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Earth has three convection cells per hemisphere, resulting in each hemisphere having a band of desert at the end of the Hadley cells.

Given a planet with the following characteristics:

  • Twice the radius of Earth.
  • Rotates every 24 hours. (So the linear velocity of rotation is doubled, but the angular velocity is unchanged.)
  • Gravity is 1g (achieved by some means out of the scope of this question).
  • All other characteristics like atmospheric composition are the same as Earth.

Would it still have three cells per hemisphere, or more? Everyone seems to agree faster rotation with constant radius would increase the number of cells, but is it the linear or angular speed that matters?

The best answer I have been able to find on that is https://earthscience.stackexchange.com/questions/992/what-factors-determine-the-number-of-hadley-cells-for-a-planet and while it's not clear what the symbols in the answer mean, it does seem to say that increased radius holding all else constant, does increase the number of cells. Some sources also seem to argue that one of the causes for air sinking at the edge of the Hadley cells is Coriolis force imparting sideways momentum; I'm not clear why that would cause it to sink, but if it does, that would also support the conclusion of proportionality to planet radius. If it's a matter of having time to radiate energy away to space so as to get cold enough to sink, this would also support the conclusion.

There does seem to be a consensus that the number of cells per hemisphere must be odd. So if it's not 3, the next candidate is 5.

Does that mean that on the planet with the above parameters, each hemisphere will have two bands of desert (at the latitude where air descends and warms up)? So, a band of hot desert, a band of cold desert, with a band of wet climate in between?

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    $\begingroup$ I think this question is best suited for either Astronomy or Earthscience $\endgroup$
    – Alastor
    Oct 26, 2022 at 8:26
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    $\begingroup$ I don't have a full answer, but the link you provided presumes that cells would form where Coriolis force turns the N/S momentum into E/W momentum. On Earth, that winds up being at the 30 degree and 60 degree mark. When you're on a spinning globe, planetary rotation turns linear momentum into angular momentum as you increase latitude. $\endgroup$ Oct 26, 2022 at 16:41
  • $\begingroup$ I'm pretty sure the formation of deserts is more complicated than just planet radius. You also have multiple questions in this thread. $\endgroup$
    – ITM_Coder
    Oct 27, 2022 at 18:33
  • $\begingroup$ IMPORTANT: does it have the same axial tilt as Earth (23.5°)? $\endgroup$ Nov 1, 2022 at 9:37

2 Answers 2

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About winds and cells

A rotating planet with an atmosphere has 3 types of cells per hemisphere:

  • Hadley cells around the equator: sun warms hot air that raises and - in the north hemisphere - moves north while cooling. As it moves north, it also moves east due to kinetic energy. Indeed, consider that stable air at the equator moves with the planet, so it has a given linear velocity. As it moves north, the linear velocity to stay in place (zero speed relative to ground) is reduced because circumference of the planet is reduced. As a consequence, airmasses moves east to dissipate their kinetic energy.
  • Polar cells, where air cools down at the pole and - still in north hemisphere - goes south (no choice, you are at the pole) with no kinetic energy, so it lags behind the rotating planet, making it moves to the west.
  • Ferrel cells, a secondary or indirect type of cell appearing between the primary ones that are the Hadley and polar cells.

Let's call Bambam the proposed planet with twice the radius of Earth as main difference, other elements remaining the same, including energy received from the sun (this one impact the quantity of air raising at the equator). On Bambam, the winds at the equator will have double linear speed so quadruple amount of energy ($E_{c}=\frac{1}{2}mV^2$). Assuming air cools down at the same speed, it will need the same linear distance, so half the radial distance or latitude. The Hadley cells will not be the double side than Earth's, but probably only slightly so due to the energy levels and friction (got lost in calculation here, but will happily welcome contributions).

In practice, Bambam will have, per hemisphere, the room for two distinct secondary Ferrel cells at the border of the primary Hadley and Polar Cells. Betwee the Ferrel cells is enough space for a third-level turbulence that could be a cell on its own, or maybe multiple smaller ones. As mention, a stable system implies an even number of them to not end up with opposite winds in the same place.

Deserts

Bambam will likely have cold deserts bordering the polar cells due to the dry cold air. At the border of the Hadley cells, desert presence will heavily depend on ocean distribution, as this changed the amount of water in the hot air. Topography is also a key factor and can on its own create deserts.

For the secondary Ferrel cells, expect even more instability and less features defined only by the cells.

To conclude, Bambam will have multiple secondary-order cells, but those are unlikely to create neat deserts latitudes on their own, topography and ocean playing a key factor here.

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  • $\begingroup$ I'm not sure (hence why I'm not writing an answer), but I thought that the Hadley cell formation had to do with planetary rotation speed. Spin fast enough, and you form a second set of the cells in each hemisphere; size isn't relevant to the problem, if I recall correctly. $\endgroup$
    – Palarran
    Oct 29, 2022 at 12:36
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    $\begingroup$ @Palarran The initial Hadley theory (north-south air circulation) did not consider rotation and the Coriolis forces (see britannica.com/science/Hadley-cell, but other sources claim the opposite). It proposed only a single big cell per hemisphere, which is not observed. Ferrel proposed a more adapted model, giving its name to cells beyond the first one. $\endgroup$
    – Uriel
    Oct 29, 2022 at 14:25
  • $\begingroup$ Well, reading the original archive.org/details/112Hadley shows that rotation is taken into account by Hadley, so not sure about Britannica statements. $\endgroup$
    – Uriel
    Oct 29, 2022 at 14:34
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Prelude

I didn't know what a Hadley cell was until now. For what the Hadley cell is, you might go to Wikipedia.

