Thermodynamics for Airship Pilots
At the Imperial Wartime Academy of Airship Pilotage in Aniara, there’s a question the instructors like to spring on first-year students.
“You’re the navigator,” the teacher says, tapping a riding crop idly against his thigh or scratching one of his long ears. “You’ve just finished a bombing run on Izmir when your captain receives orders to make all possible speed to Nysa.”
By this point the student is nodding somberly, trying to look sharp while simultaneously picturing the whole continent in his mind’s eye. The city of Izmir sits about twenty-eight degrees north of the equator, a hundred miles west of a mountain range; Nysa is some six hundred miles south-southwest from there.
“What altitude should you request from the engine room?”
The student squirms. He’d expected something about fuel reserves, maybe, or the time of the trip, or Nysa’s infamous flame cannons. Not altitude.
Thinking of the world map, he takes a guess. “High,” he ventures. “To get above the clouds that pile up by the mountains near Izmir.”
“The clouds? What clouds?” The instructor looks as though he might choke, though in truth he’s heard this answer a hundred times. “The wind comes from the east at that latitude. Izmir is in a rain shadow. No.” The instructor raises his voice, making the rest of the class straighten up. “You stay low between Izmir and Nysa, for three reasons.”
He paces as he lectures.
“One: between the equator and thirty degrees, the air moves northward at higher altitudes and southward close to the ground. Two: the trade winds that blow east to west are only at low altitude. Up high, they blow the other way. All possible speed, remember? We’re heading south-southwest, so we go where we can ride the wind rather than fight it.
“Oh, and three: get too high near the thirtieth parallel and you might get caught in a west-to-east jet stream, in which case we’ll never see you again unless you can successfully circumnavigate the entire globe and come back around.”
The student buries his face in his hands, wondering why he ever signed up for this damned school, and the teacher thinks of the bottle of Dorian wine stashed under his bed…
How do I know all this? Well, it helps that I invented Aniara and their airships. But I didn’t invent atmospheric thermodynamics, no matter what my four-year-old believes. Our Aniaran instructor is talking about large-scale patterns of air circulation known (here on Earth) as Hadley cells, and I’ve decided that they’re the same on his world as on ours.
As speculative fiction writers, we often start with an end in mind. Brandon Sanderson famously developed the world of the Stormlight Archive because he wanted to write about warriors in power armor wielding huge swords. Rowling had to explain why entire prep schools full of tweens with unlimited arcane power were roundly ignored by humanity at large. Tolkien wanted Mordor to be a forbidding, inaccessible badland, so he boxed it in with three mountain ranges in a neat rectangle, which if you spend a few minutes on the Wikipedia page for plate tectonics… yeah.
But the real world is full of patterns and wonders that we can transport wholesale into our imaginations. Have you heard of the Severn Bore, a phenomenon where booming waves thunder backwards up the River Severn? How about the Little Ice Age, which gave us both the witch trials and frost fairs on the River Thames? Or the northern lights, which might be Valkyrie armor or caribou spirits depending on whom you ask?
Assuming that our made-up worlds work more or less like Earth isn’t a limitation, it’s an opportunity.
First, use the fundamentals of Earth’s climate to keep your fantasy realm grounded. When background details like the weather work like they do on Earth, it gives us instant familiarity. There’s a reason Tolkien started the Lord of the Rings in the Shire: we instinctively root for a hero trying to save world when that world is like ours.
It also lowers the barrier to entry for new readers stepping into your world. For example, most deserts sit around 30 degrees latitude, the spookily named Horse Latitudes, even though it’s hotter at the equator. (Why? Hadley cells again!) If you’ve got deserts all over the place, that’s just one more thing to explain when you could be spending that word count on your awesome magic system.
Next, throw your imaginary peoples and creatures into the mix. If I’ve got airship pilots, they need to know about Hadley circulation; otherwise they risk taking the long way home. Dragons do, too—maybe that’s how they get from village to village so quickly, like real-world airlines using the polar jet stream to shave hours off eastbound flights. Intelligent creatures learn the rules, then set them to work.
Finally, if your readers know your world is basically like Earth, it’s all the more wondrous when you do break those rules. A river that runs uphill stands out instantly. So does a desert in the jungle, a lone mountain on a flat plain, or a stationary rain cloud. Even if it’s totally mundane to your characters, readers will sense something magical is afoot.
So what’s up with Hadley cells, anyway? In short, hot air at the equator rises until it’s cooled enough that it can’t go any higher. More air keeps coming up beneath it, which forces it out towards the north and south poles. Meanwhile, the rotation of the Earth underneath it gives it an eastward drift. By about 30 degrees latitude, it’s cooled enough to sink back down to earth, where it completes the loop by heading back towards the equator with a westward drift. (As for the deserts, if the air at the 30th parallel is sinking down and away, it’s not blowing around the place bringing in moisture off the oceans!)
Simple, right? Here’s the real secret: you don’t need to be an expert to use this stuff. (My degree is in music, for crying out loud.) There are tons of resources online for worldbuilders and imaginary cartographers, and they’re well worth your time. Getting a handle on the basics gives you everything you need to construct a world familiar enough to your readers that they’ll instinctively understand when you’re following the rules and when you’re breaking them.
(c) Nathaniel Webb
About The Days of Guns and Roses:
Gifted Emily Sledge has just graduated at the top of her class from the secretive and dangerous Harkness Academy.There’s just one problem: she doesn’t have a job lined up.
Never one to give up, Emily ditches a promising interview to chase a mysterious message in a bottle. But instead of landing her the perfect job, the clue leads her to a forgotten castle where a band of young romantics prepare to make their final stand against an unknown enemy from beyond the Veil.
Isolated and in danger, Emily must choose between her career and the strangers who need her help. But all is not as it seems in the Castle Forlorn, and Emily has to grow up fast if she wants to survive threats that she can’t just hit with a hammer… all while the clock ticks down to the final invasion.
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