulb here refers to the bulb of the thermometer, the part that houses the mercury that rises with the temperature. If it is exposed, as it usually is, it measures the temperate of the air around us without taking into account humidity.

Wet bulb temperature, on the other hand, is measured with the bulb of the thermometer wrapped in a wet cloth or wick. As water from the cloth evaporates, it cools down the thermometer. Essentially, what this is measuring is how hot a human would feel even after taking into consideration the cooling effect of evaporating sweat.

But how does this have anything to do with humidity? I hear you ask. Well, my friend, as the humidity in the air increases, the ability of water from the cloth, or from sweat, to evaporate decreases. This reduces the ability of the body to cool down.

The higher the humidity, the higher the wet bulb temperature is for a given dry bulb temperature reading.

Ok, but why is this important?

The human body is a well balanced machine. Our life is sustained by chemical and biological reactions that operate in a very narrow range of temperatures. Most of the time, our bodies are capable of maintaining our temperature through a process called homeostasis.

An important part of this process is cooling down by sweating — this is simulated by the wet bulb temperature reading. When the wet bulb temperature rises above a certain level, we become incapable of cooling down by sweating. Or by pouring water over ourselves — because the humidity in the air is so high that it does not allow this sweat or water to evaporate.

A combination of high temperatures and high humidity can be lethal. Various studies have placed the wet bulb temperature at which human health is seriously endangered at between 32C to 35C. At the upper limit it becomes lethal even to healthy people. 35C is often considered the level at which humans cannot survive outside for long periods of time — this wet bulb temperature is equivalent to a dry bulb temperature of 40C and a relative humidity of 75%.

Other combinations of dry bulb temperatures and humidity can also produce this wet bulb temperature. As soon as the wet bulb temperatures approach 32C, extreme caution needs to be practiced — you need to minimize time away from air conditioning, stay in the shade when you do go out, and stay hydrated. If it hits 35C, it is considered unlivable even for healthy people.

How can I know the wet bulb temperature

In most weather forecast apps, there is a setting to turn on this reading. Turn it on. I personally check this value every day — not that where I live is likely to hit high wet bulb temperatures, but it is a good reminder just how close to unlivable we are getting.

I live in Cairo, surrounded by desert on both sides. So our humidity tends to be lower than the tropics. India and several locations in Latin America are more in danger of high wet bulb temperatures than we are. But better safe than sorry. I check the value of the wet bulb temperature every day and stay extra hydrated when it goes up.

Below is a screenshot of Carrot Weather on my phone showing the current temperature and the wet bulb temperature as well.

As you can see, we have a dry bulb temperature of 31C, but a wet bulb temperature of 19C. Safe values for now.

What can I do about it?

Other than staying hydrated and minimizing time away from air conditioning when the wet bulb temperature rises, the only thing you can do is start supporting efforts to stop climate change. Look into solar panels, get an electric car. Avoid airplane trips for short distances — take the train instead. Lobby your local politicians to take actions against climate change. These are some of the things you can do to avoid making dangerous wet bulb temperatures a more common occurrence for many places in the world.

That’s it folks, I hope you learned something new and that you put this information to action. Stay safe.

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