# Could extreme climate zones exist next to each other?

I would love to shape my planet in a way, that allows me to have an extremely cold, icy desert, next to an extremely hot, sanddesert.

By "next to", I don't mean a few hundred miles apart, with gradually equalising climate. I'm talking about a foot-march of, at most, 4-5 minutes between the two. Light a cigarette on the edge of the desert, be finished with it when reaching the icy desert.

Both areas should engulf roughly 140.000-180.000 square miles.

Is there a way an area like this could form naturally, without humans fiddling around with stuff and without magic?

If there is no other way around it, I'd be willing to use mountains to accomplish that, but I'd like to keep both areas as more or less flat-grounds.

You may choose the planet's positioning around a star of your choosing, its size, number of moons, all these shenanigans.

Edit: as suggested, I'll place a couple more constraints to narrow the question down a bit:

1. He who makes it plausibly possible to get rid of the height-problem (only option being vertical distance as opposed to horizontal) will be awarded 10 imaginary upvotes from me! That does not mean, answers that encourage the vertical distance are not welcome.
2. Underground-systems should, if possible, only be used, if both areas are actually underground. That makes underground-systems useless, probably, but I'll let myself be surprised.
3. Tidal-locking of a planet, if usable for this question's purpose, is obviously allowed.
4. These 2 Areas do not need to be habitable. The rest of the planet should, though. [by people that evolved there, no visitors]
5. It does not have to be water-ice. Any ice that allows for this situation to happen is welcomed. Bonus points for ice that does not kill people that want to cross it.

Edit 2: Temperatures

You may assume temperatures as follows.

Icy Desert:

• in extreme cases dropping to -90 to -120 °Celsius.

• should average somewhere between -60 to -70 °Celsius.

Border:

• anything that helps you (and therefore me) come up with a solution for the problem.

Sanddesert:

• in extreme cases rising to up to 80-100 °Celsius.
• should average somewhere between 70 to 80 °Celsius.

Should you have an idea that really could work, but needs small corrections on those numbers, add them to your answer, they'll definitely be acknowledged.

• Not with those restrictions, no.
– user20762
Jun 6, 2017 at 13:27
• If you don't care about anything other than the temperature difference, this might be possible with extreme elevation on a low gravity planet that still has a relatively thick atmosphere and heavy geological activity in the hot part - maybe with parameters like this. But life as we know it would certainly be impossible there. Do you mean water ice? Maybe it would help if you could put more restraints on your question. Would you be ok for example with one being a vast, underground cave system? Jun 6, 2017 at 13:51
• "Light a cigarette on the edge of the desert, be finished with it when reaching the icy desert." not sure about that, but "Light a cigarette on the edge of the icy desert, 20 seconds fly and be in the desert." sure - mountain plateau in a desert. Jun 6, 2017 at 13:54
• I observe that the cold place on earth is on top of an ice ridge in Antarctica - and that only gets down to -92C, and the hottest place was the Lut desert in Iran - which has got to +70C. In other words, your desired ranges are already outside Earth norms - putting them close together is going to be even harder. Jun 7, 2017 at 14:07
• @Olaf Klausson You say the areas do not have to be livable, does the planet have to be livable? Adjacent extremes are possible on a planet with almost no atmosphere.
– John
Jun 8, 2017 at 16:56

Space Mirrors/Lens

You may be familiar with frying ants with a magnifying lens...If you don't focus to a point, but rather an area you'll get a spot that's warmer and a big area that is cooler.

Focusing the power of the sun on a fairly small area of the world would increase the temperature of that area... if your world is naturally cold, these "artificially" warmed areas could well be but a short walk from the colder "normal" areas of your world.

Imagine if the picture below is a lens miles across in geosynchronous orbit over your warm desert, but not quite as focused... so the surrounding part of the desert is constantly in shadow and slightly colder than normal for that part of the world, while the center is much warmer than average.

An interesting side effect of this would likely be a constant wind blowing towards the hot region, as the hot air in the center rises it'll suck in the cold air from the surrounding region... which would like lead to some pretty exciting weather at the very center of the region.

