# Thin Air in Aylar Worlds



## Sheilawisz (Mar 17, 2014)

As some of you know, many of my Fantasy worlds feature a species called Aylars as the most important characters of my stories.

The _Aylaris Lethalius_ are complex creatures and maybe I will create another thread about them, but this thread discusses the main difference between Aylar worlds and Earth: Their worlds have a different air, but the difference does not have a chemical nature.

Aylar world's air is about 80% Nitrogen and 20% Oxygen, which is quite similar to Earth's air... However, they have less air above their heads and that means that the air at sea level in an Aylar world is equivalent to the air found in Earth at an elevation of about 3700 meters or 12130 feet.

I have been using the site Altitude.org and their Air Pressure calculator to try and create an altitude/air pressure scale for my Aylar worlds. In Earth, an elevation of 3700m means that you have only 65% of the air that is available at sea level...

That means that they have a little less than two thirds of Earth's air at sea level, and I used that formula to calculate how quickly the Aylar air would become thinner with an increase in elevation.

First, these are the numbers for Earth Air:

900 meters: 90%
2000m: 80%
2500: 75%
3100: 70%
3500: 66%
4300: 60%
5750: 50%

7400: 40%
8000: 37%
8800: 33%
9500: 30%

Now these are the numbers for Aylanya (my most important Aylar world). Please consider that the percentages refer to their own sea level air, not Earth's:

600 meters: 90%
1300: 80%
1600: 75%
2000: 70%
2300: 66%
2800: 60%
3600: 50%

4900: 40%
5300: 37%
5800: 33%
6300: 30%

According to these calculations, a simple elevation of 600 meters (1970 feet) in Aylanya would be equivalent to some 4800 meters in Earth, the elevation of Mont Blanc. Aylanya's 1300m would mean 5500m in Earth, which is beyond the record altitude for human survival at long term.

Human visitors in Aylanya would better get some oxygen tanks at a modest elevation of 2300 meters (7550') because that's equivalent to Earth's 7000m, and my other Aylar worlds like Lallhalya are designed to have even thinner air.

What do you think of these numbers? Do they sound correct to you?


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## Ankari (Mar 17, 2014)

What's the reason for the thinner air? Why do these worlds have thinner atmospheres? Atmosphere is formed by the gravitational pull of a planet. Gravity is the product of the size of the planet. The smaller the planet, the less of an atmosphere it can retain. Did you factor this into your world building? Besides the size of your planet, you have to consider the lack of gravity, and how beings on such a planet would interact with the world.

A lower gravitational pull will create taller beings with lighter bone structures. They would be able to jump further distances. Waves would be larger, and buildings can be taller. Also, people can pick up heavier objects.


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## Sheilawisz (Mar 17, 2014)

@Ankari: Aylanya is the same size exactly as Earth, and also it features the same gravity. The difference is that it has a smaller atmosphere since the planet was formed, even though it can potentially hold as much air as Earth.

Lallhalya is more of a Fantasy world. It's not a planet like Aylanya or Earth, but an Alaska-size island located in an endless ocean... Anyway, this thread is focused on the calculations for Aylanyan Air alone. Do you think they are accurate and believable numbers?

It's interesting to analyze how the thin air would affect wildlife...


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## Penpilot (Mar 17, 2014)

Sheilawisz said:


> It's interesting to analyze how the thin air would affect wildlife...



No expert, but one thing to consider is atmospheric pressure. Outside of gravity pulling us down there's the weight of the air pressing against us in all directions.

Things that come to mind are the boiling point of liquids will be lower. Also I think flight will be harder because there's less atmosphere to create lift, so if you have flying animals, there maximum size will be smaller than what we have here on earth.


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## Sheilawisz (Mar 17, 2014)

Thanks for your comment, Penpilot.

Aylanya is a world with many species of dangerous birds, and they have dragons as well. The dragons are not particularly large, and they tend to live at high altitude in the Northern mountains.

