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The question: What realistic environmental signs would telegraph to the occupants of a floating landmass that their home was slowly losing altitude?

The planet: I'm working on a story that takes place on a gas planet, but with breathable air (I'm sure it has a solid core below its dense cloud layer somewhere, but it's not relevant to the story). It's close enough to its star for the planet to have a well-lit atmosphere during the day, and probably doesn't get totally dark at night, due to its gaseous nature. The length of a day, and the gravitational pull, are the same as ours on Earth. There are no land masses as such, but there are incredibly slow, floating sky whales of epic proportions (like 10+ miles long) that people live pastoral lives upon as if they were land masses (neither the humans nor sky whales are native to this world, so don't worry about how they evolved to be this way).

The event: At one point, the whale the protagonists are living on slowly begins to lose altitude. Not in a way that's immediately noticeable, but gradually over years and years. My question is how would the occupants know this was happening? They know they live on a giant flying creature, and have seen others in the distance at different altitudes. The whales live thousands of years, and this one's behavior has remained unchanged for all of recorded history. The people revere them as gods and don't suspect they ever change course (and would probably believe it was the apocalypse). How would they realize something was wrong?

The signs: I know that the air pressure would increase, raising the boiling temperature of water, but what else would happen? Would the sun's path across the sky change? Are different weather patterns more likely at lower altitudes? Basically, what believable signs would alert the occupants to the problem before a catastrophic pressure increase killed everyone on the island/whale?

UPDATE: Thanks for all the great suggestions! I don't want to select a "correct" answer, because everyone pointed out different things that are just as relevant. To summarize what I've learned, and plan to use:

  • Scientific observations (such as barometric pressure) collected by the protagonist.
  • Anecdotes from older residents or visiting traders.
  • More intense thunderstorms.
  • Strange animal migrations (to higher ground or never returning).
  • Vertical migration of natural flora.
  • Shorter days, and sunset shadows lower on the wall, as the horizon rises.
  • Failure of delicate crops due to less light and heat.
  • Weight scales off a little due to increased gravity (very subtle).
  • Rise in respiratory health problems due to change in atmospheric gases.
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    $\begingroup$ Do they have instruments advanced enough to detect a subtle increase in the boiling point of water? Such an advanced civilization would keep a lot of records. How does that fit in with that pastoral lifestyle? $\endgroup$
    – Raditz_35
    Commented Feb 15, 2018 at 14:35
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    $\begingroup$ I was thinking for them that water would just take longer to boil. $\endgroup$
    – Shanenopolis
    Commented Feb 15, 2018 at 14:36
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    $\begingroup$ p.s. I like the world building. I hope you get enough science in there to make is at least plausible. Sounds fun. $\endgroup$
    – Len
    Commented Feb 15, 2018 at 16:57
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    $\begingroup$ @DarkHippo Actually gravity would most likely drop rather than increase as you move towards the core. (Gravity drops, pressure rises, at the very center gravity is zero). $\endgroup$
    – Tim B
    Commented Feb 15, 2018 at 17:31
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    $\begingroup$ @TimB That depends on the mass distribution of the body in question. Sure, once you're inside some of the mass, the magnitude of the gravity pulling you towards the body's center will decrease (some will now be pulling you UP). However, if you are on a "floating mass" descending in some planet's atmosphere, assuming the atmosphere's mass itself to be negligible for purposes of gravity, then by definition gravity will increase proportional to the square of the change in distance (or decrease when moving outwards/upwards). $\endgroup$
    – Darren Ringer
    Commented Feb 15, 2018 at 20:17

12 Answers 12

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Clouds

Different types of clouds sit on different layers based on altitude, air pressure, and relative humidity. You mentioned your atmosphere has breathable air, does it also have water vapor in the atmosphere? Are there rain clouds? Methane clouds? Sulfur Dioxide clouds? Maybe an enterprising character notices that the moisture collectors are collecting more methane than before, or that when they head into a storm cloud, they get more violent turbulence than before. Depending on how far up they are, maybe they are used to riding out some of the more violent storms that happened lower in the atmosphere, but the protagonists have a rude awakening when a storm that they previously wouldn't have paid mind to 10 years ago tragically destroys their town.