• I see that as an incredibly beautiful, tidally-locked solid quartz crystal moon, which is focusing the sunlight always onto the same place on the planet, which would also have to be tidally-locked to its sun, right? How cool. Jun 6, 2017 at 23:48
• The lense moon mustn't be a perfect sphere however, as it needs to be at the L1 point between planet and its star. For the focal point to lie almost perfectly on the planet surface star, planet and moon would be way too close to each other then. Jun 7, 2017 at 10:26
• With this temperature gradient on such a large scale, There will likely be storms strong enough to rip a man's flesh from its bones. Jun 7, 2017 at 11:00
• @N2ition The moon thingy is never going to work. Unless you have like 4 Jupiters in your system to block ALL the incoming asteroids (and even then a crap ton are going to make it through). The asteroids impact the moon, dust forms, there goes your idea. On the other hand, if you keep it clean, then yeah ;) Jun 7, 2017 at 12:38
• Actually the day/night border on a small body without atmosphere CAN do that without the mirror. There are craters on the moon, where the edge is fully lit by the sun (almost permanently) and the center is in complete darkness, shaded by the ridge. This gives a temperature difference of more than 150 °C from the ridge to the bottom. Jun 7, 2017 at 14:42

# Yes

As long as you're happy to spend those 4-5 minutes travelling nearly vertically downwards at uncontrolled speeds.

Though that's particularly large for a high plateau you can play around with that in your own time. Put the plateau on the seaward side of your dry desert. The mountains stop the wet sea winds reaching inland, the rain falls on the mountains before, or as snow on, the plateau and gives you a low lying dry desert beyond.

• "140.000-180.000 square miles". This would not be stable for example on earth (but on mars, a planet that couldn't retain oceans). But I have no proof. How would such a structure even form? We get to magic and so on pretty quickly again. Keep in mind that even at 1°C per 100 metres (which is an overestimation), you need a difference in hight of about 4 km for a drop in 40°C Jun 6, 2017 at 13:57
• It might be possible in the interior of a Pangaea type continent, the majority of the interior of which being desert. A plateau that size? I couldn't say. You're looking at a major geological fault to create it, perhaps a small plate being pushed up by the surrounding pressure, far from stable on geological timescales, but humans don't experience geological timescales. Jun 6, 2017 at 14:04
• Well, it might be possible if the planet has very little water (so that the desert would be way below earth's sea level) - where we can go to early Mars again for example. But I still doubt this could exist with those parameters on earth even for a short time (not that you suggested that). Interesting issue for sure. I wish I had the time to check out if this was really possible and how. Jun 6, 2017 at 14:18
• This is exactly what I was thinking. Just give all the soldiers hang gliders. Jun 6, 2017 at 14:27
• @Raditz_35 With a different composition of the atmosphere (e.g mainly Argon) and a higher g you get a "better" Γ=-g/c(pd) (e.g. ~-40K/km for Argon and 2g) Jun 9, 2017 at 12:19

A magma pool could peek up in a cold space.

I don't know any real life examples of cold and hot deserts, but Iceland has natural hot springs even in ice and snow.

Theoretically, some vent creeping up could dramatically warm the ground. You'd rather need something to reflect heat and/or still the air to get it hot. That temperature gradient would create quite a wind at the intersections. This could account for some sand. Seems likely you'd have nasty fumes and black sand, though.

I haven't worked out all the details, but that seems another approach.

(If both are underground, one might make the entrance be an ice cave in the arctic and the magma pools inside a natural cavern. That heat could even account for some of the ice tunnels.)

• The lava would have to be dangerously close to the surface, and 374mi in length and width. I like this idea, until the volcano erupts, then its a planet eco-system killing dust fills the air. Jun 7, 2017 at 3:25
• @cybernard: You mean like Yellowstone? ;-) Jun 8, 2017 at 11:08

If the planet has no rotation, then the dark side versus light side would be very different indeed for a short travel. This would also result in an uninhabitable planet because the extremes of weather due to huge temperature differentials between hot lighted and cold unlighted regions. Jet stream effects could account for a transition zone tens of miles wide but not as narrow as you are envisioning. So you have basically two choices. A) Depend on suspension of disbelief B) Use an abrupt change in altitude as others have already suggested.

Another unusual possibility occurs to me. If a volcanic eruption created kimberlite pipes that acted as heat pipes after cooling due to a large amount of erupted metal then you could have underground water seeping into some of the kimberlite pipes and vaporizing to steam. This would add heat to a given area. It is possible that a steam curtain could be formed that acts much like the air curtain effect at the entrance to office buildings and shopping malls. By venting high speed air steam vertically there is much less heat exchange from side to side of the curtain. So it is possible (given a very unusual set of planetary features) to have a hot and cold area very close together.

