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In my world I want a massive region (7 million mi²) to be as flat as the Great Plains. There are no trees or hills; basically it's just a grass sea.

I am trying to figure out what kind of weather patterns would develop; such as wind speed, rainfall, and climate. I also would like to know, in a region that large, how hostile to life would it be?

The region is bordered by the ocean to the east and mountains to the west/northwest. If my world had an equator¹, it would probably be just a little above it. I don't believe there will be any bodies of water in the region, except some streams maybe.

¹to answer some questions the world TECHNICALLY has no equator because it is more cylindrical (unless I have no idea what equator means-- which is likely). However as it is a fantasy setting with magic I have more or less hand waved the fact that given the normal laws of physics it would crush it self into a ball.

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  • $\begingroup$ Where does it say it is the size of North America? I'm not seeing it. $\endgroup$ – JohnWDailey May 7 at 12:44
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    $\begingroup$ This would be a "Continental climate" like the Russian Steppes $\endgroup$ – Thucydides May 7 at 13:53
  • $\begingroup$ Okay I edited the question to be a little (a lot) to make it make more sense. My apologies for the initial confusion. It's been a while since I was active. $\endgroup$ – Gray9 May 7 at 14:22
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    $\begingroup$ Wait... no equator? This world isn't spinning? This is an important detail. $\endgroup$ – PipperChip May 7 at 14:46
  • $\begingroup$ I apologise formthe confusion as it has been a while since I looked at my notes. I genuinely don't remember what shape it is. I thought not was flat but it could have been cylindrical or pacman shaped. Either way I know that gravity's normal laws were more or less thrown out the window on this one. $\endgroup$ – Gray9 May 8 at 3:03
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In the most general terms, you need to understand some things to get a good answer. Yes, simulation tools exist to create specific weather patterns, but they rely on general principles. Some things to consider:

Grassland Formation

You already know you want a massive grassland climate. So it must have the climate that grasslands have right here on earth (assuming earth-like biology, of course).

Grasslands form in nutrient-poor and low-rain areas. Periodic, naturally-occurring fires tend to promote grasslands. Therefore, this massive land area needs to be dry enough to prevent forests, wet enough to support grass, and have the right mix of nutrients in the soil.

There are examples of massive grasslands on earth: the Great Plains (mostly grass) and the Asian Steppe being two great examples. They get occasional rains and storms, but are mostly sunny most of the time.

Hadley Cells and Eyeball Earths

Remember convection: hot liquids tend to rise, cool liquids get 'sucked in' below the now-vacant space where the hot liquid was. This happens on a massive scale in atmospheres. Instead of going from pole to equator, though, it forms zones/bands of moving air. These are called Hadley cells!

The question indicates that there is no equator. This can be due to several things: maybe the world is a ring-world (like in the Halo video game series) or the more likely Eyeball planet.

Eyeball planets are 'tidally locked' with their sun(s), so the same bits of land/ocean are always facing the sun. This results in climate oddities!

Combining this with Hadley cells means you could (in theory) have rings-of-climate with the "under the sun" point and "furthest from the sun" points acting like a hot/cold poles and the center point of the rings. Combine this with Hadley cells, and you get permanent winds which could (in theory) support a ring of grassland climates with the specified area.

Odd stuff would likely happen in Eyeball planets, though.

  • The Hadley cells would result in a (permanent?) cold wind from the dark side to the day side.
  • Water on the day side could be a source of rain/snow which feeds into the grasslands, although maybe not directly on the grasslands.
  • Plants, including grasses, would always want the faces of their leaves/blades facing the sun, so every blade of grass would be arranged in the same direction.
  • Travelling to/from the day/night side could be a part of daily life for animals.

Eyeball earths: they are really amazing!

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Unless trees don't exist, this is dodgy. Even in the actual Great Plains, there are trees, especially around bodies of water. (This includes streams and ponds; you don't need something like the Great Lakes.)

Assuming your world isn't entirely crazy¹, you can probably get fairly substantial grasslands which, as you go further north, turn into taiga (e.g. Canada and Siberia). Rainfall will likely be modest except near the coast.

I would recommend mocking this up in Azgaar's FMG which provides some automatic climate tools. Here's what I came up with after a few minutes fiddling:

Heightmap Biome map

The light green drab is grassland, the dark fern green is taiga. Note that you will naturally get forests (bright green) closer to the equator and around rivers which turn into temperate rainforest near the coast.

Note also that the scale of this map is 0° - 60° and it still only has about half of what you want (and that's counting the taiga). I think you will need a landmass about three times the size of North America to get what you want. If you want warm grassland, i.e. not taiga, you're going to have to either go much bigger or else somehow give your planet much less temperature variation by latitude.

I won't comment on how likely this geology is or isn't.

(¹ As PipperChip noted, what's this "no equator" business? Is this a ringworld or other such artificial construct?)

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  • $\begingroup$ No equator? More likely an Eyeball Earth than ring-world: en.wikipedia.org/wiki/Eyeball_planet This presents many interesting things, such as radial climate zones, plants growing to face only one direction, odd air currents/Hadley Cells, eternal night/day, and more! $\endgroup$ – PipperChip May 7 at 15:07
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You are describing La Pampa. Very humid plain in Argentina, with periodical shallow wetlands (useful to contain the water) and sparse trees. Tempered climate in general, with mild Winter. Again, humid, with abondant rains in all seasons.

https://en.wikipedia.org/wiki/Pampas

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