Practical Planetology


So you’ve made a world and you want it to be believable!  Here’s some handy tips to add a little science to your science-fiction, and make your Spelljammer worlds live and breathe and feel very real.

No Planet Has Only One Climate: Unless it’s a Rock

I spoke about this briefly in Worldbuilding for Dummies.  Let’s say you have a desert planet.  Why is it a desert?  If it genuinely had no water, how would life survive on it?  Remember, even Athas had water, even Arrakis had water; and both of those worlds have sensible, consistent in-story reasons why they remain deserts, even though life survives there.  Nothing has a universal, consistent weather pattern unless it is an airless asteroid; and only Type A bodies can get away with that excuse in the Spelljammer rules.  (Even the Rock of Bral has some weather!)

People Don’t Build in Random Places for No Reason (and Neither Do Aliens)

Nothing irritates me more that badly-placed settings.  So you have a random dungeon full of monsters on a barren moon.  Really?  Who put it there?  What for?  How do they supply it?  Where do the monsters get food from?  Do they have agriculture?  Where do they get water/sunlight/dirt/air if they do?  How do they get rid of their garbage?

The only dungeon full of monsters for monsters’ sake I’ve ever seen that’s made sense is Undermountain.  Undermountain works because a) the guy who created it is a crazy immortal archmage equivalent of Dr. Moreau; b) the magical and ecological conditions of the Forgotten Realms can support it; c) there are enough local interested parties – Waterdeep, Skullport, the drow, the Underdark – to screw around with it that in many ways it would be self-sustaining; d) its reputation for vast treasures has made it kind of like the Realms’ equivalent of El Dorado.

Consider what kind of trade your random star-tree civilization might engage in.  Consider how that flying castle restores its resources (it has to somehow!)  Consider why someone wanted to build something in that spot in the first place!  In the real world, we like to build in pretty places with easily-accessible resources.  People only build in weird, isolated and sparse places if they have compelling reasons to want to be left alone (which usually makes them weird or dangerous).

Let’s consider this example: A city of ghasts would want to build close to people they can eat; but the people they eat tend to frown on that (and their reek) so they drive them out (or move).  Cities of ghasts would only be possible if, perhaps, they took slaves or were sent tribute from the surrounding terrified communities; if they had a portal to other places; or if there were once other communities or a great battle on the city site (which means they’re all starving and desperate and a million times more dangerous).

Even dwarves need a source of water in their floating mountains; even a colony of beholders has to eat and breathe.  How are they doing this?  That likely dictates their surroundings.

Weather Comes from the Ocean and the Mountains

In very basic terms, weather is generated by high pressure and low pressure air fronts, and how they interact with each other.  Clouds brew over large bodies of water by means of cycles of evaporation.  High pressure areas absorb moisture; low pressure areas condense or drop it in the form of rain or snow or possibly sleet and hail; winds blow from high pressure areas to low pressure ones.

When weather fronts carrying a lot of humidity reach higher ground, the lower temperatures force them to drop their moisture in the form of rain, snow, et al.  The higher the ground, the more the moisture drops; which is why much of our sources of fresh water originate in mountain snow packs.  But of course, ground just steadily raises altitude (on average) the farther away from the ocean you go; hence we talk about “sea level” altitude.

So if you look at a map of the Earth, you will see that our biggest deserts and savannas are either a) just on the other side of a tall range of mountains from the prevailing ocean weather front, due to the weather fronts dropping all their moisture climbing the opposite side of that mountain range, or b) so far inland that all the rain has fallen by the time it gets there.

I might use my own country of Canada as an example.  If you look at a map of the country, you will see that the West Coast (just north of the area that Americans call the Pacific Northwest,) is a coastal rain forest.  Why?  Big ocean (the Pacific, which is deeper than the Atlantic) dropping rain on the West Coast, whose weather fronts must then climb the Rockies, the Kootenays, and the many other interconnected big mountain ranges of Western Canada.  Just on the other side of those mountains?  The Okanagan Valley; a semi-desert, but only because it has a very big lake in the middle of it, otherwise it would be a full-fledged desert.  Most of the rest of the Interior of British Columbia is either desert (Kamloops area), dry alpine (Terrace) or tundra.  The areas that aren’t desert or tundra have the enormous Fraser River running through them.  (You can see similar landscaping in Colorado and California).  The Canadian Shield (a big lump of rock mostly supporting Ontario & Quebec) is different because of a mix of polar winds, Atlantic weather fronts, different terrain, and the Great Lakes and the St. Laurence River, which are big enough to generate their own weather.  As a result, they get mild summers and a lot – A LOT – of very cold winter snow.  (Okanagan Lake isn’t quite that big, though it does help!)

