Wet Bulb Temperature


"Wet bulb" temperature is the temperature + relative humidity at which water stops evaporating off a "wet" thermometer bulb. If air is sufficiently humid (saturated w/ water vapor), evaporation will no longer cool the bulb, and it gets continuously hotter. [...]

Dry air has essentially infinite capacity to absorb moisture, so, humans can survive in very high temps if the air is dry [...] But, the wet bulb is not about heat, per se. It's about the absorptive capacity of air. A wet bulb temperature in the mid-80s F can, and does, kill humans. Heat waves in the EU & Russia in 2003 and 2010 killed over a hundred thousand people at ~ 82 F. [...]

If sweat won't evaporate, our body temp rises, continuously. And when body temp hits ~108, we're dead. For a vulnerable person in wet bulb temp, this takes much less than an hour. Naked. In the shade. [...]

So, what does this have to do with you? Well, up until last ~ 40 years, wet bulb temperatures were extremely rare on this planet. But that's over, now. We're already seeing multiple wet bulb temperatures per year in multiple locations. By mid-century, parts of the Southeastern U.S will see weeks of wet bulbs every year.

This is quite bad. Thousands of people will die on each of those days.

This has ... implications. Human habitations get very difficult to manage when thousands and thousands of people are dying every day, for weeks, every year.

This is actually an active area of climate science and resilience study.

So, the moral of the story: Many of the places humans currently live on the planet are on their way to being functionally uninhabitable by humans. They will have to move. Some may try to "adapt," and some may pull it off. But this will be exceptionally difficult.

If we do everything right, starting now, there's a chance we could return some of those places to habitability for future generations, in the 3rd millenium or so. But as of today, we've already roasted most of them. Carbon emissions have a very long life in the atmosphere.

So please, help your city prepare for the refugees. Depose the NIMBYs in your city government. Defeat the car-stans who deny that all of this is happening.

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26 Responses:

  1. Scott says:

    The opening scene of https://en.wikipedia.org/wiki/The_Ministry_for_the_Future is a worthwhile visceral dramatization of wet bulb temperature.

    • phuzz says:

      It almost feels more like a horror story than sci-fi at that point, even though it's entirely plausible.
      In fact that's the worst part, that it's not only plausible, but likely in the near future.

  2. mdhughes says:

    See also Greg Egan's Perihelion Summer.

  3. Jim says:

    I for one would like to see a cost-benefit comparison of abotic and diatom dual-use desalination and carbon capture from seawater. Google scaled up the abiotic reactors as Project Foghorn and then mothballed it because green hydrogen was too expensive even though its cost was plummeting then, has continued to plummet, and is expected to plummet for decades to centuries.

  4. tfb says:

    I think there could be no better demonstration of how fucked we are than that this thing originates in some long series of tiny chunks of text which then have to be stitched together. Someone has basically written a program (or, worse, done it by hand) to stitch together the ethernet packets that are what Twitter provides (actually, they're packets but they're sequenced, didn't XNS provide something like that?) into a TCP stream of characters which you can read. A society which accepts this is not a society which is going to deal with climate change.

    Oh, but it goes on: the person is using wet-bulb temperature in a nonstandard and, actually, wrong way, and then goes on to treat the thing they actually mean (relative humidity) as if it's all that matters, when it isn't.

    So. The wet-bulb temperature is the temperature read by a thermometer covered by a water-soaked cloth, so it's the actual ambient, the dry-bulb temperature, - the effect of evaporative cooling from the water. It is never greater than the dry-bulb temperature, but at the point when the relative humidity reaches 100% it is the same. At that point no evaporative cooling is possible (convective and radiative cooling still is possible).

    And it's completely fine for RH to reach 100% (so for wet-bulb temperature to reach dry-bulb temperature) so long as the temperature is low enough because convective and radiative cooling can keep you cool. As a silly example: if you fall into the North Atlantic in the winter the RH is 100% (you're in the water) and indeed you will die pretty quickly unless you are wearing very special clothing. But you'll die of cold, not heat. Less extreme examples are times when people wear wetsuits: these really make the RH inside the wetsuit 100% (if you've ever smelt the inside of a wetsuit after you've been wearing it for 10 hours while working hard you will realise this), and yet people wear these things (I have worn these things) for hours without dying, because the ambient temperature is so low (and often but not always because they're in very cold water, which is really good at the conductive/convective thing). Not sure if a wetsuit will keep you alive for long in the winter north Atlantic: it will make things better but I think you probably still die of cold, just after a few hours.

    What isn't fine is if RH reaches 100% (you can't sweat) and the ambient temperature is high enough that convective / conductive cooling is no longer sufficient to keep you cool. This also depends on things like wind strength (windier is better) and how much sunlight you're exposed to (less is better: the TOA solar constant is ~1.3kW/m^2, surface is lower but still a lot), what colour clothes you are wearing (not sure how that goes since you need to know what 'colour' they are in IR as well), how conductive the clothes you are wearing are (more is better), how much convection they allow (more is better: wear loose clothes), and probably other factors I've not thought of.

