Dry, thin air is forgiving; as many a mountaineer could tell you, temperatures that would fry or freeze with humidity and heaviness just don’t bother you at altitude. The reason? There’s just not as much substance to conduct and convect heat away from the body, so the sun, as long as it shines on you, keeps you warm, and the sweat evaporates readily. On an alien planet this effect could easily extend to much more pronounced levels than on our own world.
On Earth “the death zone”, the altitude beyond which even acclimatized individuals will not survive long-term, is generally considered to be 8000 meters and up, or starting at 26,200 feet. The corresponding pressure level is 356 millibars, around a third of sea level. But a wrinkle that science-fiction worldbuilding often misses is that this altitude is not deadly due to lack of pressure, but rather lack of oxygen. Our atmosphere is 78% nitrogen; we only metabolize the 21% of our air that’s oxygen. Take the non-oxygen components out of the air, and you’re left with a breathing mix that provides as much metabolic fuel as at sea level but is only 200 millibars or so in pressure, lower than the beginning of the “death zone”. The partial pressure of oxygen that corresponds to “the death zone” should be 21% of 356 millibars, which would equal 75 millibars, assuming a pure oxygen atmosphere.
“The death zone”, as far as I’m aware, is a somewhat overwrought concept, in as much as people have summited Everest without supplemental oxygen; at Mount Everest’s summit the pressure is as low as 300 millibars total, with the partial pressure of oxygen being 63 millibars up there. But even if you could extend the threshold at which a human would succumb to hypoxia, the Armstrong limit is around 63 millibars, depending on your exact body temperature: at pressures lower than this, “ebullism” sets in and the body’s fluids start to boil away, a much more serious problem.
But almost right down to the Armstrong limit it seems humans could adapt long-term, assuming a pure oxygen atmosphere. And this is extremely low pressure compared to Earth at sea level: barely more than a twentieth as much. What would be the sensation of such a world? Well, as part of my worldbuilding for my fictional planet Cerberus, I’ve already covered some of this. But the biggest upshot is that as long as the sun is shining on you, you’d warm up, and heat would not be conducted nearly as much away as even at Mount Everest, let alone your average high desert. Which for an Earth-analogous sun, means that a given temperature, you’re going to heat up much more easily. Ergo, the temperature at which you’re most comfortable in the sun decreases with ambient air pressure.
The intensity of sunlight versus being in the shade matters more and more with lower pressure (culminating with the vacuum of space, of course, where a surface could easily be a hundred degrees above zero on the sunlit side and a hundred degrees below zero in the shade); shade becomes brutal, but surfaces heat up much more easily. A dark (i.e. radiation-absorbent) surface on a sunlit slope would easily rise above the freezing point even with air temperatures that we on Earth would consider brutally cold. There, it would be perfectly comfortable. And if it heated up enough during the day, it would stay warm through the night. Perhaps not warm enough for constant thermal comfort…but that’s when you consider a consistent day-night cycle like Earth receives in the mid-latitudes. Even Earth itself defies this pattern.
In certain circles Alaska is famous for the cultivation of huge plants: the areas that have suitable soils for farming have surprisingly rich productivity. The reason? The growing season is short in terms of the number of days, but each day has a lot more sunlight. At latitudes that high, summers have constant daylight. So there is up to twice as much possible sunshine for plants to grow…or for humans to enjoy. I myself experienced a deeper and more rapid suntan in northern Norway than I ever have in California…because when I slumbered in my car, the sun was above the horizon shining on me through the glass instead of it being dark out, so the cumulative exposure time was double (it helped in my particular case that Norway had unusually clear weather when I was there, but it illustrates the principle).
You even see this in climate statistics. Fairbanks, Alaska records 334 hours of total sunshine in an average June; San Diego, California averages 242. Substantially less. And that’s not just because of “June Gloom”; Fairbanks’s 334 hours in June is more sunshine than any month averages in San Diego. The reason, of course, is Fairbanks’s high latitude: 66 degrees (just short of the arctic circle), compared to San Diego’s 32 degrees. San Diego isn’t really cloudier than Fairbanks; the sun is just above the horizon much less during the summer.
Imagine how warm an exposed dark surface could get with such continual daylight…with the low air pressure minimizing conduction and convection, it could easily be 20 or even 40 degrees Fahrenheit below zero by air temperature, even as ground temperatures and surface temperatures of any vegetation were well above freezing. Flowers could bloom and grapevines could grow off of that slope and into the sunlight even as ice fog suffused the air. Very classic sci-fi, yet the physics are plausible. Human beings in the sun would need a jacket to break the wind and insulate, but the clothing could be light indeed as they tended vibrant lush vineyards…as long as this whole environment stayed sunlit.
Fairbanks pays for its summertime cheat code in the winter; in December Fairbanks on average records 36 hours of sunshine…across all 31 days of the month. Not because it’s particularly cloudy but rather because the sun isn’t above the horizon much that time of year. On a slope like we’re worldbuilding, the ice fog would still be there, but ambient air temperatures would be much colder than summer’s -40 degrees…and being in shade the vast majority of the time, the surfaces would become extremely cold no matter how dark and radiation-absorbent they were.
Already this is fascinating enough, and would enable summertime gardening on certain planets that we’d dismiss as far too cold to possibly be habitable. The thinner the air, the more the world becomes dominated by microclimates driven by solar geometry, soil color, and wind geometry driven by the landscape. We see shades of this in the high deserts of Earth, but there is probably an entire class of planets out there that take it far further. And while the Antarctic vibes of seasonal nunataks opening up into a superbloom may well be very common, there’s an even more delightfully alien possibilifty: a peak of eternal light.
What does this mean? Well, it’s simple solar geometry. On any planet, at the poles, the sun circles around you horizontally on any given day; on ours, specifically, it corkscrews up to 23 degrees above the horizon by midsummer and corkscrews down to 23 degrees below the horizon by midwinter. And then back again. This motion is due to the axial tilt of our planet, which not coincidentally is also 23 degrees. Without any axial tilt, at the poles the sun would circle you at the horizon, every day, endlessly. Geometrically, you’d see the top half of the sun above the horizon and half of the sun below. So it would be weak sunlight indeed.
But consider terrain; the horizon has an apparent “dip” when you increase altitude due to the curvature of the planet, so as viewed from a sufficiently high altitude, such as on a high mountain, the sun will distinctly circle above the horizon. At a low angle, but above the horizon it will be. At the very summit, therefore, an object will always be sunlit from some direction or another. Ergo, our hypothetical dark slope could be permanently warmed up by the sun. So you need not cultivate only plants under a greenhouse or plants that are hardy against extreme winters; even tropical plants dependent on steady, warm, frost-free temperatures could grow outdoors in ambient air…as the sun shines on them through ice fog and the thermometer in the shade reads 40 degrees below zero. The human gardeners too would be perfectly comfortable.
Now, 40 below zero is only a very rough ballpark estimation, but with sufficiently thin air, close to the Armstrong limit, the effect could probably be pushed much further, especially if you’re dealing with a continuous warming of the surface by sunlight as opposed to merely episodic sunshine in the summer. In rarefied but still breathable air on a peak of eternal light, I wouldn’t be shocked if plant life grows unaided even at -100 degrees Fahrenheit.
Outer space presents the ultimate “banana belt” environment, and not just in full vacuum on a habitat; there could be (and likely actually are) planetary surfaces where “impossible” temperatures are in fact hospitable, to human life or perhaps even native life. Tune the conditions just right, and oases of warmth and lushness could be found shockingly far away from the sun. All due to the magic of sunlight and air pressure…