I do consider that the thinner air makes flying more difficult, because the birds would need a greater effort to generate and keep a good lift- Maybe they would feature longer wings, in order to be able to hover better in the thin air.

However, here in Earth we have a bird species called RÃ¼ppell's Vulture, which is the highest flying bird of the world. This vulture can fly as high as 11300m (37000') above sea level, that's so impressive... According to my calculations the Ruppell would be able to fly as high as 5300m in Aylanya, and probably it would be the highest flying creature in that world as well.

I did not think before about the lower boiling points for liquid, and especially water, in a thin air world (their tea and other infusions would be terrible!). I need to think more about that.

Do you think that my numbers for the Aylanyan air pressure/elevation are accurate?


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## Snowpoint (Mar 17, 2014)

Sheilawisz said:


> Maybe they would feature longer wings, in order to be able to hover better in the thin air.



The problem with larger wings is that they weigh more, requiring more lift. I don't know how it all works, but maybe the birds are more like humming birds... but with long, barbed beaks... for hunting?


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## Penpilot (Mar 17, 2014)

Sheilawisz said:


> Do you think that my numbers for the Aylanyan air pressure/elevation are accurate?



Like I said, I'm no expert but they seem accurate enough for you to write stories around. I found this page that lets you extrapolate barometric pressures. The Barometric Formula Maybe it will help you some more.


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## Wormtongue (Mar 18, 2014)

Ankari said:


> What's the reason for the thinner air? Why do these worlds have thinner atmospheres? Atmosphere is formed by the gravitational pull of a planet. Gravity is the product of the size of the planet. The smaller the planet, the less of an atmosphere it can retain.



Venus has 92 times the surface pressure as Earth, even though it's slightly smaller.  You have to be 60km above the surface of Venus to experience Earth sea level pressure.


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## Ankari (Mar 18, 2014)

Wormtongue said:


> Venus has 92 times the surface pressure as Earth, even though it's slightly smaller.  You have to be 60km above the surface of Venus to experience Earth sea level pressure.



Yes, but this was explained by the content of the atmosphere, and not because the atmosphere is larger. Venus's atmosphere is nearly all CO2, nitrogen and sulfur dioxide. The gravitational pull of venus is nearly that of earths, but the density of Venus's atmosphere is off the charts because of the molecular content.


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## Sheilawisz (Mar 18, 2014)

The atmosphere of Venus has a mass much larger than Earth's atmosphere, indeed.

I believe that a planet's atmosphere depends on several different factors: The mass (and gravity) of the planet, how strong its magnetic field is so the atmosphere does not get blown away by solar winds and also how much gases were available when the planet was formed.

My theory is that Aylanya did not have a lot of gases to form an atmosphere when it was a young planet, but its magnetic fields are strong enough to protect the atmosphere that it already has.

Anyway, do you think that my calculations about Alyanya's air pressure/elevation sound correct?


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## Queshire (Mar 18, 2014)

The work you've put into it is really neat, but from a heartlessly pragmatic view point I don't think it's neccesary if your equation is accurate or not, and I don't know enough about the subject to say if it's accurate or not. The natives of your world wouldn't know about the thinner atmosphere as that's just how it is for them, so IC it'd only come up if there was a visitor from another world. OOC it would only come up as justification for why things are different from the real world and thus repair a broken willing suspension of disbelief. Now, nothing I've heard so far seems to suggest that it'll be a common occurrence for you to have to deal with a broken willing suspension of disbelief and for those whom it does break their disbelief, how many are going to have the required scientific knowledge to say "This is WRONG!" For such microscopically few people, eh, well you can't please everyone.

Too long, didn't read; yeah you should be good.


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## Sheilawisz (Mar 18, 2014)

Thanks for your comment, Quesh.

The story about Aylanya is called _Entre el Hielo y el Cristal_, and it's a first-person biography style novel. The narrator is a teenage Aylar girl that describes her life and her world to a friend while she visits Earth, so when she starts to mention the many differences between both worlds, the thin air is one of the things that she explains.