Even if the world you have created has geometric-shaped mountains, they are going to affect your weather and they might create deserts.  Glyth likely will have mild changes in weather by comparison because only 20% of its surface is covered with a liquidish substance; but small particulates encourage water to gather around them, making a heavier raindrop more quickly, so the acid rain described is likely a self-perpetuating steady drizzle.

Going back to my real-world example, you’ll note that much of Canada doesn’t seem to follow those rules.  Like, what about the Prairies?  Why isn’t it entirely desert?  Well, first of all, it’s pretty low ground (really flat, remember?)  Second, consider the differences in air pressure between the North Pole and the Equator!  So most of the winter, cold weather fronts blow down the corridor to wet the otherwise dry Prairies, and they remain fed by the Rockies and the Canadian Shield.  The drawbacks to this arrangement are that Winnipeg, Manitoba, which is only a couple of hundred km from the geographic center of Canada, is scoured by bitter cold Arctic winds in the winter (you think Chicago is the Windy City?  HAH!) and much of Manitoba is routinely flooded when the mountain packs melt in the spring.  Manitoba also often suffers tornadoes because of the competing weather fronts literally stirring up trouble (more on how that works in the next section).

Atlantic Canada, not being of exceptional altitude, get a lot of rain, most of it favourable, but nothing like the West Coast rain forest.  Except that it is often bombarded by heavy snow in the winter (think about what happens when that Arctic wind meets the Atlantic!) and it is often assaulted by hurricanes; more in the last ten years, I might note.  Why is explained below:

Weather is Affected by Rotation and Magnetism

The rotation of the Earth, and its magnetic field, determine a great deal of our world’s weather.  Do you notice that hurricanes brew in the Atlantic in the Northern Hemisphere and the Pacific in the Southern Hemisphere?  It has to do with the way the planet spins and the prevailing wind currents as they shift between the Poles and the Equator.  The planet rotates opposite of the direction of sunrise and sunset; it rotates from West to East.  So while both the Pacific and the Atlantic generate big weather fronts, in British Columbia, that’s more likely to manifest as torrential downpours, blizzards, and tsunamis, because the weather rolls with the direction of the Earth’s rotation.  In the meantime, off of the East Coast of North America, we keep experiencing hurricanes and tornadoes, because the weather fronts generated by the Atlantic travel overland opposite to the direction of the Earth’s rotation; and if the winds are strong enough, that will make the whole works spin like a top.

If your world rotates so that the sun rises in the west and sets in the east, that means that the hurricanes will be on the West Coast, not the Eastern one.  If it rotates North to South, you could end up with superstorms out of The Day After Tomorrow that mix the worst of polar winds and the natural rotation of your world to create Fimbulwinter!  (Basically, you end up with Hoth!)

Now, weather tends to be blocked by angular objects such as mountains, so a geometrically-shaped world would be an interesting case.  First of all, how would the shape of the planet affect its rotation?  (Want to find out?  Spin a die of the appropriate shape like a top and watch what it does in the brief blink before it stops.)  Secondly, I wonder if weather would easily cross from one facet to another.  I bet that nothing but huge tornadoes and hurricanes would cross a line, which would probably form from spinning winds throwing themselves again and again onto a corner until it eventually comes loose and blows away.

If this weren’t complicated enough, your planet’s magnetic field also affects weather.  The scientific explanation currently en vogue is that cosmic rays hitting out atmosphere release electrically-charged particles into the atmosphere that gather moisture around them, drawn and held by the static cling.  The end result of this is more clouds in the atmosphere, especially where it’s hot (such as the Equator) causing an increase in the world’s temperature (a.k.a. Global Warming) and precipitation.  The stronger your world’s magnetic field, the less cosmic rays enter your atmosphere, and thus the less cloud cover you have.  We also know that solar winds change weather in sudden waves and cause electromagnetic interference.

Fortunately, this is Spelljammer, not a David Weber novel.  It doesn’t have to make perfect scientific sense.  But borrowing from that knowledge can make the world feel more convincing.  How would moving air pressure systems work on a desert world?  Siroccos might transform into gigantic bone-scouring sandstorms that would destroy cities!  And what would happen to spelljamming helms under the effects of a solar flare filled with electromagnetically (and probably magically) charged particles?

Land is Shaped by Forces

Okay, so you have Geometric Shape world.  Why is the land arranged in geometric shapes?  The PCs may never find out; but you should know, just in case.

In the real world, various forces shape the landscape, including but not limited to: wind, oceans, precipitation, erosion, running water, tectonic shifts, volcanic eruptions, climate change, magnetic shifts, glaciers, tsunamis, floods, gravity, solar winds, vibration, meteors, landslides and the moon.  Maybe the Gods too; why not?  How did your world come to look as it is?  It wasn’t just created that way; and if it was, why haven’t any of the above-named forces changed it?

