This world is an alternate Earth similar size, mass and distance from its star to our Earth. The atmosphere is similar in composition to Earth except at 5000m altitude the pressure is two bar and the temperature is typically 20-30 degrees C.

Humans live on a very large high plateau 5000m above the swamps and shallow seas below (sea level at 50 degrees C and 90%+ humidity). Snow is known but is seen only very occasionally on the very highest peaks. They do not usually descend very far into the very hot and humid lower areas, but have decided that an expedition should be mounted to explore these areas.

What measures and mechanisms can they take to combat the heat and humidity and extend the area they can explore? Most areas are covered in dense tropical rain forest on rough downward slopes. The temperature and humidity increase with the distance descended.

Their technology is limited by an extreme lack of metals, so no electrical apparatus and no significant* metal artifacts, but they can use anything else that could be made without electric power or significant* amounts of metal.

*Roughly a kilo of metal can be spared for the expedition if required. 10kg of metal would be considered a vast expense.

Additional context I was planning to use the Canadian Humidex as a guide for human survival limits my crude estimates for how long a person might survive being : Orange zone days to weeks, red zone hours to days and above that minutes only https://www.canada.ca/en/environment-climate-change/services/seasonal-weather-hazards/warm-season-weather-hazards.html#toc7

Update Perhaps I was not clear enough. The issues to be overcome are mainly high temperature and high humidity. The humans have little metal so no industrialization as we know it, but they have had the enlightenment they can experiment and invent things so there's lot of other scope - sealed suits using rubberised cloth, cooling by letting alcohol in a backpack boil (or even diethyl ether) bellows to modistly compress humid air below its dew point and then expand, quick lime to absorb moisture etc etc chemistry is your friend here.

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    $\begingroup$ I believe humans can technically descend to about half the height without serious health problems, and even make a living down there accommodating to increased heat and moisture. Anything below would require them getting a lot of water in order to sustain water loss from sweating, 50C with 90% humidity is about as high as one can get in a Russian banya, low levels, and this condition is sustainable for at least one hour for a normal human, yet sleeping inside will likely kill you. Adapting to high temperatures might take too much for human species IMHO. $\endgroup$
    – Vesper
    Commented Sep 8, 2022 at 9:27
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    $\begingroup$ Additional difficulties would arise from increased pressure, if at 5000m the pressure is two bar, at 0m it would be about 3.8 bar, since atmospheric pressure decreases exponentially from height, and gravity of your planet is about 1.0G so the direct comparison with Earth's curve applies. Adding this much might cause unknown (to me) health problems to unprotected humans, especially when humidity and heat are added. $\endgroup$
    – Vesper
    Commented Sep 8, 2022 at 9:35
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    $\begingroup$ Found a nice article, here tandfonline.com/doi/full/10.1080/10803548.2019.1699728 there's a lot of modelling data for how a normal human heats up under hot conditions, for various humiditu and heat values. Probably it can answer your needs somehow, in order to imagine what to depict for your humans down there. $\endgroup$
    – Vesper
    Commented Sep 8, 2022 at 9:41
  • $\begingroup$ Yes this is true. this reference saimm.co.za/Journal/v105n06p387.pdf suggests that at 5000m below Earth's surface the pressure would be around 1.6atm so crudely perhaps 3.2atm at sea level, however I suspect they will struggle to get that far down. $\endgroup$
    – Slarty
    Commented Sep 8, 2022 at 9:42
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    $\begingroup$ According to the data my fellow citizens reported in a chat (so not verified), in a typical Finnish sauna the relatively humidity is around 70 per cent, in a coolish sauna (65-75 degrees centigrade). When a sauna is heated up to around 100 degree centigrade, the relatively humidity drops quickly (some reported 20%). This suggests to me that 50 degrees and 90% relative humidity is survivable as it is. The problem would be that you won't get much done. We stay in a sauna may be max 15 minutes at a time, and then take a break to cool off (about an hour total). Sustained visits become a problem. $\endgroup$ Commented Sep 15, 2022 at 8:50

5 Answers 5


Stay high.

balloon over forest


Your people make hydrogen balloons and use wicker baskets, like the explorers in Jules Verne's Five Weeks in a Balloon. They stay at an altitude comfortable for them. Really they are interested in other lands that are of an altitude comparable to their own and if they find such that is where they will land. Lowlands will be safely surveyed from above.