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    $\begingroup$ It's a function of air pressure, temperature, and humidity. Check this link out for more info. But the point is, as long as you have a breathable atmosphere, you're going to get at least some variation in cloud cover. I'm not sure what your planet's atmosphere is composed of or what your altitude is, but no matter what you're going to get some cloud variation. $\endgroup$
    – Hans Z
    Commented Feb 15, 2018 at 17:04
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    $\begingroup$ Seeing clouds that used to be below now being around you is going to be a much more obvious sign than it being darker or the tiny fluctuation in gravity. $\endgroup$
    – Hans Z
    Commented Feb 15, 2018 at 17:06
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    $\begingroup$ Thanks for this, I will definitely use more intense storms as part of the plot. Maybe the older generation tell stories of how you used to be able to see the lightning flashing far below, but now it strikes in town. $\endgroup$
    – Shanenopolis
    Commented Feb 15, 2018 at 17:47
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    $\begingroup$ Never thought I would see an answer with a header of 'Butts.' I feel completed. Nonetheless, I can't really find a reference for the word. searching 'storm butt' yields a lot of random noise. $\endgroup$
    – Jakob Lovern
    Commented Feb 15, 2018 at 18:43
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    $\begingroup$ Damn, I blame cloud to butt $\endgroup$
    – Hans Z
    Commented Feb 15, 2018 at 18:48
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By their awareness of other whales and the knowledge of the traders and pilgrims who pass between them.

If the whale is slowly sinking over years or decades then the residents are unlikely to notice other than old people moaning, and old people always moan. It's reasonable to ignore anything they're saying about the sun being brighter and the stars clearer when they were young. Over the time periods you're implying, the changes are so slow and subtle they can be written off as faulty memory.

It's only the people moving around between the whales who will notice that this particular whale is considerably lower in the atmosphere than others.

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    $\begingroup$ I suppose one comment by a passing trader might be dismissed, but perhaps when taken with many other signs, it might point to the real problem. $\endgroup$
    – Shanenopolis
    Commented Feb 15, 2018 at 14:39
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    $\begingroup$ One passing trader might pass comment, but as the years go by it's going to be more of a thing to them, visiting the low whale. Not that it's falling, just that it's noticeably lower than the others. Older people might remember a time when that wasn't the case, it depends how long you want it to take. $\endgroup$
    – Separatrix
    Commented Feb 15, 2018 at 14:43
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    $\begingroup$ Good point. Longer intervals between trade caravans might be significant to them. $\endgroup$
    – Shanenopolis
    Commented Feb 15, 2018 at 14:44
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Since you said the gravitational pull is just like earth's than you would probably notice because the air pressure would increase. Not by much, of course but if you had a barometer and you took readings regularly, then you would notice the increased air pressure. It would have to be consistent as air pressure varies from day to day.

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    $\begingroup$ In the novel Nightfall a group of Astronomers notice that the simulation they use to predict the motion of the stars is just a bit off. This is a clue that their information on the makeup of the solar system is incomplete and something big is going to happen. I can imagine a similar situation in this world where the weathermen notice their forecasts for barometric pressure (and rain) are going askew and this leads to the discovery of their home gradually sinking. $\endgroup$
    – thepizzaelemental
    Commented Feb 15, 2018 at 18:19
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    $\begingroup$ By extension, wouldn't the water boil more quickly/slowly at a different altitude? Wouldn't expert cooks notice the difference? $\endgroup$
    – kikirex
    Commented Feb 15, 2018 at 19:53
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    $\begingroup$ I imagine a cook who can’t believe what they’re seeing running to other professional chefs. The chefs meet up and compare their measurements over a few weeks, and then they collect the whole town/village, all in their professional chef outfits, telling them: "Folks, we have a problem. Water isn’t boiling as it’s supposed to be, and it’s weird.". And that’s the start of a dramatic development about that sinking world. Absurd/Amazing. $\endgroup$
    – Jonas Schäfer
    Commented Feb 17, 2018 at 10:24
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What about changes in the behaviour of the animals on your floating continent?
It's well documented that animals seem to be able to notice the signs of natural disasters like storms or earthquakes long before humans do, so perhaps the animals instinctively react to the changes in altitude.