• 4-5 min walk = a mile MAX Jun 6, 2017 at 14:49
• You beat me while I was writing on the jet stream idea. :) The op says solar system objects are changeable--couldn't that allow for a different size/strength/location jet steam than Earth's? With then different variation of weather effects on either side, especially if at ground level? Jun 6, 2017 at 15:12
• @N2ition Sure, differences in planet size inclination etc will change the amount of and dispersion pattern of energy the planet receives. And ultimately, weather is just a word that describes instantaneous conditions of movement of heat and fluids due to absorbed energy. Jun 6, 2017 at 15:24
• @Raditz_35 I know that :P Saying that 4-5 min walk is a mile MAX is like saying that humans are 20 feet tall MAX. Technically true, but kind of misleading. Jun 7, 2017 at 13:06
• @MissMonicaE If you live on a tidally locked planet that has such a large temperature gradient for no physical reason, I promise you that you will walk that mile in about 5 minutes Jun 7, 2017 at 13:11

A Natural Nuclear Reactor did exist here on Earth. Another one, much, much hotter, a little pathologically shaped, but still natural, could bring about the conditions you want, by heating a well-delimited area which would otherwise be much colder. Think of a well-defined, horizontal layer of soil made of uranium.

I didn't do the in-depth math necessary to be sure, but I think you could have just enough soil above it to shield the surface from radiation without undoing the heat island. If not, well, you did say it doesn't have to be habitable.

Story bonus: A particularly heat-conducting rock (or maybe artificial debris) protrudes from the surface. Your character lights the cigarette by touching the tip on it.

Arizona has some pretty "extreme" climate zones within the state. While it doesn't go from a hot desert to a frozen desert... we do go from a desert climate to a forest/temperate northern half of the state rather abruptly. Phoenix and the lower half of Arizona is literally lower in elevation. To get to northern AZ, there is a plateau like example picture below. This plateau draws the hard line between the 2 biomes.

• Day/night temperatures also differ more the higher you go. Jun 7, 2017 at 11:11
• But ggiaquin, +1, good answer. It's the first option I thought of, good to see not everyone resorts to far-off ideas! Basically you're often looking at about 5°F per 1000 ft (in situations where the terrain is well mixed with nearby elevated air... (minimal solar heating, decent wind)). Think we could take care of making N Arizona desert by further reducing moisture levels (by additional terrain features, less moisture on the planet, etc). Unfortunately for our world, even for the Arizona 25°F average change, that'd be around a mile... vertically... a tougher/long walk. Jun 7, 2017 at 14:47
• Perhaps we can get around the trouble by changing the dry adiabtic lapse rate. Since it's $-g/c_p$, either increase gravity or change to a gas with a lower specific heat capacity? Though I cannot begin to imagine the other complications that'd bring as adjustments to weather/living/etc. Jun 7, 2017 at 14:48
• A couple useful links: elevation of Arizona and higher-resolution temperature map (select Arizona on the state drop-down). Jun 7, 2017 at 14:55
• @JeopardyTempest yes, even for a car trip north, it takes several minutes to go up the elevation change. Jun 7, 2017 at 15:41

I'm thinking outside of the box (literally). It could be possible if your planet is a cube. I know that is not very realistic, but that is the only way that I can consider such a strong climate change. You would go from the equatorial face to the polar face. On a spherical planet, I don't think it would be possible. The wind will try to equalize the temperature.

You may choose the planets positioning around a star of your choosing, it's size, number moons, all these shenanigans.

• A disc would work even better. It would also make it really hard to cross the border. Unfortunately, both sides would be either too hot or too cold. :( Jun 6, 2017 at 16:37
• @Feathercrown I think if it were possible to have a debris field like the rings of Saturn somehow solidify and create in disc shape, the gravity would pull you towards the disc and let you literally walk from potentially a side of complete sun light to complete darkness in 5 minutes. You should submit that as an answer.
– Neil
Jun 7, 2017 at 10:16
• @Neil It would need to be spinning pretty fast, too.... I'll think it through a little more and then submit it, thanks! Jun 7, 2017 at 13:13
• Wouldn't cube not be even dwarf planet? Jun 9, 2017 at 23:32

This question is so interesting, I asked my 11 year old about ideas too. Here it is:

The planet is very tiny. The desert at the equator is very hot, and the person starts there and walks 5 minutes to the polar desert, where it is very cold. There is a river between the two deserts where the people could live. They would cross the river on a bridge to get from one climate to the other.