This voyage would be preceded by long study of wind pattern on their plateau such that explorers can leave on the wind and expect to return.

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    $\begingroup$ You could even add a pedal-powered propeller made of wood and leather for help through calm skies. $\endgroup$
    – bta
    Commented Sep 8, 2022 at 23:19
  • $\begingroup$ +1 A very good answer - one which I was planning to use this later in the story, but I was hoping to see if there was any way to progress on foot or wheel first. $\endgroup$
    – Slarty
    Commented Sep 9, 2022 at 7:47

Dazed and confused :

I'm sorry to say your humans are not in a great position. Most of the assumptions in other answers assume the people are different from normal humans - better evolved for the temperature and pressure. I'm not making those assumptions. I'm going with what we know of actual humans. If these people are different, we have no frame of reference to give a science-based answer.

They do have one advantage. They have (assumedly) become well-adapted to the increased relative oxygen of your planet, and the effects of increased pressure in general. This might improve their tolerance as a result of pressure. With all that humidity, there isn't likely to be a lot of direct sunlight due to the oppressive cloud cover.

But they are pretty much out of luck. They are already at the edge of oxygen tolerance. The kind of temperatures they would experience at those levels of humidity will kill them quickly.


Nitrogen narcosis doesn't have full effect until around 15 bars, but at three bars (which your people will experience quite quickly) there are at least some neurological symptoms and impairments.

Prolonged exposure to excess oxygen starts to have toxic effects at 1.6 bars. at this level, the partial pressure of oxygen is about 8 times that at standard temperature and pressure. We'll assume your people can tolerate at least this level of toxicity, since their society is already at this level. But they may be suffering some neurological issues as a group, including a tendency towards seizures and vision problems (especially myopia).

Most of the research on oxygen toxicity is looking at people who are either exposed to high levels of oxygen for brief periods, or who adjust the partial pressure to physiological levels in a gas mix. These won't be options for your people. They will have serious neurological complications, including serious seizures. Prolonged exposure will cause hypertension and acute lung injury, which ironically will cause lack of oxygen and resulting organ damage.

You might be able to rig up something that causes your people to rebreathe air, effectively reducing O2 concentration, but then you start to face CO2 toxicity. It's tricky to parse out how much of CO2 toxicity is due to low oxygen, but at sufficient pressures, it won't matter. The CO2 will out-compete O2 in the blood, and I'm guessing your people will be acutely vulnerable to hypoxia (being so adapted to high oxygen).

Temperature & Humidity:

A lot of scientific research has relied on assumptions about the body and it's ability to tolerate prolonged heat. But with climate change, studies are being done to validate these assumptions. The old results say people can't sustain anything higher than about 46C at 50% humidity - well below your 50C and 90% humidity. The new results are less favorable and frankly a bit grim. Your body sweats in response to heat. But this only works if the sweat can evaporate. At the temperatures & humidity you're talking about, sweat will simply drip out of people and do nothing to cool them. At the temperatures you are discussing, humidity would need to be less than 10% for sustained endurance.

enter image description here

But wait, it's worse. At increased pressures, water evaporates more slowly. So sweating is less effective than it would otherwise be.

So your explorers will die of heat stroke, choking on their own fluids, too confused to do anything to help themselves (which would be pretty much nothing but going back home).

Is There Hope?

Unless the other conditions on your planet are different than on Earth, then not really. So you need to adjust your planet to help your people.

The biggest challenge is temperature. So Your intrepid explorers need to keep cooler. How do you do that without tech?