I have decided to leave my calculations as they are, they sound good enough.

With the help of the Altitude site I have calculated all the way up to the point where there is virtually no air, which is 33300 meters for Earth and 22000 for Aylanya. The altitude/pressure numbers for Lallhalya are crazier, but I do not really worry about that because that world is part of a pretty impossible universe to start with =)


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## chrispenycate (Mar 18, 2014)

With an equivalent gravitational field the pressure will drop off at the same rate percentagewise as on Earth. That's for gasses of equivalent molecular weight, but you'd specified that. To get a faster drop off with altitude you need a higher gravitational field. Sea level pressure can be higher or lower than Earth, that's only dependent on absolute quantity of gas (which can vary wildly, even within the solar system, the only region we've studied so far.

Ignore the magnetic field for the moment; Venus doesn't have one, IIRC, and it's not going to effect many molecules, anyway. Just charged particles, and most of them come from outside.


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## Sheilawisz (Mar 19, 2014)

Thank you, Chrispenycate.

You mean that the air pressure at Aylanya's elevation of 9300 meters should be 31% of its own sea level pressure, as it is in Earth, and not the 15% that it is according to my calculations?

I considered that idea when I started to design the Aylanyan atmosphere, but it occurred to me that, if Aylanya is holding an air mixture chemically almost identical to Earth's but with a smaller mass and weight, then its volume would be smaller too and the top of such an atmosphere should reach a much lower elevation.

At least, that's what would happen if the atmosphere was a liquid... but with gases, I am not sure anymore. This is so difficult to calculate! How do you know?

I'll be thinking about it =)


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## chrispenycate (Mar 19, 2014)

A liquid has a surface; a gas just sort of dwindles into vacuum. Given you've got less total gas it will reach your chosen threshold level – one molecule per cubic centimetre, or whatever you like - at a lesser altitude, but the drop off rate will be the same (slower for a less dense gas, a tiny smidgin higher because of the gravity fall off with distance, but look at the thickness of the atmosphere layer relative to the diameter of the Earth; and the 'd' in dee squared is from the centre of the Earth).

A liquid is incompressible (very nearly. I'm afraid it's engineering approximations, close enough for rock and roll, that you're getting from me, not true scientific precision. I don't care about parts per million, or I'd have to worry about less gas giving less greenhouse effect, meaning more day/night temperature cycling… so I simplify) so pressure = depth times density. A gas compresses, so the density is a smooth increase from edge of vacuum to lower edge of atmosphere= ocean or land. So pressure drop off is closer to being a square law. Of course, the fact that temperature decreases at higher altitudes increases the rate somewhat; but this is true however much gas you have. In fact, the drop off in pressure might be a tiny bit slower than Earth's, because it's not cooling so much from altitude. Or a tiny bit faster because gravity doesn't drop off quite as much. But you're surely not going to inflict this on your readers, nor will there be many as pedantic as myself


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## Sheilawisz (Mar 20, 2014)

I see that you have a deeper understanding of these concepts than I have, Chrispenycate.

Taking this into consideration, the air pressure at sea level in Aylanya should be 50% of what it is in Earth, so they could have 60% at 4300 meters (an elevation that is important in the story) which would be equivalent to 30% of Earth's sea level pressure found at 9500 meters.

In this scenario, the Aylanyan sea level would be equal to 5750 meters in Earth... I am not sure about accepting that, this air seems a little too thin for the animals that inhabit their sea level.

I want to keep the numbers that I already have... for some reason, I love the idea of Aylanyan air getting dramatically thinner even with a modest increase in elevation. Do you know of any possible theory that could explain the sharp drop in air pressure, while keeping a sea level pressure equal to Earth's 3700 meters?

As you have said, it's not necessary to explain all the theory to the readers of my story, but I want to know these details for myself even if they are not mentioned by the narrative.

In case that nothing can explain the dramatic drop in air pressure, I can leave it as a mystery.

Thanks a lot! XD


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