• Such a tiny planet would have no atmosphere, no rivers, and no weather. Jun 7, 2017 at 13:17
• @gerrit Thanks, makes sense or there would already be tiny habitable planets. Well, op does have reality-check tag. :) Jun 7, 2017 at 14:05
• @gerrit If the tiny planet had a surface gravity the same as earth then would it hold an atmostphere? Or would something else go wrong? Jun 9, 2017 at 3:25
• You mean just like the tiny earth-like planet in Rick & Morty? Jun 9, 2017 at 10:58

# A Big River

Oceans regulate temperature. It's described pretty well in this post on quora. A massive river could do it, one which has eroded away or filled in a fissure hundreds of feet underground, or which runs quickly enough to pull away all extreme temperatures underground or downstream. Perhaps parts of it lie entirely underground, which is where the little cigarette break might take place. Perhaps there is a flourishing oasis right at its borders, further blocking airflow between the deserts by its towering foliage.

# Surrounding Geography

Mountains have a profound effect on wind patterns. Check out a wind map of North America. Notice how the Rockies and Appalachians look different from tornado alley. The surrounding geography could blow a stream of air directly in between the two deserts, preventing any equalization of temperature. This site on gap winds talks about how certain mountain ranges effect the weather.

The closest thing to what you are thinking about would be a narrow canyon carved through a desert by a river. Four of five minutes of climbing down (or up, but it would be much harder :p) a stairway carved in the wall of the canyon and you go from desertic climate to a much more mediterranean one. Still, don't expect snow down there.

• If the height difference is enough, the upper one would be the cold one. Jun 8, 2017 at 19:15

One possibility that comes to mind is using shadows to cool of your icy desert. In the middle of your hot desert could be the shadow from a stable orbital dust disk (like Saturn's rings).

This kind of disk does not sit still, so some suspension of disbelief will be necessary, and yours will need to be really dark to blockout most of the sun light.

Also, the sun is not a punctual source of light. To get the abrupt transition you want your planet will have to be very far away from it's sun in order of it to appear to be a point and your penumbra region be small (otherwise you will not be able to quickly go from hot to cold). But for the hot desert to be hot even so far from the star said star will have to output a lot of heat... a small hyperdense blue star is what you want!!!

• I think this idea makes the most sense. What do you mean by the disk doesn't sit still though? Could it not be perfectly coplanar with the planet's orbit around the sun? I'm imagining the shadow realm to be exactly on the equator with zero tilt in the planet. Of course there would probably have to be no moon as well... Jun 6, 2017 at 22:45
• the disk must be thin to be stable, so if it is coplanar with the orbit around the sun it will cast no shadow on the planet. But if it is not coplanar then it will experience a torque from gravitational pull and the area it shades will move. This movement will create a larger area of transition between cold and hot deserts...
– ivbc
Jun 7, 2017 at 0:01
• By coplanar I mean the disk is on the same plane as the planet's orbit. It would be around the equator basically. So it would definitely cast a shadow. But I see what you mean with the gravitational torque. Perhaps a moon could be in such a perfect orbit that it cancels out the torque? :D Jun 7, 2017 at 16:43
• If the disk is thin this shadow would be very small.
– ivbc
Jun 7, 2017 at 17:04

Okay, so this one is a little more out there, but if you're willing to deal with an immense man-made structure, you could set up a Dyson sphere. This is a structure that completely surrounds a star at a distance that is habitalble. Imagine a sphere with the diameter based off of the Earth's orbit surrounding the Sun. Inside that sphere, you could have a ring or band(s) that would orbit inside. Sorta the same idea as @ivbc, but being a manmade ring, it might work better.

The lack of sunlight in the area beneath the internal object would that are much colder. Additionally, since there is nothing blocking the other area, it would be constantly bathed in sunlight, making it scorching hot. I'm not sure if either area would habitable to humans, but maybe the Dyson sphere was set up along time ago and broke, or the people who set up like it really, really hot and don't need day/night patterns.