Maybe some of them start moving to higher ground then they have ever been seen before, or perhaps those that are capable flee entirely to another whale. Migrations of birds that never return would certainly be noticed, people may not realise why but it would probably be enough to make them start wondering and looking further into what is happening (so they may notice some of the less immediately obvious signs pointed out in other answers like the brightness of the sun and temperature).

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    $\begingroup$ Migrating birds that never return was actually something I had in mind, but some terrestrial animals moving to higher whale-ground is also a good idea, thanks! $\endgroup$
    – Shanenopolis
    Commented Feb 15, 2018 at 17:43
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A few differences you may notice:

  • Darker: As you get more atmosphere above you it will get darker (depends on how dense the atmosphere is though as to how far down you go before it is noticeable).
  • Colder: You're getting further away from the sun's heat. This will depend again on the structure of your atmosphere.
  • Distance to other near by objects will change: If there are any other floating objects around they will get further away (if above).
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  • $\begingroup$ Though the change would be subtle, I could probably tie the increased dark and cold to something else, like the crops failing. $\endgroup$
    – Shanenopolis
    Commented Feb 15, 2018 at 16:52
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    $\begingroup$ @Shanenopolis I imagine it would effect crops, it just depends on whether anyone can make that link between failing crops and a drop in altitude. $\endgroup$
    – Lio Elbammalf
    Commented Feb 15, 2018 at 17:00
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    $\begingroup$ Wouldn't it get warmer as you have more atmosphere to trap in the sun's heat? That's how it works on earth. $\endgroup$
    – Hans Z
    Commented Feb 15, 2018 at 17:05
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    $\begingroup$ @HansZ You are partially right. Here is Jupiter's atmosphere for example: en.wikipedia.org/wiki/Atmosphere_of_Jupiter#/media/… $\endgroup$
    – Tim B
    Commented Feb 15, 2018 at 17:35
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    $\begingroup$ @TimB that's pretty interesting. Learn something every day. $\endgroup$
    – Hans Z
    Commented Feb 15, 2018 at 17:37
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Something that I haven't seen other answers mention is poor health. As they lose altitude the pressure is going to increase, yes, but the composition of their air is also going to change. Over time they are going to start breathing more of the heavier gases in their atmosphere, which will result in more cases of illness related to an improper air mixture.

Nitrogen narcosis is an example of a condition that divers have to be wary of, caused by higher pressures. People are definitely going to notice when the weakest members of their community start hallucinating.

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    $\begingroup$ huh... I considered something similar, but for the whale creature, not the people. Maybe it works for both. $\endgroup$
    – Len
    Commented Feb 16, 2018 at 14:57
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If its "not in a way that's immediately noticeable" then I doubt the average citizen of the place would detect the difference; unless they could see that they are physically lower by comparison to another landmark, another physical object in the distance.

And even that might not get their attention, since after all its a living creature they're living on. They're probably very used to it going up and down and what not.

Look to the scientist types in your story. They'll be the ones who are keeping information that they compare from time to time which could tell them something's wrong. Gas giants atmospheres are very interesting and the changes in the gases themselves could tell them a lot. It would be one of the big focuses of their scientific community.

Also, the creatures bio-rhythms would be of great concern to them. They wouldn't want it to get unhealthy, since they are, y'know, living on it. Its health is their health quite literally.

