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I have a homebrew D&D setting in which the Gods went insane and started battling each other en masse, wreaking havoc on the planes of existence in the process. The last surviving gods had a moment of clarity, and they patched the broken pieces together into a patchwork of different planes of existence with quasipermeable borders between them. These borders mostly keep the ground or water from falling into an endless abyss and stuff like that, but creatures, temperature, and some weather effects can make it through.

The planes that got broken and cobbled together were:

The Material Plane - basically similar to Earth

The Plane of Ice - magically sucks the warmth out of everything in it, mostly endless ice and snow with occasional bits of rock

The Plane of Magma - magically heats everything inside it, seas of magma dotted by the occasional mountains with lots of volcanoes

The Plane of Dust - magically dessicates exposed water, covered in sand and dust and silt, occasional rocky outcroppings

The Plane of Smoke - hot (but not as hot as Magma), roiling poisonous air with no gravity and very little ground (only chunks of cinders)

The Plane of Steam - also has no gravity and no ground, but it's more variable in temperature and less poisonous than Smoke, and mostly consists of roiling stormclouds with regions of hot steam, cool mist, thunderclouds that electrify everything, rainclouds so thick they form bubbles, etc

The Plane of Mud - a slurry of mud and slime and occasionally acidic sludge, with the most solid regions tending to be swamp or bog-type conditions, and the less solid regions are like if you had an ocean with the consistency of a milkshake

What would be some of the weather effects noticeable around the borders between these planar chunks? If you had one or more temperature-defined planes nearby, how would that affect prevailing winds? Apart from just "stuff spilling over from the next planar chunk", what kinds of things would you expect to happen?

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    $\begingroup$ This is very difficult to answer. For example if there is a border between the Plane of Ice and the Plane of Magma, how can we calculate which "wins" in the battle of heat. From a complexity point of view there seem to be seven factorial (= 5040) possible borders between all the different planes depending on your geometry. I think we need a much more tightly defined question. $\endgroup$ Jul 10, 2020 at 22:47
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    $\begingroup$ If these biomes are magically sustained by the power of the gods (the dust terrain can't be washed out, the magma terrain never cools off...) then obviously the answer is "whatever the gods feel like". $\endgroup$
    – Cadence
    Jul 10, 2020 at 23:33

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Massive changes in environmental conditions create weird weather conditions. Without a map, it's hard to know what regions border which other regions and what weather patterns would result. So this answer gives an overview of some of the more general principles.

Fire creates its own weather

The word of the day is pyrocumulonimbus. That's a storm cloud created by a massive fire (or a Plane of Magma). Here's an example from Wikipedia:

Storm cloud created by fire

As the superheated storm clouds crossed the border, they'd wreck havoc and create serious storms. You can learn more about fire-generated weather here. Now if The Plane of Magma bordered The Plane of Dust, you'd have to decide what happens to all that water vapor.

Extreme cold also influences weather

If you have extreme cold and you introduce (relatively) high heat, the results can be intense. This clip from The Simpsons is obviously exaggerated but the gist is right. The combination of hot and cold air normally can create an occluded front. But if you have magical limitless cold constantly interacting with something much warmer, you're likely to have severe weather. Here's National Geographic's explanation of how tornadoes form:

Tornadoes form when warm, humid air collides with cold, dry air. The denser cold air is pushed over the warm air, usually producing thunderstorms. The warm air rises through the colder air, causing an updraft. The updraft will begin to rotate if winds vary sharply in speed or direction.

So the borders of the Plane of Ice will likely have a permanent storm.

Gravity is really important

Our understanding of weather depends on gravity. If The Plane of Smoke and The Plane of Steam don't have gravity, there's no saying how weather will work. This question has a useful discussion.

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Nature is nature, with weather being created as complicated systems involving the four classical elements. The patchwork plane left behind when your gods smartened up and fixed what was left are patches of the world stapled to patches of world ruled by two elements of the four. D&D calls these the Paraelemental Planes.

The elemental dominance of the elements of each of the seven planes that remain likely fall upon these lines:

  • Material Plane -- All Four Elements
  • Plane of Ice -- Water/Air (Radiated Cold)
  • Plane of Magma -- Fire/Earth (Radiates Heat)
  • Plane of Dust - Earth/Air (Removes Water)
  • Plane of Smoke - Fire/Air (Poisonous)
  • Plane of Steam - Fire/Water
  • Plane of Mud -- Earth/Water

Now what does this mean when it comes to weather?

Planar Cosmology

With gods putting the seven worlds back together in a bit of a crazy quilt of a world, the first real question is how did they do it?

This is relevant because at a very simplified level, the weather that we have is fuelled by the sun and the differences in heat capacity between Earth, Air, and Water. The rotation of our planet around its own axis and the sun help spin these currents and create weather. It is, of course, fiendishly more complicated than that but the point is a high-level overview.

The Patchwork Globe

The Material Plane is still a round world, only with patches of the six Paraelemental Planes to fill in the gaps left when the gods destroyed potions of it beyond recovery.

The planes dominated by Fire (Magma, Steam, and Smoke) would add heat to the ecosystem overall, with a temperature gradient getting warmer as you get closer to the planar boundaries. This extra heat will likely drive weather locally in those areas and make it dangerous in these border areas in their own ways. As the hot area, there would almost certainly be some manner of air flow around the border to try and equalize the heat.

  • The Plane of Magma puts out the most heat and would likely create the strongest weather in the border areas. My expectation would be that the higher heat difference would create the strongest winds but any storms will be more so windstorms, like tornadoes, as opposed to rain and thunderstorms
  • The Plane of Steam with aspects of both fire and water should create the strongest storms near their border since a lot of heat and water will flow across these boundaries. Expect more rain-based storms.
  • Not a lot will want to live near the boundaries near the Planes of Smoke due to the higher toxicity of the air within the paraelemental plane. Acid rain would be likely as the poisonous gasses dissolve in the waters of the material plane and fall.

The Plane of Ice, with its supernatural cold, would work similarly, but most likely in the opposite direction as the fire-based planes. As a plane of water, there is a lot of ice that can hold heat. My inclination is to think that it would snow more in the Plane of Ice

This does not go into the potential chaos of the paraelemental planes bordering each other.

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Perodic predictable cross-planar interactions

Let us make it easy and sensible. Let us say there is a rhythm to the interaction at the junctures between planes. This could be analogous to tides. At low tide materials from Plane A will enter adjoining Plane B. At high tide materials from Plane B will enter Plane A. The tides are predictable but they not necessarily with the same period as our tides. They could have a period of days or weeks; whatever makes the game fun.

Exactly what happens to these materials from one plane when they enter the other will make up your weather. It occurs to me that certain interactions might render portions of an otherwise unnavigable plane navigable - for example if Dust enters Mud, the mud might solidify to the point your characters can make their way across the top. If steam enters smoke the water might bind up the soot and gas and render the place habitable.

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