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I need maps of climates and prevailing winds for my desert-dominant world. It has one continent stretching in a band around the tropics and into the subtropics, making up about half the planet's surface. There are two polar seas. The planet has a 20 degree axial tilt, Earth-like atmosphere, is slightly smaller than earth, and spins a little faster (not quite a 20 hour day).

Using Climate Cookbook, Mark Rosenfelder's PCK, and Google, I've been researching the patterns that govern wind and climate in our world. I think I have a very basic understanding of the way air moves, the Coriolis effect, seasonal pressure differences over land and sea, and earth's pressure belts. I'm ready to draw them all in on my map. But alas, the only reference I have is Earth, and Earth's continents are broken up by major ocean. These seem to scramble up what would otherwise be neat little belts of pressure and turn them into swirling pressure cells.

Now, what is likely to happen on a planet with a continuous strip of land around the middle? The north and south shores of my continent are currently dancing around the lines of my planet's subtropical convergence zones. Will I have low pressure cells over the gulfs in winter and the peninsulas in summer, or will the more uniform nature of the land give rise to an unbroken pressure belt over land and sea alike?

In short, will more consistent land mean that the areas of pressure will also be consistent: a few, large, unbroken strips as opposed to many jumbled patches like on Earth? Currently my maps (summer and winter versions) show quite a few pressure cells neatly arranged around their respective belts, but I'm second-guessing myself.

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    $\begingroup$ A continuous wide belt of low-lying land would see a continuous wind blowing from the west, much like the continuous ocean spanning the 40 and 50 degrees southern latitude on Earth; but land is seldom a continuous plain: moutain chains would break this into complicated pieces. Without a full-scale simulation no serious answer can be given. The closest thing Earth has to your world is the Sahara; try finding wind patterns over the Sahara and generalize from that. $\endgroup$ – AlexP Jun 17 '17 at 13:19
  • $\begingroup$ Your 1st question may be too broad. Additionally, it's unclear if you're asking about how pressure cells will form or how pressure belts will form (the latter is a subset of the former). $\endgroup$ – KareemElashmawy Jun 20 '17 at 15:13
  • $\begingroup$ Ah, I apologize; "what is likely to happen" was not meant to be a question in and of itself. I will clarify the question. $\endgroup$ – RLuebke Jun 21 '17 at 1:22
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You're broadly correct in the neat pattern of convection cells the same way as it would be on Earth, the devil is in the details. Broadly the world weather isn't going to change that much, seasons are going to be slightly less different, the tropics a little closer to the equator, and summer and winter will be a little longer, that's just because you've reduced the axial tilt. The biggest difference is going to be seen at sea-level with onshore winds being the dominant influence, the land is going to have slightly greater low pressure than the ocean does under a given insolation regime, monsoons are going to be the normal at the continental margins too. Longer summers means higher sea surface temperatures so where the sea comes close to the equator you have the potential to generate massive cyclones on a regular basis. Coriolis is still going to break up the broad pressure belts/convection cells into cyclonic systems regardless of surface topography but winds will be stronger because of the reduced surface friction.

As a note such a planet might reasonably have much higher CO2 levels in the atmosphere, a larger single continent means less weathering of freshly uplifted rocks due to less weather penetrating the interior, such weathering sucks a lot of CO2 out of the atmosphere, especially in the Himalaya.

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Pressure Belts

As far as pressure belts are concerned, you'll see the same types of equatorial bands as we see on Earth, Jupiter, and Saturn. Pressure belts form due to the spherical shape of the planet unevenly heating the atmosphere at different latitudes. This phenomenon is independent of the underlying geography (provided the geography isn't piercing the atmosphere).

Hadley Cells

Going one step down, Hadley cells form from the directions of the pressure belts. Like the pressure belts, they form based on the planetary geometry (spherical, tilt) and will be quite similar to Earths. Since Hadley cells extend across the height of the atmosphere, the underlying geography won't affect their behavior.

Pressure Cells

Take another step down, and you'll reach the pressure cells. These are much more complicated and difficult to predict because they cover a host of phenomena and are dependent on intrinsically sensitive meteorological factors.

Formation

Pressure cells form from cold and hot air masses in the upper atmosphere (and typically from polar regions for high pressure cells). High pressure masses pushes air out creating a large clearing of clouds over a massive area. The Coriolis effect then shifts this into a cyclone mass. That in turn can transform into any number of other cyclones depending on the cyclogenesis.