Other than that I might change it so that it IS immediately noticeable; sudden drops from time to time, before the creature rights itself, would be very dramatic; akin to earthquakes.

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    $\begingroup$ @Shanenopolis atmospheric pressure. Your science protagonist notices that the pressure has slowly increased over the years. $\endgroup$
    – RonJohn
    Commented Feb 15, 2018 at 18:24
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    $\begingroup$ @Shanenopolis in the 1640s, Torricelli developed the mercury barometer. en.wikipedia.org/wiki/Barometer#History $\endgroup$
    – RonJohn
    Commented Feb 15, 2018 at 19:38
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    $\begingroup$ @Shanenopolis, slightly off topic question here: Your characters live on a giant whale in a gas giant. How'd they get there? I presume got there via spaceship and have since de-evolved from that level of technology? If so, here's a thought, they wouldn't have lost ALL their tech or their understanding of physics. $\endgroup$
    – Len
    Commented Feb 15, 2018 at 20:00
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    $\begingroup$ > evidence she can accumulate to prove her case Is this some moral story about how the idiots don't believe the scientist who says that we are falling? :) (In any way related to global warming and such?) $\endgroup$
    – Kaz
    Commented Feb 15, 2018 at 23:23
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    $\begingroup$ > They wouldn't want it to get unhealthy, since they are, y'know, living on it. Its health is their health quite literally. To be fair, we're not exactly doing a great job of looking after the thing we're living on... $\endgroup$
    – Bob
    Commented Feb 15, 2018 at 23:58
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I have a slightly different idea for you, since the protagonists are aware they are living on a sky whale, could it be that they perform health checks on their whale deity? If so perhaps they might notice that the whale's blood tests have been shifting away from the last centuries results to now have a greater amount of heavier gases in it and this could lead them to suspect that they are losing altitude.

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  • $\begingroup$ There's actually a story reason for why they don't really know much about the creature. Discovering how it works and why it exists is part of the plot. However, there could be certain signs of its status that manifest in ways that impact their daily lives, and the characters might realize later on how it's all connected. $\endgroup$
    – Shanenopolis
    Commented Feb 15, 2018 at 18:25
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UPDATE: Thank you everyone for all the great suggestions! I don't want to select a "correct" answer, because everyone pointed out different things that are just as relevant, so I'm supplying this answer to summarize what I've learned, and plan to use (I'll mark it accepted as soon as I'm able):

  • Scientific observations (such as barometric pressure) collected by the protagonist.
  • Anecdotes from older residents or visiting traders.
  • More intense thunderstorms.
  • Strange animal migrations (to higher ground or never returning).
  • Vertical migration of natural flora.
  • Shorter days, and sunset shadows lower on the wall, as the horizon rises.
  • Failure of delicate crops due to less light and heat.
  • Weight scales off a little due to increased gravity (very subtle).
  • Rise in respiratory health problems due to change in atmospheric gases.
  • The permanent cloud layer below them seems to be closer.
Improve this answer
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    $\begingroup$ Hello. Not to be too mean spirited about site rules, but this site works on the question and answer principle, meaning that each question you ask is supposed to have only one correct answer. By you saying that this question has multiple correct answers, you are saying that your question should have been closed as 'too broad.' I understand that multiple answers have good ideas in this context, but in accordance with the site philosophy, you should pick only one of those answers as the correct one. $\endgroup$
    – kingledion
    Commented Feb 17, 2018 at 13:37
  • $\begingroup$ I understand that since I'm new to this site, I don't fully understand all the rules yet, but I feel like I disagree with your assessment. The correct answer is all these things. No one listed all of them, so no one deserves ALL the credit for answering the question. Another user suggested I do this, so it can't be uncommon, and it makes sense. Are you suggesting someone else should compile this list? That seems rather arbitrary. $\endgroup$
    – Shanenopolis
    Commented Feb 17, 2018 at 18:11
  • $\begingroup$ @kingledion: I don't see how that could possibly work on a SE site like this one. One correct answer only, really? $\endgroup$
    – Lightness Races in Orbit
    Commented Feb 17, 2018 at 19:33
  • $\begingroup$ @Shanenopolis The rules for closing a question are that if there are multiple correct answers, the question to too broad and should be closed. This includes if you ask mutiple questions and therefore get multiple answers. This has been a topic of conversation since the site launched, see two meta discussions here and here each dating from the two weeks after site launch. $\endgroup$
    – kingledion
    Commented Feb 17, 2018 at 22:38
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    $\begingroup$ @kingledion: That is not a general SE wide principle that I've ever heard of. The SE model acknowledges that, although there may be one factually correct answer to an objective question, there are always multiple ways to describe and explain that answer, which is why we have a voting system and why the OP chooses the answer that helped them the most (notice how is never referred to as "the correct answer"). Anyway, as for how Worldbuilding is run, I don't know your community so will take your word for it :) Off to bed now... $\endgroup$
    – Lightness Races in Orbit
    Commented Feb 17, 2018 at 22:48
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Color of the sky.

