Worldbuilding Guide Part 7: Atmospheres & Oceans

Links to previous parts: Part 1, Part 2, Part 3, Part 4, Part 5, and Part 6
You may have noticed that some sections in the previous part call for parts of your planet like Atmospheres, Oceans, and other such things that were not addressed. This is because of poor planning on my part, but at least part 6 isn't the length of a novel.
== Earthlike Atmospheres ==
Earth's modern atmosphere is composed of 78% Nitrogen, 21% oxygen, and 1% Argon, with trace amounts of basically everything else one could think of. However - it was not always like this. Its initial atmosphere was likely very thick and composed of hydrogen & helium. This was later blasted away and replaced with a secondary atmosphere composed of cometary material and volcanic gases (SO2, N2, NH3, H2O, CO2).
Earth then cooled, oceans formed, life evolved, and blue-green algae began producing oxygen via photosynthesis, producing Earth's current atmosphere. This is generally how the atmospheres of habitable planets form.
Earth's atmosphere is divided into five layers: the troposphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere. The temperature variances in these layers are pretty complicated and almost totally irrelevant, so we wont go over them here. Anyway, thats an atmosphere.
Because of how insanely complicated the equations for atmospheres are, I usually use this spreadsheet from Artifexian. Basically plug in the relevant stats for your planet, check which gases will remain in the atmosphere, and then plug in some percentages. Done!
Eh... no. For earthlike planets you're generally quite limited in terms of which gases are present. In fact, it is a good idea to keep things the same or close to the same as Earth. Oxygen (so animals can breathe), CO2 (in trace gas form, so plants can breathe), and Nitrogen, so amino acids and DNA can be created. Even Argon is almost guaranteed to be there in some form, as it is created from the decay of certain isotopes of Potassium common in a planetary crust. 
After filling in those four, just chalk up the remaining tiny percentage to unspecified trace gases unless you really want to drive yourself insane.
This atmosphere has a weight, and that is felt as pressure. There are lots of units one can use to measure this, but the most intuitive for most people is the atmosphere, which is equal to the sea level pressure on earth. The habitable ranges for sea level atmospheric pressures depends on the nitrogen/oxygen content, described by this graph:
For habitable planets, the partial pressure of nitrogen must not exceed 3 atmospheres, O2 must be between 0.16 and 0.5 atmospheres, CO2 must not exceed 0.02 atmospheres (although it should be below 0.005 atmospheres for no physiological stresses), and Argon, despite being an inert gas, should stay below 1.6 atmospheres. Stray outside these and you die an agonizing death. At least at sea level. You might be able to do something really cool with height, having only the highlands be habitable. I would read the hell out of that! 
For the completionist, here are some maximum partial pressures for various noble and trace gases.
*He: <80 atm (extrapolated)
*Ne: <5 atm
*Ar: <1.6 atm
*Kr: <0.46 atm
*Xe: <0.21 atm
*NH3 (Ammonia): < 0.0001 atm
*CO (Carbon Monoxide): < 0.0001 atm
*H2S (Hydrogen Sulfide): < 0.00002 atm
*CH4 (Methane): < 0.05 atm (toxic limit, flammable at lower pressures)
*NO2 (Nitrogen Dioxide): < 0.000025 atm
*O3 (Ozone): < 0.0000001 atm
*SO2 (Sulfur Dioxide) < 0.000005 atm
=== Side Effects ===
High oxygen worlds are really fun. Things will burn faster, hotter, and more easily. More oxygen also makes respiration easier, so theoretically speaking you could get very big creatures, such as what happened during the Carboniferous period, when you had millipedes 2 meters long.
=== Scale Height ===
The spreadsheet linked above computes the average atomic weight, which no one gives a flying fridge about, and the scale height, which is very important. The scale height is the distance over which the pressure and density of an atmosphere fall by a factor of 1/e (e = 2.718..., an irrational number that we use for this for some reason). For every additional scale height, the pressure falls by an additional 1/e. Essentially, the larger the scale height, the more slowly an atmosphere tapers off to nothing. The scale height can be computed like so:
H = (RT)/(mg)
R = Molar Gas Constant (kg m2 s-2K-1 mol-1)
T = Temperature (K)
m = Molar mass (kg mol-1)
g = Surface gravity (m/s2)
Something useful we can find with this is to find the atmospheric pressure at a given altitude, which we can use to make some very interesting settings. One that my friend suggested is an incredibly high-pressure world where the only habitable regions are the ultra-highlands, necessitating the development of powered flight very early on in history. Fun! 
By the way, when the pressure is almost zero, you're in outer space and you can start throwing down space things there. 