• How exactly does this answer the question? Jun 6, 2017 at 17:10
– beth
Jun 6, 2017 at 17:16

A doughnut world could do it. This older synopsis of it somewhat describes what your looking for even. The bigger issue for this solution is finding a stable doughnut world. sci-sho space (YouTube) did an episode on this possible, though highly unlikely possibility.

https://curiosity.com/topics/yes-a-donut-shaped-planet-is-technically-possible-curiosity/

The planet has a jet stream at ground level

Set up your planet with orbit, lunar pull, topography, magnetic fields due to rock composition, prevailing winds, and bodies of water such that the weather patterns form to include a ground level jet stream with much more dramatic weather differences on either side. Orbit: Earth's Extraordinary Journey, is a three episode tv documentary available on Netflix that can get you started on Earth's orbital and lunar weather pattern background information for a jumping point to your planet's characteristics.

Regarding rock composition and magnetics affecting weather systems, this is just from my own observations of local weather patterns, but I will check around later to see what I can find on that, or if others with mineral knowledge already know perhaps they can contribute.

Your person would have some considerable difficulty keeping the wind from ripping the cigarette out of his mouth while crossing the jet stream, but after 5 minutes of exhausting blizzard wind conditions, he emerges from the hot dessert into the icy dessert.

• Local magnetic fields are unlikely to cause major differences in weather patterns under most ordinary circumstances. Now, if there was an intense change in the magnetic field in the magnetosphere you might get some spilling of radiation into the atmosphere from outside the planet with some local side effects but that’s quite different than what would be likely through mineral composition.
– user39094
Jun 6, 2017 at 15:52
• @ColGraff Maybe local observations are more to do with the local topography than rock composition then. Although there are hills, rivers and lakes near the city, none are on huge scale I thought necessary to direct the storm systems path. I wonder if anyone would be able to give reality check on a ground level jet stream. Jun 7, 2017 at 14:12

There are many places here on earth where you can find temperature and rain extremes very close to one another. I am going to focus on the Island of Hawaii.

A great example of how to climate extremes can be in the same area is the Kohala district. The Waikaloa Coast on the Northwest part of the island gets 0-10" of rain a year. Mt Kohala on the North coast gets 120-160" of rain a year. Both places are only 11 miles apart.

At the coast are remnants of dry forests, and near the summit lies a cloud rain forest.

Approximately 40 miles West Southwest from Mt Kohala is 14,000' Mauna Kea, where it regularly snows.

The Island of Hawaii has 4 out of the 5 major climate zones in the world, and 8 out of 13 of the sub-zones in an area 94 miles long. The temperatures remain pretty consistent throughout the year in each zone. As an example, I have friends that live in a zone which is the consistent weather of a Seattle summer all year long. The snow on Mauna Kea is consistent all year long. If you wanted a template to have extreme zones next to one another, this seems like a good place to use as a template of what is possible and what can exist in your readers mind.

Not naturally, but a giant artificial wall/mirror on the equator of a tidally locked planet would do it.

Although I admire the creativity of some of the answers, since you want a science-based answer, I'm going to disappoint you and answer that this is not possible in any natural way.

The high and the low plateau, such as in Separatrix' answer, comes closest, but is not really in the spirit of a 5 minute walk. In the absence of water, the dry adiabatic lapse rate on Earth is 9.8K/km. On a heavier planet this could be slightly more, so let's be generous and consider a planet where this is 15K/km (gravitational acceleration 1.5g). Then a 30K difference would require a 2 km drop. Not exactly a 5 minute walk uphill, certainly not on a planet where gravity is 50% stronger than on Earth.

N2tions 11 year old suggested a tiny planet. A tiny planet would have no atmosphere and thus no weather or climate, so this doesn't work.

More fundamentally: temperature gradients go along with pressure gradients, and where there are large pressure gradients, there will be strong winds. Those winds (advection) work to equalise temperature differences. If you start off with a huge temperature gradient, there will be huge winds; and soon the temperature gradient will be much smaller.

If you are willing to step aside from natural climates, one way to achieve it would be to build a huge mirror along the equator of a planet tidally locked with its sun. I don't know why anyone would build such a wall. If the planet is Earth-like, this wall will have to be at least 30 km high such that no significant amount of air will flow over it. The insulation is doable; a 60K temperature gradient is handled regularly by walls in cold climates on Earth. You don't want to have any holes in the wall, so your 5 minute walk will be in a tunnel. This will need airlocks as the pressure is going to be very different between the sides, to the degree the atmosphere may actually freeze on the cold side of the planet. It goes without saying that the engineering challenges of this wall, consisting of diamond, graphene, and handwavium, will be immense. As user N2ition points out in a comment, the wall will still need to be maintained/fixed when punctured by meteorites or other (natural) disasters, as those would lead to serious airflow in particular if occurring at lower altitudes (as the pressure differences between the sides may be very large).