  • Caves: If your world is riddled with caves, these might provide a retreat from the oppressive heat. Caves will have an overall lower temperature if you are far enough into them. The temperature is likely to be at the mean-average temperature. Around here, with seasonal variations, that can be quite low. This may not be the case on your world. If your planet had previously experienced a prolonged cold period where deep permafrost formed (as happened on our Earth during the Pliocene) the underground temperatures could be considerably cooler than surface temperatures.
  • Seasons: You can adjust the temperature and humidity with seasonal variations. So if there is a wet and dry season, humidity can go down. If there is a warm and cold season, then temperature (and likely humidity) will vary. This might put more of a range on exploration, since they would need to get back to livable temps before the seasons change.
  • Rivers: A cold river coming off of a mountain range will provide a retreat to lower temperatures during the peak of daily heat. In this case, they can explore until the river warms up too much.
  • Long nights: Your daily temperatures may fluctuate greatly. So if, for example, you have a night that lasts for a long time (like several days), the temperatures may drop considerably by the end of the night. If your caves are cool enough, the explorers could retreat to caves in the day and look for their next cave at night. I see lots of great drama as the temperatures climb and the pressure-addled explorers become increasing desperate to find shelter.
  • Mountain ranges: given higher elevations your explorers can retreat to, they could move from mountaintop to mountaintop, braving the heat at the coolest points of the day and season. A network of high-altitude camps and small settlements could let them move from elevation to elevation.


In the realm of possible but not too real, there are solutions to at least some of your issues

Fish Oil: ...or any other migrating species that arrives on your plateau but lives in the high pressure & temperature zone. Snake oil has a nice ring. Some species produce oils that make a membrane more or less rigid, more or less permeable, and the like. You could justify modest increases in the tolerance for pressure and temperature by saying that the people consume the fats of these species and passively acquire increased tolerance of these environments (in much the same way that fats from cold-water fish remain more soluble in cell membranes, which is why fatty acids from fish improve resistance to heart disease, "hardening of the arteries").

Drugs: It is not outside the realm of possible that your people have discovered medicines that reduce the effects of oxygen toxicity or nitrogen narcosis. An anti-seizure drug purified from a plant could prevent seizures, and a drug that lower blood pressure would help counteract hypertension. Drugs that do the opposite to neurological effects counteract these. A drug that dysregulates temperature control SOUNDS like a horrible idea, but something that reduces metabolism while simultaneously preventing sweating (under conditions where sweat isn't cooling you off anyway) might help. A drug that reduces the swelling associated with organ damage to heat stroke might mean your people get overwhelmed by the heat but suffer less long-term damage.


Make them Lizardy

enter image description here

50 degrees is too hot for normal humans to survive for days on end. But these are not normal humans are they? They have evolved to live at 2 bar pressure on a planet that is even higher pressure at the surface, and is outrageously hot and humid for the most part.

It stands to reason these creatures are better adapted for extreme heat and pressure than you or me. Their metabolism works at a wide range of temperatures, like Earth reptiles, and unlike Earthlings whose bodies just stop working if we go up or down by three degrees.

I suspect your Lizardarians evolved from something that lived on the 50C surface and only turned into stupid crybabies recently, when their ancestors climbed onto the plateau and got too comfortable. Wah wah, it's too hot down here. Let's go back to the plateau.

While they might not LIKE going downhill, they can tough it out with endurance training. Then it just becomes a matter of how much food and water can they bring with them.

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    $\begingroup$ I'm giving you +1 just for that picture. Eat your heart out, John Travolta! $\endgroup$
    – JBH
    Commented Sep 9, 2022 at 5:33

They walk... there's no difference here between divers in Earth's ocean and your people other than your people can breathe

Assumption: Your "humans" evolved on your planet. You don't say that they colonized it and the question doesn't read like they did. Colonizing humans with technology would have brought factory implements that would allow the creation of ceramics, solving the problem entirely. So... you're calling them humans, which really means your evolved species simply looks a lot like us.

What's 2 bars of pressure to a people who evolved to live in 2 bars of pressure?

