Worldbuilding Wednesdays: Rain Patterns

Welcome to Worldbuilding Wednesdays!  Every Wednesday, we spend what is probably far too much time walking through our worldbuilding process.  In this week's post, we finally focus on something besides the planet we're working on:  its atmosphere.

What We Have So Far

The world is ten times the diameter of Earth, and ten times the mass.  It is 99% hollow, and instead of orbiting a star, it has three satellites:  a "red" moon, a "blue" moon, and what we've decided to call the Lantern, a moon-sized satellite that looks as bright from the surface as our sun does from Earth.  Thanks to a combination of the world rotating slowly and the Lantern orbiting quickly, a single day-night cycle on this world is just shy of 2 Earth-days long, and roughly every four of those day-night cycles, all three satellites line up; that's what we'll be calling a week on this world.

We've also introduced the idea of visualizing what's going on, using a random hill by the coast on our new world.

As you'll soon see, chances are that it's going to be raining on that hill.

Atmospheric Science for Beginners

We didn't address it before, but we should now:  Is it possible to have an atmosphere like Earth's on a planet like ours?  As it happens, the answer is yes!  ...Kinda.

You see, thanks to the fact that we used nice round numbers like "ten times the diameter" and "ten times the mass," math related to those two numbers gets really easy.  So if, for example, we wanted to say that an atmosphere with roughly the composition and surface pressure of Earth's was present on this new world, the big concern would normally be that, with 1/10 the gravity of Earth, the atmosphere would fly into space.  After all, that's what happened to Mars' atmosphere: it was too small to hold onto air molecules, and they slowly drifted away.  We would do a calculation of this planet's escape velocity to see if it was low enough for air to escape... and we would discover that the "tens" from our other numbers canceled out, and the escape velocity is exactly the same on our planet and Earth.  The atmosphere is saved!

It is also really, really thick.

Air pressure is the result of us all sitting at the bottom of columns of air.  The column of air stretches from the ground to the edge of space.  Air isn't very heavy, but a column that big does add up.  And on a planet with the same pressure but 1/10 the gravity, the only way it would have the same pressure is if the atmosphere were ten times taller.  On Earth, the atmosphere is 100 km thick; on our planet, that would mean the atmosphere is 1,000 km thick.  Conversely, climbing into the air 25 kilometers (near the height of Mount Everest) would feel similar to driving through a suburb in Colorado (closer to 2.5 kilometers).

Imagine that you're back on that hilltop.  You watch the tides go in and out in a complicated pattern, thanks to the three satellites.  The Lantern and both moons hang in the sky, which seems to go on forever.  Low-hanging clouds move in.  Above them, a larger, flatter set of clouds floats serenely by.  Above them, a third set of clouds, impossibly distant, streaks the edges of the air with white.  Those tallest clouds could be up to 800 kilometers away, and if you can see them at this distance, they may well be the size of a small country.

As you try to imagine the distance between you and the most distant clouds, drops of rain begin to fall.

We Hope You Like Fog

Clouds will be absurdly easy to form on this planet compared to Earth.  1/10 the gravity means that droplets of the same size found on Earth are ten times more likely to become airborne.  The planet's large size and slow rotation lend themselves to soft but remarkably consistent weather patterns, so once a raincloud forms, it will be in the air for quite some time.  When we did the mathematical modeling for the weather patterns (because we're nerds who like that sort of thing), we noticed two consistent things:  A continent-sized rainstorm that would linger for years at a time, following the equator and reforming shortly after it dissipated; and areas of twice-daily or more bouts of fog.  The fog would form out on the choppy, stirred-up-by-three-moons seas, then roll into the coastal areas, sometimes for hundreds of kilometers.

Overall, this means that our new world is wet.  It rains almost daily.  Never for very long, since water droplets would have to be ten times larger before dropping, and never very hard, since the terminal velocity of a rain drop is about 2.5 times lower than on Earth.  Rain would consist almost entirely of large, soft drops of water, about the size and mass of a pea (though they would weigh about ten times less).  In those few places where it snows, flakes are huge, averaging 2 inches across.  Clouds are omnipresent, but the harshest winds will top out at 15 knots, and cloud-to-ground lightning is rare (lightning inside clouds is slightly more common than on Earth; there's more stuff banging around in the clouds, even if it isn't banging as hard as inside Earth's clouds).  With low winds and regular rain, umbrellas are going to be very common on this world.

