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Comment by donatj | Hacker News Reader

I grew up poor and without AC in Minnesota where we regularly get spells of 100°F+ (37.7°C) at VERY high humidity thanks to all the lakes. It’s survivable, it’s absolutely not pleasant though and I wouldn’t wish it on anyone.

Fans and wet rags help some. I used to fill a 2-liter bottle with cold water and hold it against my chest. A lot of sleepless nights. When we got a local second run $2 movie theater, it was a literal godsend.

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You do not understand the issue at hand. Whatever you experienced was way less severe, because evaporative cooling obviously was still possible. There is a point where you simply cannot cool yourself passively. That's the point where survival is at risk regardless of health.

https://en.wikipedia.org/wiki/Wet-bulb_temperature

Having a rag of cold water on you will cool you regardless of evaporation. It is a thing that is cooler than the environment. The water from our taps at least is a pretty constant like 50°F

So you have a supply of water at 50°F - that’s 10°C (an optimistic temperature in many places). And you are using it to try to maintain a body temperature around 37°C. Let’s assume you do so by transferring heat from your body into the water enough to raise its temperature by 25°C - that’s probably overly optimistic given that it will also be warmed by the environment and the rate of transfer will be reduced as the temperature increases, but it’s a working number.

The specific heat capacity of water is about 4kJ/kg/°C so a liter of that cold water can absorb 100kJ of energy in the process of warming up. Once it’s warmed up it can’t do any more cooling.

Your body, when not working hard, produces about 200W of heat. That’s 200J per second.

So that liter of water can absorb your body’s heat output from about 500 seconds of being alive. That’s a little over eight minutes.

So that means the cooling needs for one human for one day can be met by about 170 liters of cold, 50°F water. That’s about 44 US Gallons.

If, however, that water can evaporate - well, the latent heat capacity of vaporization of water is over 2 thousand kJ per kg.

That’s why the wet bulb temperature matters. If water can’t evaporate, that cold water can’t absorb body heat except by just warming up.

The Great Lakes region of the US and Canada is a truly special place when it comes to fresh water. The experiences in this area can't be extrapolated to elsewhere. You can get hypothermia in the middle of a summer heatwave if you spend too much time in the water more than 1km from the shore. Even if you stay at your local beach, most people would find the water uncomfortably cold. In Chicago today, the high temperature is going to be around 18° or 19° C. The surface temperature of the water at the beach is slightly less than 4° C.

> Your body, when not working hard, produces about 200W of heat. That’s 200J per second.

Minor quibble, 2000 (kilo)calories / 1 day = ~100 Watts.

Fair - I took a number for an active, awake human. You don’t produce that wattage all the time; resting and sleeping have much lower heat output; average power over a day is probably more like 100W (hence that being a reasonable average ‘healthy calorie intake’ number which I assume is where your 2000kCal comes from.)

Focused resting probably reduces it even further. You're not just "not working hard" in a situation like this, you are barely moving and probably slowing down your metabolism if anything.

Hmm, taking this in the opposite direction, what if you stop relying on water for evaporative cooling?

What if I cover myself in alcohol or gasoline some other easily-vaporized substance? Would that still evaporate at a satisfactory rate when the wet bulb temperature is high?

Depends. Are you a smoker?

It looks like Puron (the replacement for Freon) is non-toxic (and non-flammable), but it also wouldn't be a liquid at any temperature you'd need cooling at. I'm guessing there are other chemicals which have vaporization temperatures in the 150-200 degree range, and are neither toxic nor flammable, but assuming ethanol works it has the advantage of being fairly easy to come by in a pinch.

Thank you for doing the math!

At least in my area in central Europe, 10°C would be wildly optimistic in summer. I also presume when submerged in cold water, your body will start heating up, at first. So there is probably only a narrow temperature window for effective cooling. Not to mention, you may have to work and be awake to exchange the water, and being constantly wet for a prolonged time may come with it's own set of health problems.

I imagine there could be a situation with a deadly wet-bulb temperature, but below body temperature, where a large body of water may work to cool the body by convection, no?

A large body of cool water works great at moving heat way from you, obviously - an ‘acre foot’ of water is 1.2 million liters - 300,000 gallons. So a fairly small one acre lake that’s ten feet deep has 12 million kg of water, which can absorb 48GJ of heat before it rises in temperature by a single degree.

The big question though is how long will that lake remain cooler than the air in the face of wet bulb temperatures around body temperature. Thermal equilibrium will win in the end.

Yeah, but the question rather is, would let's say 32°C water, 5°C temperature gradient, work to cool the body, if completely submerged? When ambient conditions are deadly wet-bulb 32°C, would it still cool the water to its original 32°C?

So, my thinking would be, if the heat transfer from the body to the water and from water to air (through large surface) is sufficient. In a shaded pool, so all heat transfer from the outside is by convection and conduction.

I guess, if it would be sufficient cooling, you could also "just" drink the water and remove hot sweat from the skin, or dispose urine, instead. In theory, never mind electrolytes.

In Atlanta the tap water in summer is more like 22°C (72°F), which is coincidentally the average nighttime temperature.

I could imagine that a place where the nighttime temperatures are in the mid 30s (°C) would not have any cooler water available.