Answer: nothing. 2 bars to them is 1 bar to us. We evolved in it, we experience it. Doubling that pressure doesn't stop us from breathing it. It just means that coming back down to one bar requires some time. Normalizing everything (divide-by-2) means that the pressure at your planet's "sea level" is relatively about 1.9 bar - which wouldn't stop us from exploring.

Given time your people would learn about the bends and build ways to ascend back to the plateau slowly. Otherwise, they simply repel down the proverbial cliff and explore away. They adapt fairly quickly to the higher pressure, although long-term effects can be a problem (but that's not what your question is asking about).

Nope, JBH... They're not colonists and they didn't evolve here...

OK, let's invent the specific condition that your humans are survivors of a calamity. They arrived in escape pods and so might have brought knowledge, but not equipment or significant resources. Let's even assume that space travel is so common that, unlike today where astronauts pretty much all need advanced training and education, they at least understand something like ceramics as the "I'm a new private in the Army" level. Given enough time, they'd figure out how to use ceramics better to handle the pressure.... But back to the point...

The air pressures really aren't that big of a threat.

Experienced divers can safely dive to a depth of 40 feet (12.19 metres) when exploring underwater reefs. ... William Trubridge – a 30-year-old New Zealander is the first man to dive 396 feet metres (121 metres) without any assistance. ... The maximum depth reached by anyone in a single breath is 702 feet (213.9 metres) and this record was set in 2007 by Herbert Nitsch. He also holds the record for the deepest dive without oxygen – reaching a depth of 831 feet (253.2 metres) but he sustained a brain injury as he was ascending. (Source)

But your people have the ability to breathe...

So, let's use that first number of 40 feet for experienced divers exploring reefs. What's the pressure? Converting PSI to BARs... 3.9 bars.

Walking around to explore the environment is well within suspension of disbelief so long as you deal with the effects of long-term exposure to high pressure oxygen. (Link provided as an example, not a full solution.)

Yeah... I can't see why any condition of humans can't just climb down to the bottom and explore away.

Addendum: Some comments have been made that I've not given due diligence to temperature and humidity. I could be wrong, but it is my opinion that those comments were made by people who have never experienced high temperatures and high humidity. I have. So has my father. So has anyone living on an island near the equator.

My experience was in Houston, TX. 100+ degree weather and 70%+ humidity. It was miserable! But I never felt threatened by heatstroke save once: helping a friend move and forgetting to drink water constantly (dumb, dumb-dumb, dumb!). I lived for years in Dallas, TX and Austin, TX. 100+ degree weather and 50%+ humidity (air conditioning in January...). I never even heard of anyone suffering heatstroke that didn't fail to drink lots of water. Workers worked, joggers exercised, and people played in those conditions regularly.

My father's experiences were in the Philippines. 100+ degree weather and 100% humidity for a week on each of several visits. He's still with us at 85 years of age having never suffered heat stroke. And the Philippines natives don't seem to have had trouble with it for who knows how many centuries if not millennia.

Folks, heat and humidity need nothing more than three things: time to acclimate, drink lots of water, and rest when you tire. It's not rocket science and the temperature and humidity will not work against the body while it sits in the shade drinking water.

Atmospheric pressure, on the other hand, can cause problems, especially if experienced for a long period of time. But even that can be overcome with acclimation.

So, my assumption remains. Did these people evolve on this planet or not? Because if they did, we're not actually talking about humans from Earth. But even if we were. This isn't a life threatening situation unless people do something drastic — like being forced to run from the saber-toothed Munga beast in the early afternoon. That might cause a few people to be eaten.