Cyclogenesis

Cyclogenesisis predominately dependent on the scale of the meteorological phenomena. The smaller it is, the more likely it's dependent on geographical phenomena. As far as your pressure cells are concerned, the underlying geography will determine their geometry strongly if the cell is small enough (i.e. Mesoscale). If the cell is large enough (tropical), then underlying geography will affect much less so. The exact nature though, will depend on the geography, the meteorological state of the air mass and its neighbors, and various other meteorological factors that are extremely sensitive to the meteorological conditions.

Take Tornados for instance. They are formed by Mesocyclones (cyclonic low pressure cells) formed within strong thunderstorms. When a low wind collides with a higher altitude wind travelling perpendicular to the low wind, the low wind 'shears' the higher wind and causes the air to spin. Then, when the spinning air mass further into the low pressure cell it's pulled upwards, creating an updraft near the ground. This forms the funnel cloud. Then, if rainfall increases in the storm, a strong downdraft drags the mesocyclone with it. When the mesocyclone reaches below the cloud base, it pulls in cool air from the updraft region previously mentioned. The subsequent combination of cool and warm air forms the rotating wall. When the updraft increases in strength, a low pressure area is created near the ground which pulls the mesocyclone further down. This manifests as the funnel of condensation commonly known as the Torndao.

Notice how many factors played into it? This is why tornadoes largely occur in large flat areas. These areas provide the space necessary for powerful wind shearing to occur. Furthermore, the area also needs low pressure cells to form commonly. As such, this doesn't occur in deserts (another flat area) because such low pressure systems don't typically form in deserts.

Other Geographical Effects

Similarly, Mountainous areas will block small cells and funnel the air mass and winds alongside the range (or around the mountains). Coastal areas can (and will) feed high pressure cells. Deserts produce hot air in the day, and cold air at night; next to a coast they will cause a daily shift in air masses back and forth, etc.

Largely the formation of cells is dependent first on atmospheric conditions, then geographical conditions.

Errata: I didn't cover tornadoes, storms, and other phenomenon because their formation is much more dependent on how a given storm moves and a host of other factors.

Conclusion:

Ultimately, meteorological processes are chaotic processes, therefore they are absurdly sensitive to every condition and parameter that effects them. As a result, even with the knowledge of how the systems form, we cannot fully predict them. Case in point: predicting tornadoes, tropical storms, earthquakes, et cetera. You don't need to fully predict every aspect of your world. By all means use artistic license where necessary.

What you do know is how the geography can effect the local weather generally. Your Pressure Belts and Hadley cells should look similar to (if not identical) to Earth's. Your Pressure Cells on the other hand are impossible to forecast here. They will form and breakdown with storms and high/low pressure systems. The only statistically predictable phenomenon you have is the change in the local heat capacity of the equatorial strip with respect to the orbital tilt; but, that will only affect local weather phenomena, not the atmosphere miles above it. At the very least you'll have a North/South oscillatory shift in the pressure cells as the North/South coastlines are heated unevenly (tilt). Otherwise, the land area is likely too small to suggest other statistically reliable weather phenomena.

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  • $\begingroup$ I hate to reduce your answer to "it's complicated and difficult to predict", but is that basically what you're getting at here? I think I understand the pressure belts, Hadley cells, and other geographical effects, but there's a lot to take in. I'm still uncertain about my wind/pressure/climate mapping because I have things like low pressure cells over my tropical desert (under the ITCZ), and climates that don't quite line up with Koppen, so I'm worried I'm doing something wrong. $\endgroup$ – RLuebke Jun 21 '17 at 1:38
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Wind comes from convection in air, i.e. when cold air (for instance, over a sea) meets hot air (for instance, land.). Given that, in your world, there is both a northern and southern sea, there would probably be winds blowing from the north and south, converging along the equator. This would create a continuous high pressure zone over the equator.

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    $\begingroup$ See en.wikipedia.org/wiki/Wind#Causes_of_wind - convection is one among many. $\endgroup$ – Mołot Jun 19 '17 at 11:03
  • $\begingroup$ Yes, but on a world where the main continent is the tropics, convection would be a major factor. The outer regions of the continent are cooler than the inner regions, so convection would be a significant cause of weather patterns. There would be other wind sources, however my response is dealing with the pressure system itself. $\endgroup$ – Dofork Jun 19 '17 at 11:50
  • $\begingroup$ Could you elaborate on this? Would there not be other pressure belts? If I understand right, there's a low pressure zone around the equator on earth, with adjacent high pressure zones, and then low pressure zones, and finally high pressure poles. Why might this world not be the same? $\endgroup$ – RLuebke Jun 21 '17 at 1:52

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