An Orbital is a miniature ringworld used by the Culture in Iain M. Banks's Culture series. Wikipedia has details on their basic structure, but to simplify, the main difference from a Niven ringworld is that they're much smaller and orbit the sun like a planet instead of encircling it.

What I'm interested in is the weather patterns that would arise on an Orbital -- the Culture could generate weather however it'd like, of course, but what would a "natural" Orbital be like, without direct interference into its weather?

Additionally, what would be different if the Orbital had very tall "bulkhead" mountains separating areas?


Some notes on forces affecting the weather.

Tidal forces from the star would affect the atmosphere. There would be very fast and stable wind in the stratosphere. Without bulkheads the dominant wind direction would be opposite to the rotation.

If the bulkheads were high enough they would stop these winds from forming. More probably the bulkheads would be high enough to limit weather but not high enough to stop stratospheric winds. Assuming a composition similar to Earth, this would mean 10km (or more for added safety) high bulkheads. In situation like this I'd assume the top half of the troposphere would be dragged by the stratosphere opposite of the rotation which would have to be balanced by the surface winds in direction of the rotation. The wind speed would depend on the number and height of the bulkheads. So you'd presumably have a near constant wind of the desired speed and stable direction.

One bulkhead would have cool dry air being sucked down. I'd presume the designers would place a sea here to moisturise and warm the air. Since the sea would have the same amount of sunlight as every other part this would work well. Combination of cool dry air above warm moist air would probably create lots of localised thunder storms and even tornadoes, which is another reason to have a sea here instead of habitable land. I don't see large storm fronts or hurricanes being formed, so ships and boats should be able to avoid storms.

The other bulkhead would have warm moist air driven up. This should result in nearly constant rain in the bulkhead mountains. I doubt anyone would want to live in this area, but it would be a good place to build hydro-power.

The area in between would have localized thunderstorms at night, rain on the windward sides of mountains and arid areas behind them. These rain shadows might be stable enough to form deserts, I'd assume arid grasslands exist. Without bulkheads this would be what all of the orbital would be like. Weather would be dominated by the effects of the mountains on the stable wind.

Since rivers and winds would run in opposite directions and both the river flows and the winds would be near constant, sailing ships should be quite practical form of low tech travel. And there would likely be large navigable rivers running from the rains of one bulkhead to the sea at the other bulkhead. In fact, since the winds run at different directions at different altitudes aerial sails or glider aircraft would be much more practical than on Earth as long as you can reach the boundary between the winds. This might actually be important for a low-tech "inheritor" civilization as I doubt orbitals would have huge hydrocarbon reserves. Without bulkheads river directions would be random and wind direction would be same regardless of altitude.

In addition to the tidal and bulkhead effects, there would be effects from the day-night cycle. These should be similar to the effects here on Earth? Although the higher speed the line between day and night moves might make effects negligible outsides coastal areas and areas with large lakes. Basically, air above water warms and cools slower, so there would be winds toward land at the morning and toward water in the evening.

All guesswork obviously...

  • $\begingroup$ «There would be very fast and stable wind in the stratosphere. Without bulkheads the dominant wind direction would be opposite to the rotation.» are you sure it wouldn't be a longitudinal wave, seemingly blowing towards the dense part on one side and away on the other? $\endgroup$
    – JDługosz
    Oct 30 '16 at 1:54
  • $\begingroup$ @JDługosz No, I am guessing it would depend on the specifics of the tidal effect. I am simply assuming that the rotation would be fast enough in relation to the gravity causing the tidal effect for the wind to be much larger than the wave. I think that is usually reasonable since the point of these structures is usually to provide centrifugal gravity which requires relatively fast rotation and strong tidal effect would cause significant structural stress. So engineering suggests my assumption would usually be correct. $\endgroup$ Nov 1 '16 at 7:53
  • $\begingroup$ Can you explain more about how the tidal force produces constant wind? $\endgroup$
    – endolith
    Oct 2 '20 at 16:26
  • $\begingroup$ Also Look to Windward does mention a jetstream on Masaq` $\endgroup$
    – endolith
    Oct 2 '20 at 20:07

What did you design it to do?

If you've got glacier areas, for example, then you've put in cooling apparatus. Automated cooling/heating apparatus will provide a significant driver for your weather patterns.

Water areas? Also drive weather. If you made your orbital all-desert, you're going to get a different effect than a world-river orbital, vs. an orbital with huge oceans (cooled at the deep ends?), vs one with shallow oceans. How connected did you make your water masses? Is there thermal exchange designed into them?

Did you include forcefields to effect your weather, by creating baffles in the air? Break up repeating wind-currents? Are they switched, or always on? Baffles for your ocean currents? Are you inducing waves or tides in your oceans? How? Those choices will effect your weather. (What effect did the designers want to have?)

You've got a hive mind that controls the forcefields and other systems. Are you telling me you purposefully took weather control away from it? It's in it's self-interest to not let big weather problems disrupt it's ongoing activities.

  • 1
    $\begingroup$ Of course, but I was interested in the "default", so to speak, so I could know what I'd have to keep in mind. $\endgroup$
    – user6383
    Jan 10 '15 at 17:23
  • $\begingroup$ Did you mean "hub Mind" instead of "hive mind"? $\endgroup$
    – endolith
    Oct 2 '20 at 20:09

Ok, I'm not a meteorologist and I have basically no idea how weather works. But I had an interesting idea, so I'm posting anyway.

Without the mountain "bulkheads", the surface is roughly one-dimensional, as opposed to roughly two-dimensional on our earth. That could be a very important factor. On a 2D surface, a vector field can have elaborate vortexes, but in one dimension the only solution is "everything goes in the same direction".
Now this is pure speculation but it seems conceivable to me that the weather would stabilize in this environment and not be chaotic. There would be local vortexes of course, caused by mountains e.g. but they could be static, like the whirls of a river.

  • $\begingroup$ The surface is 1000 to 6000 km across, half the diameter of Earth. It's pretty two-dimensional... $\endgroup$
    – endolith
    Feb 4 '21 at 14:44

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