Last summer there were reports of the water of Florida's beaches being bath-tub warm...

Because Minnesota is a cold place. Many of the places where 'wet bulb' events are most likely to happen have only hot water available.

The tap water in Phoenix can exceed 100 degrees, people will fill a bathtub in the morning so their home A/C and evaporation will cool it off by the evening.

Even river water in India or Southeast Asia can exceed 100 degrees. There is no cold water to wet the rag with unless you have a deep well or refrigeration.

> The tap water in Phoenix can exceed 100 degrees, people will fill a bathtub in the morning so their home A/C and evaporation will cool it off by the evening.

Eww! They probably have chlorine in their water, huh? I imagine, otherwise the microbial game would be out of control.

What you describe is not universal. I recommend you visit Phoenix in July and open a "cold" water tap.

Do you believe everyone has that available on tap?

Nice living in the wealthiest country in human history, ain’t it.

Do you think this is the case for the billions of people closer to the equator or in poorer countries?

Heat is far more easily managed than -20 deg C. Cold deaths being far more than heat deaths proves that point as well.

It's true the cold is more deadly, but heat is more dangerous. A 50F delta from room temperature is dangerous for heat but easily manageable for cold. Unfortunately, cold deltas from room temperature can easily be 100F, which is less survivable than extreme heat.

Guessing here but doesn't most people die to cold due to an accident, emergency of some sort or drunkenness? Ie a problem for you, not for the society at large.

If you live in a place that gets a bit below freezing every winter -20 is not a problem. I recently survived -33C and have rarely experienced less than -20C, life went on as usual, like going to the store and working.

But if the areas in the article suddenly had -20C that would be a better comparison on how bad it would be.

> I recently survived -33C and have rarely experienced less than -20C, life went on as usual, like going to the store and working.

Was it -33C at the store and at the office?

Try living outside at -33C without the opportunity to go inside.

I'll take heat, every time. Cold is a killer.

You think people living in those regions in the article can just go inside to solve their problem?

With shirt off, drinking plenty of water, a mechanical fan (or even a manual handheld Japanese style) I can easily live “outdoors” in mid forties for days. No problem.

-20 with no heating (not carbon friendly in such zones), i will be dead in a few days.

I dont think you understand what the article is talking about.

Wet bulb temperature is the lowest temperature to which air can be cooled by the evaporation of water into the air (sweating, evaporative cooling).

If that temperature exceeds the body's max temperature, the clock starts ticking. If you stay, you die. It's not just very hot.

The point is that at some sufficiently high humidity, fans and wet rags do not help any more, at all! Combine that with sufficiently high temperatures, and it's not survivable. And will kill people with weak circulatory systems (i.e. the elderly and babies) quite a bit earlier.

It also causes long-term damage (e.g. loss of kidney function) to healthy individuals. This is an issue that’s been tracked for a decade in mesoamerican field workers.

Yes, the fan cannot cool. But have your ever experienced the cold water tap being cooler than the ambient air temperature? They mean that they soak a rag in cold water, from the tap, and then apply that to themselves. This will 100% help cool the person down. You could run a cold bath, take a cold shower, even drink the cold water (but not too much too cold!) If you have cold water, you can survive.

That only works if the water pipelines are underground and all the above-ground plumbing is shaded. If the water heats up on its way to the faucet it's useless.

That's a big "if" in rural areas of third world countries, though.

I don’t think of Minnesota as a place where 100f+ occurs with regularity or for sustained periods. A brief search confirms my assumptions, but I’m happy to be proven otherwise.

There are parts of the US where 100f is reached by midday and persists past sunset for over a month, with high humidity.

Actually getting over 100f is decently rare in Minnesota. I’d guess most summers we do, but not for extended periods. We often get a few weeks where the high is in the upper 90s.

However, our humidity can get extremely high - basically as bad as it gets in our country. Our record seems to be a dew point of 88, and the highest ever recorded in the US is 90 (in Louisiana).

The corn sweat is the biggest contributor, but all of our lakes don’t help either.

Like Dallas in 2023, for example.

https://www.dallasnews.com/news/2023/11/05/2023-texas-summer...

You are talking about Minnesota but your description is identical to that of Atlanta, Houston, and any number of other southeastern US cities during the first half of the 20th century. A/C became widespread in the US first in the South, because it made indoor work possible in the afternoons. During that time (first half of 20th century), Northerners still considered A/C optional because the hot season was relatively short. A/C has been considered non-optional in the American South for the last 50 years. (In fact, I often find that when I visit the South in the summer I need to bring a sweater because it's not uncommon for indoor spaces to be ridiculously cool.)

Those days are over. Now Minnesota is like Atlanta in 1950, and Atlanta in the summer is now dangerous. Phoenix in summer is now deadly. A/C is a life-and-death issue in the southern half of the US (unless you're on top of a high mountain). A long absence of A/C in those places will kill you, and I don't mean figuratively.

For what it’s worth, it turns out a most of our humidity is from corn fields, not our lakes. https://www.mprnews.org/story/2016/07/22/feeling-sweaty-minn...

It’d be nice if they could genetically modify corn to pump out less water, though I don’t know if that’s feasible.

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