The great reclimatization

[linguofreak]

It should also be noted that the sizes of the atmospheric cells need not be fixed at 30 degrees each. If the world is warmer, equatorial convection is more important and the polar low is less important, so the Hadley (and probably Ferrel) cells will expand, and the polar cell will contract.

Also, keep in mind that if your day length, average wind speeds, or planet size are markedly different from Earth, you may not even have the same number of cells. A planet with a bigger radius, shorter day, or higher wind speeds might have 4 or 5 cells per hemisphere, a planet with a smaller radius, longer day, or lower wind speeds might have 2 cells, or possibly even one, per hemisphere.

[bulbaquil]

At an Earthlike axial tilt, at least, you can't have 2 cells per hemisphere; you have to have an odd number (because basically air will always be rising in the tropics and sinking at the poles).

[linguofreak]

Correction: Greater wind speeds will *decrease* the number of cells.

Also, after reading up some more on things, it seems like the width of the Hadley cell has more effect on things than the number of cells beyond it. (Basically, temperature drops slowly going poleward within the Hadley cell, quickly outside it).

Meanwhile, I made some guesses about the planetary parameters for Almea (the only hard number Zompist gives is radius), and ran them through the planetary sim at the bottom of this page.

It gives good general temperature estimates, but doesn't take into account terrain (other than a simple query for percent ocean coverage), and I don't think it models Hadley circulation either, rather, it just asks you for an equator to pole heat transfer rate relative to Earth.

I tweaked things until the average planetary year round temperature was about equal to Earth, and it turns out that:

The equatorial temperature is estimated to be about 10 F (~5 C) cooler than on Earth, so the sea level* climate at the equator would be equivalent to something like San Jose Costa Rica, which lies at about 3,800 feet.

The polar temperatures, however, are estimated to be much warmer than on Earth: Şiḍḍi is about on a level with Iceland, at 65 degrees latitude, but the temperatures at that latitude on Almea would be roughly equivalent to somewhere around Berlin (52 N), about 4F / 2C cooler in summer, and 4C / 8F cooler in winter, so I don't think Skouras and Xurno are in too much trouble.

Verduria, meanwhile, lies right around 35 S, which is where this simulator predicts Earth and Almea to have approximately equal temperatures.


*Speaking of sea level, the atmosphere will likely be thinner on Almea, being that it's only 5320 km in radius, by how much depends on a lot of factors, but the simulator I used suggested about .6 atmospheres (it allows you to choose your surface pressure, but makes a suggestion based on planet size). This is equivalent to about 14,000 feet of altitude on Earth. Depending on how quickly the pressure around them changed, what percentage of oxygen Almean air contains (it would need to be around 33% oxygen to have the same oxygen content at sea level, compared to 20% on Earth), this could have impications for the condition that the Hellenikoi and their ship arrived in. A quick pressure change would imply lung trauma to the crew and possibly significant damage to the ship. If there's the same percentage of oxygen as on Earth, then, even without rapid pressure change, they'd still arrive feeling like they'd sailed up Pike's Peak. (The thinner atmosphere could be worked in as another reason why only the Elkaril go into the mountains, if it's not oxygen rich).

Another interesting factor is that surface gravity would be about .75 g (depending on what density we assume for the planet). This has implications for biology (plants and animals may well be taller, it won't be so easy to break bones falling), geography (mountains will likely be taller), atmospheric structure (pressure will drop off less with altitude), and so forth.

[brandrinn]

Speaking of sea level, the atmosphere will likely be thinner on Almea, being that it's only 5320 km in radius...

Please remember, the atmosphere can be as thick as you please on a conplanet. Just because a planet is such-and-such in size relative to Earth does not mean that it must have a such-and-such reduction of atmosphere. Venus' atmosphere is much thicker than Earth's, despite the fact that they are almost exactly the same size. I'm not picking on you, I'm just reminding you that the atmosphere of Almea is whatever the heck Mark wants it to be, and that poses no problem whatsoever as long as its internally consistent.

The issue of gravity, though, is a good point indeed. Increasing the planet's density to keep an Earth-like gravity is a lot more difficult to justify than just dictating the thickness of the atmosphere. I will calculate tonight (I'm nearly out the door at present) what that density would need to be, but I imagine it would be worryingly higher than Earth's 5.5 grams per cc.

[bulbaquil]

ρ = m*V

ρ = m * 4π(r^3)/3

m = ρ/(4π(r^3)/3) = 3ρ/(4π(r^3))

3/4π is a constant, so effectively m is proportional to the density over the cube of the radius. Earth's radius is 6378.1 km; Almea's is 5320 km or 83.4% of Earth's. That cubed is 58% - in other words, Almea would have 58% the mass (and thus 58% the gravity) of Earth if the density were the same. You'd need Almea to have 1/0.58 (1.72) times the density of Earth in order for it to have the same mass.

And brandrinn is correct - atmospheric pressure is not merely a function of gravity alone.

[linguofreak]

Speaking of sea level, the atmosphere will likely be thinner on Almea, being that it's only 5320 km in radius...