Oh!  And, speaking of that rare snow, something we forgot to point out:

Universal Climate

The Lantern acts as our world's sun, but it does have some distinct differences.  For starters, because it orbits so close to our world, it is, guaranteed, tidally locked.  It is also, unless we alter physics again, orbiting directly over the equator.  It's a thing with close-in satellites; unless their orbit is unnatural, the tug of the planet's equatorial bulge drags them along until they're over the equator.

That means that, with respect to the Lantern, our world has no tilt.  No tilt and no real variation in distance over the course of the "year" means that our world doesn't have temperature-based seasons, and the difference between the equator and the North or South Pole is much more subdued.  If Earth has a tropical zone, a subtropical zone, a temperate zone and an arctic zone, then this world just has a tropical zone and a subtropical zone, one around each pole.

It isn't sweltering, mind you.  The increased cloud cover would serve to cool the planet, blocking more of the Lantern's light than Earth's clouds block sunlight.  The amount gets tricky to calculate; the clouds themselves won't be any denser, but they will be more common, and on average, there will be a whole extra level of clouds that Earth doesn't have, but this planet's surface area is 100 times larger, which changes the percentage covered by clouds, etc.  The functional result would be that, for at least half the planet, the typical day would be like an especially cool, wet day in Hawaii.  The lows would be about 16 °C (60 °F), and whenever it got sunny, the temperature would creep toward 27 °C (80 °F).  If you wanted to find cooler places, it would be easier to move up than it would be to head North or South; every kilometer of elevation would drop the average temperature by about a degree Celsius, so that when you got to an altitude of 16 kilometers, it would be cold enough to snow regularly.  Mountains could, conceivably, be up to ten times the height of the Himalayas, since ten times as much mountain could pile up before gravity tears it down, and erosion from the wind and rain is going to be less significant than on Earth.  You probably won't have a hard time finding mountains with snow, but the typical person, sitting on a hill overlooking the ocean, probably won't see any anytime soon.

The Hilltop

We'll close with that hilltop.  You watch the clouds roll in, and you note that they seem impossibly high.  That makes sense; the clouds are, on average, ten times taller than Earth clouds.  A cloud 1-2 km thick on Earth would be up to 20 km thick here, and that's also about the distance where things start getting blurry, so you might not be able to see the top of the cloud clearly.  As they roll in, always from the sea toward the land, a low bank of fog moves before them.  The fog moves slowly, taking an hour to reach you... but reach you it does, high and thick enough to even reach your hilltop.  It lingers for hours, finally fading as the moisture in the air is either wicked into the ground or whisked the rest of the way into the air.  By the time the fog is gone, the rain has started in earnest.  Rain falls in large, wet droplets, large enough to splash when they land.  You pull out an umbrella and set the canopy over your head, listening to the drops splatter.  As quickly as the rain arrives, it departs, moving further inland. 

You look into the distance.  There, a thin grey-white line on the horizon, is the Blanket, the continent-sized cloud that signals the start of the wet season.  On your hilltop, you only really experience two seasons:  the relatively dry season, where it rains once or twice a day and the fog tends to roll in four times a day; and the wet season, where the normal weather is joined by the light, distant, but constant drizzle of the Blanket.  The Blanket appears every 12 weeks and takes 4 weeks to disappear; this is as close to a "year" as your hilltop gets.  You dimly wonder how places that don't have a Blanket tell time, but then, given the size of the Blanket, there aren't many of those.  It's probably not worth worrying about.

Conclusion

We now have a basic idea of the weather on our world.  Lots of things can alter the basic weather patterns, most especially geography, so that's where we're going next.  That's right, folks, we'll finally be able to discuss things you can draw on a map.  Be sure to celebrate tastefully.  Until then, happy worldbuilding!