    But of course you can die even if RH is below 100% if it's hot enough. RH on the surface of Venus is very, very close to zero, but you will die there very quickly indeed.

    So there's this complicated multidimensional space of human-thermal-survivability which this person has reduced to a single value, which they've then got wrong. And they've posted it in the form of ethernet packets which have to be assembled by hand into a stream of characters. We are so fucked.

    Nevertheless: the point that they make is right even if all the details are wrong.

    Many places on Earth will indeed reach the limit of human thermal survivability for long enough periods to kill people who live there, and they'll do that within the next century. Some of those places will be in the US (which, very obviously, is the only place many people who live there care about). Many many more of them will be in equatorial regions. I am not sure but I think some parts of the Middle East will reach that limit, despite RH being incredibly low for much of it: it will just get hot enough that evaporative cooling is not enough to keep you alive.

    And there will be a vast immigration problem: hundreds of millions or billions of people will need either to move or to die. Many of them will be brown, and those people, obviously, will be left to die, because rich white people are racist fucks, even when they pretend not to be. But there will be white people, people who count, who will also need to move in their millions.

    The end result of this is, just obviously, going to be nasty fascist regimes based on preventing immigration in rich, mostly-white countries (I mean, the tiny problem that was 2008 caused Trump and the Johnsonites, a really big problem is just definitely going to result in nazis). Those regimes will fight resource wars with each other and also with the brown-people states which will have similarly nasty regimes. These states – including some of the brown-people states – will have nuclear weapons. End of story.

    We have, like, no time at all to deal with this problem. And we will not deal with it, and all of our children will die.

    Note. I just made up the apocalyptic bit at the end obviously, but the science bit is right: I'm a physicist who worked on this stuff.

    • MattF says:

      I'm also a physicist and I also once, years ago, fell into the relative humidity rabbit hole. Did a calculation that needed RH-- figured I could just ask any meteorologist what it was and move on. Wrong. Yes, the thermodynamics of water vapor solubility in the atmosphere is not obvious, but I was not expecting to get explanations that were just wrong about things that are clearly fundamental to 'weather'. So I'm not surprised you found it necessary to go on at length about this.

    • Ham Monger says:

      I appreciate this long treatise. I was reading the summary above and was pretty sure at least one of the original author and I misunderstood what "wet bulb temperature" meant.

      For one thing, there's always a "wet bulb temperature". I was pretty sure it was the difference (or lack thereof) between wet and dry bulb temperatures that mattered. As you indicated.

      • Elusis says:

        I grew up in the Midwest where it is often 100 degrees out and 100% humidity. Just as with "wind chill" (which can be equally relevant there), I understood what "wet bulb temperature" meant because I watched the weather forecast on TV. I am assuming the author of the original is... not from the Midwest.

    • Thomas Lord says:


  5. Obviously the solution is going back to underground caves where the temperature is stable and comfy most of the time.

    But 5G coverage would certainly suck

  6. Elusis says:

    Two near-future SF novels dealing with climate crisis made it onto my reading list in 2015. One was by Margaret Atwood. The other one was actually deeply unsettling in its likely prescience. I really, really dislike how often I think about it now.

  7. Cookie says:

    Crazy idea. Air conditioning and dehumidifiers.

  8. MrSpookTower says:

    If I can geek out here for a moment: Fussing with dry and wet bulb thermometers and using the raw temperature data can get a little confusing, and the weather reports usually don't publish wet bulb temps. But they do often publish the dry bulb temperature (called "temperature"), the relative humidity, and usually the dew point temperature as well.

    You could use that information to calculate the wet bulb temperature, but it's not really necessary. Dew point and wet bulb temperatures are the different measures of the same phenomena; the higher the number, the worse it is. The dew point temperature can be thought of as a roll-up of (dry-bulb) temperature and relative humidity. Here's a dew point temperature calculator (if all you have is temperature and relative humidity), and here's charts of comfort level associated with dew points. Very high dew points are deadly.

    Further, the infamous "heat index" is different roll-up of dry bulb temperature and relative humidity. (It's also often displayed as a roll-up of dry bulb temperature and dew point temperature, which is kind of the same thing.) Here's some heat index charts.

  9. Thomas Lord says:

    So, what does this have to do with you? Well, up until last ~ 40 years, wet bulb temperatures were extremely rare on this planet.

    What pisses me off is that he is using words incorrectly while talking about a life or death matter. Wet bulb temperatures are defined by a measurement methodology. On a 63 degree foggy day in Berkeley there exists a wet bulb temperature. The drift of his gist is important but, damn, guy, get it right.