Please remember, the atmosphere can be as thick as you please on a conplanet. Just because a planet is such-and-such in size relative to Earth does not mean that it must have a such-and-such reduction of atmosphere. Venus' atmosphere is much thicker than Earth's, despite the fact that they are almost exactly the same size. I'm not picking on you, I'm just reminding you that the atmosphere of Almea is whatever the heck Mark wants it to be, and that poses no problem whatsoever as long as its internally consistent.

True, and that is why I said "likely thinner", rather than just "thinner". But the escape velocity will be around 9 km/s (compared to Earth's 11), and that will mean, all else being equal, that the atmosphere will escape at a faster rate. In fact, Almea is on the low end of the "planets likely to retain a breathable atmosphere" scale.

The issue of gravity, though, is a good point indeed. Increasing the planet's density to keep an Earth-like gravity is a lot more difficult to justify than just dictating the thickness of the atmosphere. I will calculate tonight (I'm nearly out the door at present) what that density would need to be, but I imagine it would be worryingly higher than Earth's 5.5 grams per cc.

Well, you're looking at about 6.6 g/cc and .69 Earth masses for an Earthlike surface gravity at Almea's radius. Not totally out of the question if Ënomai has a high metallicity and all its terrestrial planets have really big iron cores, but certainly on the high end.

If Almea has the same composition as Earth, it's actually going to be a bit less dense (we can estimate 5, rather 5.5 g/cc, but it's maddeningly difficult to find good equations for a straightforward calculation) on account of less gravitational compression. Earth is actually a bit denser as a whole, on account of its gravity, than it would be if you broke it up into chunks, measured the density of each chunk, and took an average. Almea, being smaller, wouldn't have as much gravitational compression, and would thus be less dense.

Lower gravity with Earthlike surface pressure would make flight easier. Expect to see a lot of bird-like animals. On the other hand, the pressure I estimated before is low enough that flight would be harder, even with the lower gravity.

In general, projectile weapons will carry further, or carry the same distance with flatter trajectories. I'm not sure what influence this would have on warfare, but it might be worth looking into.

When the Almeans develop spaceflight, it will be a lot easier for them than for us. About the hardest part of spaceflight is getting to low Earth orbit, and a launcher that will get a spacecraft to LEO on Earth will get it almost to escape velocity on Almea. (That said, the next hardest part of spaceflight is the time it consumes, and while similar technology will go farther for them as far as getting off Almea, zipping across the solar system in a month will be as much Sci-Fi for them as for us).

[zompist]

That cubed is 58% - in other words, Almea would have 58% the mass (and thus 58% the gravity) of Earth if the density were the same.

Er, no, relative gravity is a function of radius too: g = m / r^2. So Almea's gravity would be about .833 Earth's.

[bulbaquil]

Oh. Right. Duh. I think I was trying to overcomplicate things.

[brandrinn]

You're closer to the core on a small planet, hence more gravity. It's an easy thing to forget when calculating gravity. As for the effects of lower gravity, I think these are often exaggerated. The human body plan is patently ridiculous for a planet with 9.8 m/s^2 of gravity, but here we are. A planet with higher gravity still could easily have bipedal creatures even more ridiculous than us, with spindlier limbs and flimsier ankles. They would possibly have better balance mechanisms or stronger bones; or maybe they would just suffer more, evolution doesn't care. As for flying creatures, I think that, too, is treated as far too simple. Saying there is likely to be a correlation between the ease of flight and the abundance of flying animals is like saying there is likely to be a correlation between living near a volcano and dying a horrible magma-related death. That is to say, "yeah maybe, but... well maybe. I mean... sure, maybe I guess." On Earth, land-dwelling tetrapods took hundreds of millions of years to learn how to fly, and animals capable of flying very efficiently and filling thousands of specific niches didn't come along until later still. If you were from the Permian Period and fast forwarded to today, you'd be amazed by how many things are flying about, and many of them are vertebrates!

This goes back to the age old issue of coincidentally lucky or unlucky body plan. Body plans change very slowly, and what body plan you have available to you thanks to your evolutionary heritage dictates what niches you can exploit, regardless of how tempting those niches may be. If flightless birds completely lose their wing bones, for example, their descendants will never fly again. Not in a billion years, no matter how much food might be waiting for them there. So, too, the evolutionary response to higher or lower gravity will depend on the dice roll of body plan. If whatever crawls out of the muck doesn't have at least two adaptable appendages, then any kind of flying descendant is unlikely for hundreds of millions of years, no matter how low the gravity. If the creature crawling out of the muck can't easily fuse its bones, then its descendants aren't going to have stocky legs with canon bones and hooves and other adaptations for heavy weight loads, no matter how high the gravity.

So I think the lower gravity of Almea can be safely ignored for the purposes of Almean biology, as long as it is done so in an internally consistent way.

[bulbaquil]

True, brandrinn... I think a lot of us forget that natural selection isn't always right, and that some species which "shouldn't" still exist do simply because, well, they kept rolling high enough on their saving throws.

[linguofreak]

You're closer to the core on a small planet, hence more gravity. It's an easy thing to forget when calculating gravity.