The color of our sky varies noticeably over about 20,000 feet of altitude change.

But I don't understand your model for the planet's atmosphere. It sounds like it's physically thick (i.e. lots of vertical km of atmosphere), but about the same pressure as ours at whale-height. You can't independently set the density & pressure at the observer, the height of the atmosphere, and the gravity: They're all interrelated because the pressure has to hold up the weight (due to gravity) of the air column (due to density and top height). What you pick for those will determine what varies fastest with altitude.

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  • $\begingroup$ I'm obviously no physicist, but if a gas planet can have pretty much any size atmosphere based on the mass and size of its core, and the barometric pressure and relative gravity would be a factor of your vertical position in said atmosphere, wouldn't you be able to imagine a planet that had the right core mass/size and the right atmospheric gases to theoretically create a "sweet spot" somewhere high in the atmosphere where pressure and gravity were ideal for humans? Or do you know of some physics reason that makes this impossible? $\endgroup$
    – Shanenopolis
    Commented Feb 18, 2018 at 6:36
  • $\begingroup$ If the atmosphere is small compared to the size of the planet, i.e. you can consider "g" to not change much, then the pressure P at any altitude is given by g * rho * lambda, where rho is the density at that altitude and lambda is the "scale height" of the atmosphere; basically, how the pressure and density exponentially drops. (For earth, lambda is about 5 miles). The OP wanted to have a large height where the pressure doesn't change much, so wants a large lambda. But then you can't have an earth-like P, g and rho; (at least) one has to be smaller. $\endgroup$
    – Bob Jacobsen
    Commented Feb 18, 2018 at 18:16
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Is your tech level high enough for a range finder?

You could use light, sound, radio, microwave, or a laser measure the distance at edge of the whale to a depths below. Over time you would chart the distance. For the radio, microwave and laser ranging that would require a tech base that could send and receive the transmissions and time the distance between them. Say 1950's tech for the basic parts, 1980's for something you could hand hold, early 2000's for some cheap and easy to use (GPS/phone).

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  • $\begingroup$ They're actually pre-industrial, and the science-type protagonist has a bunch of medieval/renaissance/steampunk measuring devices, but nothing along the lines of radar, microwaves, or lasers, unfortunately. $\endgroup$
    – Shanenopolis
    Commented Feb 16, 2018 at 6:12
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Since we as complex multicellular organisms may not be the most sensitive to changes in gravity, atmospheric pressure or gaseous composition you could use something that is more sensitive to these changes, like a fungus or bacteria on the whale. If the protagonists have contact with other whales at different altitudes they may notice whales at lower altitudes have more of the fungus/bacteria present than those above and that over time their own whale is beginning to show proliferation of the fungus/bacteria.

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