In summary, by keeping everything earthlike but slightly tweaking the percentages and pressures, we can get incredibly varied worlds! Who needs ridiculous gases?
Also add helium for abandoned advanced civilization worlds for funni
== Not Earthlike Atmospheres ==
Not earthlike atmospheres have fewer restrictions. However, it is generally a good idea to avoid having more than like 3% oxygen, as it is rapidly absorbed into the planetary surface (unless you have an ice planet). Also, avoid rare things taking up too much of a percentage of the atmosphere. 
Remember that unless the planet has a magnetic field or is under the influence of a different magnetic field, it is unlikely to have a thick atmosphere. An exception is Venus, which has such a thick atmosphere that it simply hasn't gone away yet, but there's still a large tail of gases streaming out behind it. 
=== See also ===
* Niflheim from H. Beam Piper's Uller Uprising
* Clorox from Stephen L. Gillett's World-building
== Atmospheric Circulation ==
A planet's lower atmosphere is essentially a massive circulation mechanism to transfer heat from the equator to the poles. This sounds simple, but it isn't because of the Coriolis effect.
Wam air rises at the equator, stopping at the tropopause at 12-15 km up. It then splits and moves poleward, beginning to deflect toward the east. at roughly 30º N it is essentially due east and as such can never reach the poles. It cools, sinks, and travels back along the surface towards the equator and westward. This is the hadley cell, and the prevailing winds within this are called the trade winds.
Something similar happens at 60º latitude. Relatively warm air rises to 8 km, moves poleward and eastward, sinks, and returns along the surface westward. This is the polar cell, and the prevailing winds within it are the polar easterlies because they blow from the east or something.
Between these cells is the Ferrel Cell. The prevailing winds are called the westerlies in this cell. The same structures occur in the southern hemisphere, but flipped because of the coriolis effect. 
The zones where each cells meet are named like so: The two Hadley cells meet at the Intertropical Convergence Zone at the equator, the Hadley and Ferrel cells meat at the Subtropical Ridges, and the Ferrel and Polar cells meet at the Polar Fronts. Where warm air rises, there is a low-pressure zone, and where cool air sinks there is a high-pressure zone.
To apply this to your world, simply mark the ITCZ at the equator, the Subtropical Ridges at 30º N/S, and the Polar Fronts at 60º N/S and draw in the corresponding prevailing winds. Done! 
Actually no its not. This only works for planets exactly like earth. A retrograde planet would flip the wind patterns to go the opposite direction, for example. This is neat, but what if we change the rotation rate? Generally, the slower a planet rotates, the fewer circulation cells it will have. 
Specifically: planets with 1/2, 1/4, 1/8, and 1/16 times earth's rotation rate will have will have one cell per hemisphere. Planets with 1-2X earth's rotation rate will have 3 per hemisphere, as earth does. 
Planets rotating 4x as fast will have 7 per hemisphere (equator-24º, 24-27º, 27º-31º, 31º-41º, 41º-58º, 58º-71º, and 71º-pole), and planets rotating 8x as fast will have five per hemisphere (equator-23º, 23º-30º, 30º-47º, 47º-56º, 56º-pole). No idea why planets with 4x rotation rate throw off the cool linear trend, but either way decide which is closest to your planet and mark it in as earlier.
Tidally locked planets would work like this: Powerful winds blow eastward from the subsolar point while less powerful winds would blow westward, cool winds would blow from the antisubsolar point onto the day side, and where all these winds meet there would be lots of precipitation and cloud cover. Highest temperatures would be north and south of this zone due to a lack of cloud cover. The east side will be warm and wet, while the west side will be cooler and drier. The night side surrounding the antisubsolar point will be extremely cold and dry, while the rest of the night side would be less extreme.
===DISCLAIMER===
This is obviously massively oversimplified. Real planets have pressure centers that change over time and are affected by the location of continents. If you want to achieve something this accurate, check this link. Personally I don't really care about being this accurate, and what we did above is sufficient.
===Side Effects===
As we saw with the tidally locked one, atmospheric circulation can determine wind patterns, precipitation, and biome placement, but we basically did that in part 6 because of my bad planning.
In an earthlike configuration, we can divide a planet into a hot zone, temperate zones, and cold zones. Anything which initially came from a given zone probably won't do that well in another. A plant from Peru planted in Canada will die quite rapidly, but an Italian crop planted in the US or Japan would probably be fine. Peru and Canada are in different zones, but Italy, the US, and Japan are in the same zone. 
The same thing goes for animals, so no equatorial polar bears, and people. Empires are more likely to go conquering in zones similar to their homeland. Because of this, Jared Diamond (author of Guns, Germs, and Steel) suggests that zonal landmasses are more likely to develop advanced civilizations because they provide a relatively homogenous space for people to share crops and animals and acclimatize to each other's diseases. 