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    $\begingroup$ The pressure at sea level may well pose some difficulties, but as you suggest these are not insurmountable. The real problems are the temperature and the humidity coupled with their lack of metals for technological solutions. On Earth the temperature and humidity tend to drop as you ascend a mountain ~ 6.5 deg C/km. They have the reverse (so it's 33 degrees C hotter at the bottom. At 50 deg C and 70% relative humidity humans might survive for an hour without protective measures. What can they do to keep themselves cool when climbing on rough slopes in a super amazonian rain forest for days? $\endgroup$
    – Slarty
    Commented Sep 8, 2022 at 17:18
  • $\begingroup$ Have to give this -1 for not addressing temperature stability. 37° and 70% humidity is a significant heat-stroke threat - 50° and 100% is OMG-WTF-GTFO territory. A diving bell/suit while pumping air from above might work, if we're using the diving analogy, but it's hard to see that working in the tech and metal budget given. $\endgroup$
    – Toby Y.
    Commented Sep 8, 2022 at 21:43
  • $\begingroup$ I think you have a measure of the issue. Perhaps I didn't make the question clear enough, but it is certainly a temperature and humidity issue. I think I will have to post an answer to act as a prompt $\endgroup$
    – Slarty
    Commented Sep 9, 2022 at 0:14
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    $\begingroup$ @TobyY. For whom? The people who evolved on the planet? And have you ever been in 37° and 70% humidity? I have, in Houston, TX and I survived just fine (like pretty much everyone else in the city) without even risking heat stroke. Should it be ignored? Like a thousand other details, that's up to the author. Too bad I can't down vote your comment. It's a bit uneducated. The situation isn't nearly as drastic as you think. $\endgroup$
    – JBH
    Commented Sep 9, 2022 at 5:15
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    $\begingroup$ @JBH Nope, no personal experience - but I'll point out that none of the experience you've quoted actually matches OPs specs either, even if it's closer. You're extrapolating from that experience to say 'this isn't a problem' - I'm using thermodynamics to say 'I don't believe your extrapolation is valid'. There are two important thresholds here: ~37°C, where you no longer get 'free' cooling due to being warmer than the atmosphere, and 100% humidity, where evaporative cooling due to sweat stops working. You've given figures on the edge of those limits - OP has blown past them. $\endgroup$
    – Toby Y.
    Commented Sep 9, 2022 at 8:43

Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as 101,325 Pa (1,013.25 hPa), which is equivalent to 1013.25 millibars,1 760 mm Hg, 29.9212 inches Hg, or 14.696 psi.2 The atm unit is roughly equivalent to the mean sea-level atmospheric pressure on Earth; that is, the Earth's atmospheric pressure at sea level is approximately 1 atm.


If 1013.25 millibars equal 760 mmHg, 1 mmHG equals 1.3332236 millibars. So 1,000 millibars, or 1 bar, equals 750.06173 mmhg. The question states that at 5,000 meters altitude the pressure is 2 bar, which equals 1500.1234 mmHg. Vesper's comment says that if the pressure is 2 bar at 5,000 meters it will be about 3.8 bar at sea level. Slarty's comment suggests the pressure should be about 3.2 bar at sea level.

3.2 bar would be 2,400.1975 mmHg, and 3.8 bar would be 2,850.345 mmHg.

So for the atmosphere to breathable at sea level, it would have to have a mixture of gases which are not toxic to humans at their indivdual partial pressures and which add up to about 2,400.1975 mmHg to 2,850.345 mmHg.

In Habitable Planets for Man, 1964, pages 13 to 19, Stephen H. Dole dicusses the atmospheric requirements for a planet to be habitable for humans.


On page 19 Dole writes:

To summarize, then, the atmosphere of a habitable planet must contain oxygen with an inspired partial pressure of between 60 and 400 millimeters of mercury and carbon dioxide with a partial pressure roughly between 0.05 and 7.0 millimeters of mercury. In addition, the partial pressures of the inert gases must be below certain specified limits and other toxic gases must notbe present in more than trace amounts. Some nitrogen must be present so that nitrogen in combined form can find its way into plants.

So a breathable atmosphere must contain betweeen about 60.05 and 407.00 millimeters of mercury (mmHg) of oxygen and carbon dioxide.

But a habitable planet will have liquid surface water, and thus it will have varying amounts of water vapor in the atmosphere. Dole discusses the humidity of a breathable atmosphere on pages 19 to 20. Table 4 on page 21 gives no minimum value for water vapor but gives 25 mmHg as the maximum. It also suggests that 10 mmHg might be the minimum pressure of nitrogen.