Definition

Hadley cells are atmospheric circulation patterns observed between the tropics. The air rises at the equator, moves towards the poles and then descends in the subtropics.

How Hadley Cells work

First the air rises at the solar equator, then moves towards the poles, and as at the equator, air is stationary relative to the ground below, it carries the linear speed at the equator to the subtropics, thus sliding pole+eastwards relative to the slower ground.

At the subtropics it reaches a point where it's no longer so hot and it descends. After descending, it will move along the ground either towards the poles (and forming part of the Ferrel cell) or back towards the equator. As it moves towards the equator, the ground's linear speed will increase (because at the poles you have angular speed but no linear speed, you only rotate around yourself and move 0 km every day; while at the equator you have the maximum linear speed, you move 2piEarth's_Radius (km) every day) and thus, the wind moves equator+westwards relative to the ground.

Climatic consequences of the Hadley Cell

As the air at the equator usually is full of humidity, because the Sun warms the seas, lakes and rivers, the vegetation also help keeping the air humid at night. When this humid air ascends at the equator towards higher places, it cools down very fast and also it generates a zone of low atmospheric pressure (at ground level).

When humid air cools down, the water in the air condenses a bit becoming visible as clouds. When clouds are cooled down even more you get rain. If it cools down further you get snow sometimes.

Now, when humid air cools down very fast you have your typical equatorial rains (at the equatorial climate zones).

When the hot, cooled air exits the equatorial zone, it has lost all of it's humidity, and now it will travel all the way to the subtropics, where it descends creating a zone of high atmospheric pressure (at ground level). Also, this air has lost its humidity at the equatorial rainfalls.

In school we learned (I did, at least) that when there was a high pressure zone, it meant a sunny day, and that when there was a low pressure zone it meant a rainy day.

When the air descends in the Hadley cells, you get a desertic climate, because they get constatly sunny days, with no rainy days.

The dry wind goes equatorwards again, dragging with it all the humidity it encounters towards the humid, hot and rainy equator.

Seasons and axial tilt consequences

Finally, as Earth's rotation axis is tilted 23.5 degrees, the solar equator (the parallel at which the Sun shines perpendicular to the ground) changes every day, oscilating from north to south on a yearly period. The northenmost parallel where this happens is called the Tropic of Cancer. The southernmost one is called the Tropic of Capricorn.

The axial tilt means that the solar equator is always moving, and thus the updraft air of the Hadley cell will move acordingly, bringing precious rain to places that would otherwise receive none. That is why the main subtropical climates have a dry season (when the Hadley cell's updraft isn't near their latitude) and a wet season (when the Hadley cell's updraft is just over them). The equatorial climate are a special case because they always have the updraft of the Hadley cell over them.

The deserts are areas where there's never an updraft or there's never humidity in the updraft.

Desert Listing and comprobation

I've made a list of the main (hot) deserts/desert zones in the world and the latitudes between which they are:

  • Sahara -> 12ºN - 36ºN
  • African Rift -> 4ºS - 3ºN
  • Arabian + Iranian + Thar -> 12ºN - 36ºN
  • Turkestan -> 36ºN - 53ºN
  • Takla-Makan + Gobi + Tibet -> 27ºN - 51ºN
  • Australian -> 15ºS - 35ºS
  • Atacama -> 4ºS or 8ºS - 32ºS
  • North American -> 21ºN - 39ºN

From this list, we can enumerate the reasons why they are deserts:

  • Most of them are placed between the paralells 12 to 36. These are the places more affected by the Hadley cells: Sahara, Namib, Kalahari, Arabian, Iranian, Thar, Australian, North American and part of the Atacama desert.
  • Then we have the deserts originated because they are at the center of their continent, and if there was ever any humidity in the winds, it has already rained down somewhere else before reaching there. These are: the Turkestan Desert, the Gobi, the Takla-Makan and the Tibet deserts.
  • Lastly we have the strangest kinds of desert, because they suposedly have no reason to be there, they are just next to the sea. The humidity should be there. Actually there isn't any humidity because the sea has a cold sea current that keeps the water cold, and it doesn't let it evaporate. These deserts are: the Namib and the Atacama deserts.

About cells

Why do they need to go in odd numbers per hemisphere?

The reason they go in odd numbers is because at the solar equator you will always have an updraft, and if your planet's axis is tilted less than 30 degrees, you will always have a downdraft at the poles. to maintain these two conditions you must have 1, 3 or 5 cells.

3 cells versus 5 cells

About the possibility of more than 3 cells (speculation)

I think that you could get more cells if you tilted your planet's axis to 36 degrees.

With this configuration, you will have the tropics at 36ºN and 36ºS and the polar circles (nearest parallel towards the equator where the sunlight can shine parallel to the ground) will come down to 54ºN and 54ºS.

There will be enough space for a Hadley cell between parallels 0º to 18º, an equatorial Ferrel cell between parallels 18º and 36º, an exchange Amaram cell between parallels 36º to 56º, a polar Ferrel cell between parallels 54º and 72º and a Polar cell between latitudes 72º and 90º.

On the middle of summer and winter, the polar cell may merge with the polar Ferrel cell to originate a turbulent flow similar to the one we can see on Jupiter's poles, and in the middle of autumn and spring there may be a clear differentiation between the Polar cell and the polar Ferrel cell as the hexagon shaped cloud on the pole of Saturn.

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