Andrew Dessler (@AndrewDessler): Quick 🧵 on wet bulb temperature and why it matters for human survivability.

To understand this, first some facts about the human body. To survive, humans need to keep their body temperature within a few degrees of 98°F.

At the same time, the human metabolism is generating about 100 watts of power.

Put these together and it means that humans need to dump 100 watts into the environment while maintaining this 98°F temperature.

This thermoregulation is one of the many things the human body needs to do to survive.

So how does the body do this?

The first method is what we call sensible heat transfer. This refers to transfer of heat down a temperature gradient. If the air is cooler than your body temperature, then heat will be transferred from your warm body to the cooler air.

The perfect air temperature is room temperature (say, 75-80°F). At this temperature, normal air currents in a room will bring cool air into contact with warm skin, transferring 100 watts from the person to the room air.

That's why this is the most comfortable temperature.

If the air temperature is much cooler than this, then it pulls too much energy out of the human body. In response, you can, e.g., put on a sweater. That will retard heat transfer, keeping it at the magic 100 watt number.

More interesting is what happens as the temperature goes up. As the temperature gets into the low 80s, normal room air currents will no longer carry enough heat away from your body.

One thing you can do in this case is turn a fan on. This increases flow of air through the room, increasing air flow over the skin. This allows your body to still dump 100 watts of power into the room despite the higher ambient temperature and lower temperature gradient.

The other thing your body does is sweat. Sweat works because water, when it evaporates, carries a huge amount of heat away with the vapor: 2250 J/g. To dump 100 watts of power, about 0.04 g/sec of sweat needs to evaporate (2.6 g/min).

This is known as latent heat transfer. It means that, in places that are very dry, like Phoenix, you can survive 120°F because you're sweating like crazy and the sweat can easily evaporate and pull lots of heat out of your body. I didn't say it was comfortable.

Now let's consider a hypothetical: the ambient conditions are 98°F and 100% relative humidity. Because the temperature is 98°F, the same as your body, there is no temperature gradient between your body and the outside, so no sensible heat transfer.

And because the relative humidity is 100%, you can't evaporate any sweat. So there's no latent heat transfer. With no way to dump the 100 watts of heat, the energy accumulates in your body and, via the first law of thermodynamics, your body temperature increases.

This situation (98°F and 100% RH) will cause your body temperature to increase enough that it will kill you.

This is where wet-bulb temperature comes in. It is a combined measure of temperature and humidity and it tells you how dangerous the combination is.

Values in the upper 20s are bad. Values close to your body temperature (i.e., 35 °C) are not survivable. If you are exposed to those latter conditions, you will die.

If high wet-bulb (e.g., > 30°C) conditions become common in some region, there's a real question about whether such a region will continue to be habitable.
Research on how close we are to these wet-bulb thresholds is not conclusive but it is worrying.

So keep an eye on wet-bulb temperatures and note when they get close to or exceed 30°C. That's a definite red flag. For more, see this nice paper by @climatedynamics (yes, behind a paywall, but perhaps he has a free copy).

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