• The mirror might run into the same problem as the crystal moon...asteroids causing havoc. Add to your engineering feats a barrier for that? Jun 7, 2017 at 13:57
• @N2ition True. Added this comment. Jun 7, 2017 at 14:03

Yes, here is a photograph of people bathing on Deception Island, Antartica

Temperatures go as low as -28'c while water temperatures can be up to 70'c

So in other words, hot springs!

• The photo doesn't look like either of those temperatures ;-) Jun 13, 2017 at 9:46
• If you look on google maps satellite, the island is a caldera and the 'mountains' around the rim are covered in snow, all the way down to where they meet the sea! Jun 13, 2017 at 13:33
• Yeah, I meant more the fact that sitting in 70°C water is not that comfortable, and sitting naked in -28 °C air neither. I guess at this point the temperatures are a bit nearer to each other. Jun 13, 2017 at 17:46
• The 70c water is localised to steaming fumaroles, and isn't the general water temperature. Jun 14, 2017 at 9:08

Kind of a long shot, but building on Steverino's idea. A desert region is scorched by the nearly constant sun, and can be somewhat arbitrarily hot. In the intermediate zone water exists, and leads to vegetation that grows semi-vertically (to catch the edges of the sunlight without getting scorched). Behind the wall of vegetation the sunlight never arrives, perhaps a thinner atmosphere or just limited airflow leads to the far/dark side growing very cold.

• Welcome to WorldBuilding Christophe! If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! Jun 7, 2017 at 9:57

It's not quite a five minute walk, but if you were to drive over this border, you are very nearly describing Canada's "pocket desert" in the interior of British Columbia: Okanagan Desert

What happens here is that the Cascade mountains create very wet conditions on the seaward side, and very dry conditions on the inland side. The high mountain passes also frequently get snow much later in the year than either valley side. So driving through this region (especially the Coquihalla pass) in the spring can easily mean fighting through a foot of snow before needing to go out in a T-shirt ten minutes later. As long as you're driving. And even in the spring, it can be quite hot in the desert side.

My vote is for a tidally-locked planet with a tall, steep mountain range on the night/day divide. A cave shortcut through a thin ridge could make it a quick trip from one to the other.

Canyons and a thin atmosphere work for me too, as the Martian canals worked in Out of the Silent Planet.

How about a hollowed asteroid? Do you need a lot of gravity?

From my comment I came to realize that there are places very much nearby where this kind of differential actually exist in real life - albeit without atmosphere. The Shackleton crater on the moon is close to what you would like to create.

If you put the body in a close orbit around a star and tidally lock it, the day / night border - especially with a ridgeline one it - can act as such. The lack of an atmosphere (or an extremly thin atmosphere) is imparative however, otherwise the heated gas would distribute the heat from the lit site to the unlit side with some very violent winds.

A large polar lowland/crater with surrounding mountain ranges could make for the large temperature differences you're looking for.

The surrounding mountains would block out the sunlight placing the polar lowlands in a constant shadow. With the sun never shining in the polar lowlands it would get very cold, actually most hot deserts tend to get cold at night very rapidly. Expect a stark transition between one side of a ridge in sunlight and the other in constant shade. This type of landform could easily form naturally as an impact crater.

A Polar vortex in conjunction with the mountain range could also account for a lack of ground level air mixing between the two regions allowing high temperature differences across a short distance.

The planet would likely need to be either in general hotter than Earth, have a greater seasonal tilt making a warmer summer, or some other geography features to account for the desired high temperatures of the area surrounding the crater (since they would also be in the polar region).

You can think about a swarm of planets/satellites locked in Lagrange points (L4 or L5, as the others are unstable) such that a small satellite projects a circular shadow always in the same (more or less) point.

I am unsure if this can be really done, but it's worth a try.

Using L1 would be much simpler, but that is unstable, unfortunately, and stabilizing would require "humans fiddling around".

If the shadow is large enough the center is going to be quite cold and You could think about some orographic barrier to make the change even sharper.

Not in such condition you would have a constant cold wind blowing from the dark zone.

For maximum effect place the shadow in the middle of an equatorial desert.