So a breathable atmosphere containing only the four essential gases would have a miniumum pressure of about 70.05 mmHg. Thus an extra 2,330.1475 to 2,780.295 mmHg of non toxic gas pressure would be needed to bring the total atmospheric pressure at sea level up to about 2,400.1975 mmHg to 2,850.345 mmHg while remaining breathable.

According to table 3 on page 16 and table 4 on page 21, up to 1,220 mmHg of argon would be breathable, and up to 3,900 (?) mmHg of neon would be breathable. Thus the maximum breathable pressure of argon and neon would total about 5,120 mmHg, 2,269.655 to 2,789.8525 mmHg more than needed. But the combined maximum tolerable leves of argon and neon would probably exceed human tolerance, so a much lower combined pressure would be desireable. It is is good that there over 2,000 mmHg to work with finding the maximum tolerable level.

But there would not be much need to mess around with neon and argon levels, since one of the essential gases, nitrogren, is tolerable at high pressures. Up to 2,330 mmHg.

So if there is a partial pressure of 2,330 mmHg of nitrogen at sea level, 0.175 to 450.295 mmHg of other gases like neon, argon, etc. would be necessary to bring the sea level atmospheric pressure up to about 2,400.1975 mmHg to 2,850.345 mmHg.

Assuming the atmosphere has the maximum amounts of the necessary gases, oxygen, carbon dioxide, and water vapor, that will be 432 mmHg. So to reach the required about 2,400.1975 mmHg to 2,850.345 mmHg, 1,968.1975 to 2,418.345 mmHg of various gases would be needed. Since nitrogen is breathable at partial pressues up to 2,330 mmHg, at most 88.1345 mmHg of neon, argon, xenon, & krypton would be necessary.

So if Earth humans colonize the highlands at 5,000 meters altitude, they should be able to survive breathing the air at sea level, if it has the proper mix of gases.

But only if the water vapor is low enough at sea level. The question gives the humidity at 90 percent at 5,000 meters and a much lower air pressure, and I don't kow how to translate that to mmHg of water vapor.

I note that at 50 degrees C, the specified teperature at 5,000 meters, 90 percent relative humidity would be 74.7 grams per cubic centimeter in one atmosphere pressure.


I guess it might be about 149.4 grams per cubic centimeter in two bar pressure.

More research would be necessary to say if the humidity would be tolerable to breathe at 5,000 meters above sea level, or at sea level.

Answers to this question contain formulas for calculating the scale heights of atmospheres.


And you might want to calculate the scale height of your atmopshere to see how dense it would be at sea level.

If the people of your planet are human colonists from Earth, they are likely to be quite unconfmortable living at 5,000 meters with air twice as thick as at sea level on Earth. And thus they will be even more uncomfortable with the ever thicker air as they descend toward sea level.

Of course a species of human like people who evolved at the high altitude of 5,000 eters should be quite comfortable breathing the air at that altitude. And when they descend lower to places with denser air, thatdenser air will be closer to what they are used to and and they were be more comfortable with it than Earth humans would be.

  • $\begingroup$ It's 2 atmospheres and 20-30 degrees C at 5000m altitude with normal humidity (20-40%RH say). The 50 degrees C and 100% relative humidity occurs at sea level 5000 metres below. Sorry if this was not clear $\endgroup$
    – Slarty
    Commented Sep 8, 2022 at 18:39
  • $\begingroup$ @Starty Well that makes the humdity less of a problem at 5000 meters. $\endgroup$ Commented Sep 11, 2022 at 2:58
  • $\begingroup$ Yes indeed humidity is not a problem for the people on the plateau at 5000m that's where they live, work and play quite happily. The problem comes when they venture down from the plateau. As they go down towards sea level the temperature and humidity goes up and that causes them massive problems. It's the reverse of the problem we have with mountains. For us sea level is fine and the temperature goes down as we climb up very high mountains. For them high altitude is fine but the temperature (and humidity) go up the as they descend (and increase to unsurvivable levels). $\endgroup$
    – Slarty
    Commented Sep 11, 2022 at 11:40

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