No. You can calculate surface gravity from mass and radius alone (or either factor plus density, or any other factor that allows you to calculate mass from radius and vice versa). The core, or anything else under the surface doesn't change anything unless you actually go under the surface.

That said, you're half right, if you decrease the radius without decreasing the mass, you will get higher gravity. But you only have so much wiggle-room on density for an Earth-like planet, so there's a fairly strong correlation between size and surface gravity.

As for the effects of lower gravity, I think these are often exaggerated.

Well, the big thing is that they're probably treated too much as hard-and-fast absolutes rather than rules of thumb. It's not going to happen magically just because of low gravity, but you're still going to see certain broad patterns appear. You're going to have lots of little consequences of the lower gravity that each have little influences on different aspects of the planet and its life, and those will build up to big influences in the broad picture. One consequence may fail to apply for some reason, but it's overwhelmingly likely that a fair number of them will, and that will change the flavor of things.

The human body plan is patently ridiculous for a planet with 9.8 m/s^2 of gravity, but here we are. A planet with higher gravity still could easily have bipedal creatures even more ridiculous than us, with spindlier limbs and flimsier ankles. They would possibly have better balance mechanisms or stronger bones; or maybe they would just suffer more, evolution doesn't care. As for flying creatures, I think that, too, is treated as far too simple. Saying there is likely to be a correlation between the ease of flight and the abundance of flying animals is like saying there is likely to be a correlation between living near a volcano and dying a horrible magma-related death. That is to say, "yeah maybe, but... well maybe. I mean... sure, maybe I guess." On Earth, land-dwelling tetrapods took hundreds of millions of years to learn how to fly, and animals capable of flying very efficiently and filling thousands of specific niches didn't come along until later still. If you were from the Permian Period and fast forwarded to today, you'd be amazed by how many things are flying about, and many of them are vertebrates!

As I said, these are going to be strong trends, not absolute guarantees. But the big thing is that things will work there that don't here. Also, bear in mind that the "flying is easier" bit only applies with sufficient atmospheric pressure. If the lower estimate I gave for sea level pressure holds, it will actually be a bit harder, though jumping/running/etc. will still be easier.

This goes back to the age old issue of coincidentally lucky or unlucky body plan. Body plans change very slowly, and what body plan you have available to you thanks to your evolutionary heritage dictates what niches you can exploit, regardless of how tempting those niches may be. If flightless birds completely lose their wing bones, for example, their descendants will never fly again. Not in a billion years, no matter how much food might be waiting for them there. So, too, the evolutionary response to higher or lower gravity will depend on the dice roll of body plan. If whatever crawls out of the muck doesn't have at least two adaptable appendages, then any kind of flying descendant is unlikely for hundreds of millions of years, no matter how low the gravity. If the creature crawling out of the muck can't easily fuse its bones, then its descendants aren't going to have stocky legs with canon bones and hooves and other adaptations for heavy weight loads, no matter how high the gravity.

So I think the lower gravity of Almea can be safely ignored for the purposes of Almean biology, as long as it is done so in an internally consistent way.

Well, one thing that you're certain to see, though it's as much learned behavior as biology, is different gaits. The most efficient speed for walking is determined by leg length and surface gravity, so a given creature will walk faster in higher gravity (assuming its bones hold out), and slower in lower gravity. Since Almean humans are pretty much identical to terrestrial humans except for setting flavor and background and interbreedability, they will thus tend to walk slower there than we do here (as would the Hellenikoi upon their arrival). Thus, Almeans will be more likely to break into jogging, running, and other "airborne" gaits (the thing that distinguishes walking from slower gaits is the presence of at least one foot on the ground at all times), and will be in less of a hurry when they do. If you look at videos of the moon landings, you'll notice the astronauts move almost more like slow-motion kangaroos (they use a "hopping" gate) than Earthbound human beings, although the case on Almea will be less extreme.

[brandrinn]

You're closer to the core on a small planet, hence more gravity. It's an easy thing to forget when calculating gravity.

No. You can calculate surface gravity from mass and radius alone (or either factor plus density, or any other factor that allows you to calculate mass from radius and vice versa). The core, or anything else under the surface doesn't change anything unless you actually go under the surface.

Sorry, what I meant was that you're closer to the center of gravity. I wasn't thinking about the layers of the planet, so "core" was an especially stupid choice of words :[

[rotting bones]

It's finally out: http://www.zompist.com/pck.html

OK that's it, I'm ordering both LCK and PCK.

[Mornche Geddick]

We've got the new climate maps.

These are the differences that I can see:
The Icëlani jungles have gone from the Qarau peninsula. (Does anyone know if the icëlani can live in savanna?)
Nan is still jungle, but the Rau valley is now savanna.
South Téllinor is Mediterranean, North Téllinor is steppe and desert. But there's a large river, perhaps it is the Alméan equivalent of the Nile.
In Arcél, Ayalampa is desert. But it also has a river system that might support a stable agricultural society.

[zompist]

I may adjust some rivers, preferably in a way that minimizes map redrawing.

I started a revamp of the Ereláe atlas awhile back, bringing it up to date; I'll take the opportunity to rethink a number of things.