• Earth-sun L4/L5 won't give a shadow on earth. Earth-moon lagrange points are rotating with the moon, so only will give a shadow once a month. Jun 11, 2017 at 12:19
• @PaŭloEbermann: I was speaking about a swarm of bodies, I know quite well in a 3-body you cannot attain such a configuration (L4/L5 lagrangians lie in the same orbit as the main planet, so they cannot cast shadow) OTOH I believe it's possible to find a set of locked orbits that is stable (but I might be wrong, I didn't actually work out the problem) Jun 12, 2017 at 19:27

Not on an earthlike planet.

given the size of the two environments, and they they need to be persistent, it is impossible. Adjacent stark contrasts in temprature don't stay that way for long, there will be a massive wind trying to even out the differences in temprature and pressure. As small localized hot spot in an otherwise cold environment is possible but not two similar sized large scale areas.

You need a non-earthlike planet with almost no atmosphere. Then all you need is one area in shadow and the other in sun and you can have your differences. However they will not always be that different unless the planet is tidally locked.

Deserts have a high albedo, which means that they reflect sunlight well. Glaciers also have somewhat high albedo. If there are no natural ways to keep hot/cold air from flowing from other area to other, then their border area most likely changes from time to time. This is because an influx of hot air would affect the cold area and vice versa.

You probably want to use some of these elements:

• Bodies of water: water temperature affects the air temperature. Constant warm or cold streams/currents as well as inland seas (with constant warm/cold water) can help maintain regional temperatures.
• Constant winds/airflow not unlike the trade winds on Earth: constant wind blowing from cold polar area will surely make any area chilly.
• differences in altitude and mountain ranges: somewhat intuitive, but also related to winds, since natural obstacles may affect airflow, for example blocking cold winds from entering some area. This is an ugly simplification, though. When you add humidity, air pressure and other such variables to the equation, the result may get somewhat complex.

You might also want to check out articles about the Atacama desert and its surrounding areas for ideas and concrete examples of what I've described here.

The hot part of the world could be the result of a mega-shield volcano. This volcano would have eruption points every few miles. The desert would have to be black volcanic sand. The surface itself would be heated with magma from close under the ground.

The cold part could have many long ocean inlets, or a large freshwater system. The water system, (whichever you chose) is cooled by ocean currents, that flow from the arctic. This is the opposite of the Gulf Stream, which heats Western Europe, and allows palm trees to grow in Ireland. A cold current from the arctic dissipated into an inlet could cool the climate considerably

Okay I have read many answers here and got an idea on how to achieve this (for the sake of this answer I will use the term "short climate gradient"). Have your planet be exactly like earth but further to its star than earth is to the sun and with a gravity of say 1.2g (to maintain an atmosphere much longer in height than earth's).

With all that set we need cloud cover that is so dense on a vast section of the planet with a region that is "clear sky" within the cloud covered area. This open area allows the star's heat to get through to the surface. In my mind I picture this the same way as a spotlight (in a performer's stage). The open area can be of any shape whether regular or not. The cloud cover is thick but doesn't act as a shadow on the area below it - significant light permeates through during daytime. Anyone walking (a meters like 50 to 100 - consider this as close) from the shadowed area to the open area will feel the change and this will be the climate gradient.

This weather is permanent enough to be viewed as a climate and the open region large enough that someone on one edge cannot see the spotlight's edge on the opposite side. This can work in a flat-like continent on the planet.
Pardon the English.

One possibility I thought of, would not work for geologic time scales but may work for story-level time scales - how about an underground fire? I was thinking of the Centralia underground mine fire, which has been burning for more than fifty years and may burn for another two hundred fifty years. Also, the oldest coal seam fire is about six thousand years old (Burning Mountain, in Australia), so quite variable time-frames. Or you could look into peat fires, if coal seams don't suit your story, for similar long-running underground fires. These are fires, though, certainly not geologically stable, but hundreds or thousands of years should have time for quite a few stories and a fair bit of history.

It's easier to heat things up than cool them down, so this would be set in (or at the edge of) an icy desert - either an area quite pole-ward, or else at a fairly high elevation. A desert in the rain shadow of a mountain (or a range) may be a good choice, since a prevailing wind from the icy to the heated areas should help prevent the hot air from working to equalize temperatures.