One thing that tends to be a concern in Spelljammer is that we have crystal spheres with systems that a) do not necessarily orbit a sun, and b) might have life on very distant planets from that sun.  You need to provide some other explanation as to why life exists there (maybe your planet on the outer edge of the sphere has its own little sun; this of course defies the physics of the real world but so what?  This was Garden’s solution).  Or maybe it has an alternate method of heating itself (perhaps the sphere is just warm everywhere.  Perhaps the planet’s core just has a way of radiating its heat to the surface).

Those forces could be magical too.  One of the most memorable game worlds I’ve ever played in was created by a friend of mine.  An ancient apocalyptic magical war fractured the planet and a complex spell was woven (not unlike a mythal) to prevent it from splitting.  It worked; except over many thousands of years, one of the keystones (giant pyramids) eroded enough that the containment was only partially successful; and every now and then a magical backlash would cause a dangerous magical storm that devastated the landscape and caused creatures to undergo 2e style wild magic mutations that just might kill them.  One of my favourite scenes in that campaign was fighting an army of undead in a dwarven hold, vastly outnumbered, but unable to leave because a magic storm of legendary proportions was happening outside.  Good stuff!

In Nature, Damage Recovers

One of my big bugaboos in D&D is the Anauroch Desert.  It makes no damn sense!  Okay, the land was destroyed by a terrible magical effect, and it once was juxtaposed with the Far Realm, which filled it full of weird alien creatures.  But that effect isn’t still going on, and there’s no more link with the Far Realm (which is why the remaining denizens are so ticked off).  So why isn’t it slowly being devoured by leafy green things?  Damaged land recovers, unless the effect that caused it to be damaged is still going on!  There’s no logical reason that a thousand years after the fact, it’s still a desert.  Weather patterns would not support this, for the reasons explained above.

Athas makes sense – it still has defilers and dragon kings!

Keep this in mind if you want to make your world credible.

Even Magic Has Limits (The Laws of Metaphysics)

One thing that speculative fiction publishers will tell you in their Submission Guidelines is that if you have a world of magic, then the magic should be consistent, and it should properly obey its own internal laws.  There are laws of physics, and there are laws of metaphysics.  Let’s consider Grubbian physics to be the latter, and thus, the Spelljammer Universe works!

In Greyspace, internal combustion fails.  Perhaps in your crystal sphere, no magic works that raises the dead.  My husband and I have created a crystal sphere with a flat sun that forbids Outer Planar access on one side of it (but not the Inner Planes or extra-dimensional spaces).  Terry Pratchett’s Diskworld is on the back of a giant turtle, so presumably in that Universe, planets must be carried on the backs of great beasts.  These are consistent rules.  They never change.  Life isn’t actually like that, but if it isn’t like that in a story, your players will get frustrated.

Sometimes Odd Things Happen

That being said, sometimes weird sh*t just happens!  Krakatoa’s explosion almost caused another Ice Age.  There was some mass extinction event that killed all but 20 000 human beings in the ancient history of our species.  Hell, on Earth there are rivers that run uphill (there’s a perfectly logical reason for it, but how do you think Renaissance-era people reacted to stuff like that?)  Throw something bizarre in the mix to shake it up every once in a while and consider what kind of changes it will cause!

Remember the Butterfly Effect

If you mess with one small aspect of a world’s homeostasis, it is likely to have vast, possibly unforeseen consequences.  Consider what those consequences might be!  Plugging dams make lakes; teaching a child necromancy might result in an army of undead later on; teaching agriculture to a savage village will transform their entire culture.  The latter is an endless subject for science fiction and it’s fascinating.  See the suggested reading list for ideas!


Realism should never limit you in Spelljammer; that’s what makes it so goofy and fun.  But even the weird stuff should have a reasonable explanation.  (I was wondering, for example, how Marvel was going to justify an anthropomorphic talking raccoon, but the brief description in a Universe full of superheroes that Rocket was the result of “illegal genetic and cybernetic experiments on tree creatures,” when the fact that extraterrestrials abducted Earth creatures was already established, was plausible).  Also, if you include as much realism as you can, your world will be more convincing and your deviations from the norm will make a more lasting impression!  But ultimately, never let either spoil your game!  Have fun!


Suggested Reading

The Cloakmaster Cycle

Diskworld series by Terry Pratchett

Ender’s Game and Speaker for the Dead by Orson Scott Card

Dune series by Frank Herbert (you can probably skip the “co-written” ones, which appeared after Frank’s death)

Pern series by Anne McCaffrey, especially Dragonriders of Pern

Valdemar novels by Mercedes Lackey

Frostburn, Sandstorm, Underdark and Stormwrack (3e) – good stuff for creating worlds in specific climates

Ravenloft and Planescape (2e) show wonderful examples of worlds with magical laws that differ from the standard campaign worlds

Al’Qadim (2e and Pathfinder) and Eberron (3e) are good examples of vast cultural differentiations resulting from small changes

Dark Sun is an excellent example of a Butterfly Effect gone horribly wrong




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