    In any event: How do people not know this already? Why are people surprised? It is genuinely not hard to know, for example, that large parts of the south west are expected (on present trajectory) to become essentially uninhabitable' That desertification is accelerating' That the apparently unstoppable rapid destruction of the Amazon will soon reach a tipping point of no-recovery and that those forests account for a good deal of North American precipitation. Without very rapid and immediate reduction of fossil fuel use, global simultaneous crop failures and intense water shortages are inevitable. Oh, and the major ocean circulation patterns on which we depend for a habitable planet or looking shaky. Did I mention that 100 miles of kelp forest off Northern California coast died in the space of 3 years?

    The newspapers, the YIMBYs like Matt (when he greenwashes build-baby-build for core cities), and the politicians are bullshitting, straight up. Read IPCC reports, starting at least with 2018's SR15. Find real climate scientists and their allies on social media. We do not have time left to "electrify everything". We do not have time left to replace cars with nice cush fantasy public transit. We do not have time left reboot the largely superfluous economy (which will increase emissions while not serving precarious human needs). Several dozens of people are confirmed dead from the heatwave in Oregon. The weird weather's proximate cause has been an ongoing, for years now, disruption of the jet stream -- the fucking jet stream is staggering, punch drunk. The arctic will be blue in the summer soon (accelerating heating by a lot). There's some first hints that the melting of Greenland might already be irreversible and much faster than previously modeled. We are the dog in the kitchen sipping coffee in a burning building, to put it mildly. If you hate your children, just keep assuming someone "up there" in the corridors of power is "working on it".

    • jwz says:

      "How do people not know this?" is generally a thing you should say only if you want to look like a huge jerk, but for whatever it's worth, the reason I posted this was that I had never heard about (or, really, thought about) the relationship between sweat and the atmosphere's ability to take up more moisture, and I thought that was interesting. And also, you know, apocalyptic.

      I'm sure that makes me a huge dummy in your book, though. Be sure to let me know.

      • Thomas Lord says:

        It doesn't make you or Matt L. a "big fat dummy" (and please join me in the practice of trying to check one's own ego at the door of this emergency). It's a reflection of the absolutely deadly bad general state of knowledge. You need to panic about the climate emergency. No joke.

        And I'm sorry if I sound terrified, exhausted, and angry as hell but I've had the time to start noticing what climate scientists are talking about, reading as much of the well regard literature as possible, struggling to understand what IPCC reports actually mean, and so on. There is really a news blackout. Fossil fuel burning must stop very quickly -- like, the shutdown in April 2020 quickly and then some, and for an extended period, not just a month.

        There have been around 500 confirmed deaths in Canada in the past few days. Around 50 confirmed in Portland. This is a small taste of things to come. And it's a quasi monotonic function of rising horrible over time until it plateaus when fossil fuel burning is damn near 0.

        Anything even vaguely approximating achievement of Paris goals will be gone in 5.5 years by prevailing estimates of the current burn rate and remaining carbon budget for Paris goals. Emissions have to pretty much fall off a cliff which, obviously, the governments are not going to do.

        I'm seeing well reputed climate scientists losing their shit, calling for revolution now, making noises that make you wonder if they are on a self-harm path.

        All the positive feedback effects that climate people were most worried about are empirically coming faster than modeled (right on the edge of the "worse case" part of the error bars, roughly speaking).

        You once said we'd have to do some geo-engineering. Not that, e.g., solar dimming looks like it would help, it doesn't, but regardless there's no plan or capacity like that.

        Impresario at the Restaurant at the End of the Universe.

      • tfb says:

        It doesn't make you a huge dummy. But there's a nasty thing that happens with climate-change denialists (and other denialists I'm sure: I just only know about this kind), where a bunch of people spread, in good faith, a model of how things work which smells correct but is actually wrong in critical ways. The 'skeptics' then leap on this and say 'look: model wrong therefore climate change not happening', and they can show that the model actually is wrong, because it is.

        That's why it's important, I think, to get models right: even if the explanation is simplified, it should not skip over critical detail (or should say when it does).

        (For climate change the 'plausible-but-wrong' model is the one where the atmosphere is more-or-less opaque to the infrared frequencies blocked by CO2, and adding more CO2 makes it more opaque. In fact it's already completely opaque at those frequencies, and adding more CO2 raises the height at which it stops being opaque & thus reduces the temperature & hence the outgoing power at that height. It's quite subtle.)

  10. Thomas Lord says:

    * "Portland" should say "Oregon"

  11. David says:

    I'm calling shenanigans on this. If you want people to take climate change seriously you can't just make stuff up. 100K people didn't die because it was 82 degrees. A simple search shows it was well over 100 in Europe in 2003, and the countries with the most deaths had highs closer to 120 degrees.

    Also, I live in FL where the temps are regularly 90 degrees and it's so humid it rains every afternoon. We'd all be dead if this were true.

  12. Dude says:

    In other "wet bulb" news: the heat in Tokyo, Japan is so hot that the government has advised citizens to "not exercise during the day".

    Good thing Tokyo isn't hosting the Olympics or anything.

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