The hot area would need coal seams and something for air flow - abandoned mines are common, but since we want to do this without human meddling, we can look for natural caves, or fractures in the landscape from geologic activity (like earthquakes). Caves open to the air could also serve to collect duff, burnable litter and debris - possibly from wind drifts or from bird or creature nests, maybe from hardy creatures living in the desert or left over from a time when the area was warmer (ie, if the area has cooled like in ice ages, or the continent drifted pole-ward) - to more easily set the fire smouldering, especially if the area is usually cold.

To get the seam burning, a wildfire in the burnable duff may burn long enough to light off one of the coal seams. Or else spontaneous combustion, where moisture, grain size, and temperature (happens as low as 40*C) in brown coal combine just right to get a fire going - it would be a humorous and oddly believable possibility, that was the official cause for the fire in Centralia for many years. If that doesn't suit the story, or feels too hand-wavy, then sparks from a rockfall, or else from lightning, may work better to get a fire going.

An aside - it seems kinda handwavy, laid out like this, but it's actually not too implausible. Wildfires are the natural way of cleaning out the burnable litter that collects in nature, it eventually becomes deep enough to be a fire hazard, burns clean, and starts accumulating again. And coal seam fires are quite common, maybe thousands worldwide at any given moment, and possible anywhere coal seams touch the surface (for air) - and some are very old, and so could not have possibly been due to human mining (or other meddling). And icy desert, heated up, will be a temperate, then a hot desert since the heat won't change actual rainfall. And if the heated area is at the edge of the desert, then the ecosystem on the other side could contribute - ie, grasslands would give more access to wildfires, the better to ignite your coal seam.

Anyway, you would need a border to keep the hot and cold areas of your desert working. The edge of the relevant coal seam is one option, the geological shaping could just have deposits in one area but not another, or could have deposits isolated enough one cannot catch the other afire... though the temperature gradient may be more gradual. A wind canyon may be another option - a bridge would keep the walk at five minutes, but the ground is visibly divided.

Another very nice possibility would be a river - if it was fast and deep, carved into a gorge or canyon (or even underground), it wouldn't even un-desertify your land... having it flow when the area's cold enough to be "icy" is a bit trickier, though speed and depth will help, or having it start from a hot-spring. And the fires on the other side will help keep it flowing-temperature. A river would act as a literal heat sink - preventing the fire from spreading that way, but also keeping the temperature from equalizing across the ground. The water would take the heat from one side, and flow it right downriver - keeping the other side cold and the transition area, narrow. And you could have a very dramatic effect, one side with ice at any shallow or slow edges and the other side warm, even steaming, from the ground heat.

The area would not be particularly livable, with vents of heated, poisonous gasses, ash and soot, and cracks and sinkholes appearing in the ground as the underlying structure is burned out. But it should be warm even when the area is quite cool, and the burning of soil may expose rock, which if dark may absorb sunlight much better than the icy desert's surface - especially since ice or snow reflects heat quite well - leading to much hotter days than the fire alone accounts for.

I'm not sure if the exact temperature ranges will hit your desired outcomes.

I think 70-80C can work for the hot desert, or local areas a even hotter (fire, after all). But the heat may not be even. In the beginning the burning area will start out merely warm (since it starts small and must heat the area from very cold), but get hotter as the warmed coal is easier to ignite, the surface buckling gives more access to the air, and the rock has time to slowly absorb and hold the heat of the fire - and as the rock burns bare and starts absorbing heat during the day. So, it will get hot in the sun and during the day. But I'd think it may get cooler in the shade, at night, or in winter, since the area would have to be pretty cold to keep the icy desert's temp steady.

The icy area temps given are colder than polar winter (Antarctica record low's is -89*C), which is very cold. It may be achievable with a cooler star, or a planet further from its sun, or a deeper tilt, perhaps. But it may have another problem - those temperatures are very cold for a fire to get started and keep burning long enough to warm its own area. It can work if the ground is warmed, even pretty locally, and once lit it can build on itself, but getting started is much trickier at these temps. The ground-fire will work better if the area gets warmer in the daylight, in the summer, in areas where the geology permits a bit higher temperatures - but then it may not be as cold as you want.

I think this setup, an underground fire, can produce quite dramatic results and a very fine temperature gradient for your hot-and-cold desert areas. But it will be far easier to do if the temperature gradient is not quite so extreme...specifically, not quite so cold. Even polar temperature ranges might let you edge into warm enough for the fire to start - in summer, in the sun, in a specific area - while still being very, very cold most of the time.