These are not hard limits. Look at corn, the British Empire, or any historian. Atmospheric circulation still plays a role in the distribution of flora, fauna and civilizations. Also, where cells meet, maritime barriers are created. Sailors refer to the Intertropical Convergence Zone as "the doldrums" (no relation to the thing in the phantom tollbooth) because winds there tended to be fairly stagnant or flat out not existing at all. This meant that sailors could be stuck there for days if not weeks on end waiting for winds to change, leading to a higher chance of scurvy, delirium, starvation, cabin fever, and death.
The same is true for the subtropical ridges, or the "Horse latitudes", because it is said that sailors stuck there would throw their horses overboard to conserve water. You can come up with your own names for these though. These zones will impact how the seas are navigated on your world.
Expect tropical cyclones to form on equatorial oceans between about 5 and 20 degrees north and south, and then move with the prevailing winds. Thunderstorms occur anywhere where there is a cold air mass moving into a region of warm, moist air, like in the US. They can also occur when air moves upward in regions with an immense supply of water, like around the Intertropical Convergence Zone. They can also occur in mountainous regions.
If you number your wind cells from the equator, the cells with even numbers are where you would expect tornadoes to form, as they can be seen as a turbulent eddy that occurs as the result of the other two cells, and its winds are fickle and prone to variability. Also, tornadoes form where warm, moist air flows at low levels and cool, dry air flows up high. On earth this basically only happens in the United States, which is why that country recieves a full 75% of all tornadoes on earth.
This is going to involve some lord of the rings, so bear with me. Imagine we took Mordor, scaled up the Sea of Nurnen, and made it an estuary spilling out into an equatorial ocean. This fits all the thunderstorm criteria from earlier, and when we mix that all together we get EVER-LASTING THUNDERSTORMS! Our new Mordor can expect thunder and lightning for the vast majority of its year. Each night storms rage for about 10 hours or so, and at peak lightning would strike thirty times per minute. Try doing something like this on your world so we can all gaze in awe at it. Sounds super fantastical and awesome, right? Yeah its not, it happens on earth in Venezuela, in a funky place called Lake Maracaibo. It's amazing. 
== Ocean Currents ==
For ocean currents, we need a map that has oceans, continents, and winds. To begin, draw two currents close to the equator on either side. Split these currents equatorward and poleward. The equatorward fluow will loop back around to form an Equatorial Counter Current moving eastward. Split the flow of that at the next shelf and loop back to the first currents, and you have closed your first gyre.
The poleward currents should continue northward. At or just above the latitude where the next wind cell begins, those winds drag the currents into another loop. Once that hits the continental shelf, split the flow again, equatorward and poleward. The equatorward flow will close our tropical gyres, and the poleward flow will begin our next set. Repeat until you reach the edge of the last gyre.
If you have an open ocean at either or both poles, drop a westward flowing circumpolar current. If there are any gaps, fill them with smaller gyres or logically extend existing currents. 
Color currents flowing away from the equator red, and flowing toward the equator blue. Keep the latitudinal currents that dont have any particular poleward or equatorward flow as black. Red is (relatively) warm, blue is (relatively) cold, and black is neutral. That is, warm currents carry warm water from lower latitudes to higher latitudes, cold currents carry cold water from higher latitudes to lower latitudes, and no significant heat exchange is occurring on the neutral ones.
Check to see if all the loops are closed and the spin directions make sense - that is, if one gyre spins clockwise, those bordering it spin counterclockwise, then clockwise, then counterclockwise, like a set of gears. Keep in mind that the faster your planet spins, the more air circulation cells there are, and therefore more ocean gyres. 
If your planet is a waterworld, the currents will be banded. Think Jupiter's clouds but with water. However, if you want fancy gyres like earth has, you can have your continental shelves be only a little bit submerged as the current splits at the continental shelf, not the actual shore. 
If we tidally locked our waterworld, we'd end up with a kind of spaceship lookin ocean encased in ice, with two big gyres in either hemisphere. If continents are present, the same rules from earlier apply.
=== Effects ===
Ocean currents effect seafaring societies. Explorers will discover things that ocean currents carry them to before other things, if they sail with the currents. They could also do what the polynesians did and sail upwind, because if something went wrong on the voyage it would be easy to get back home faster. This is why the polynesians reached the likes of Hawaii and Easter Island hundreds of years before they found New Zealand.
Ocean currents also cause latitudinal variations. We dealt with this in part 6. As an example, look at Florida and the Baja California Peninsula. One is dry and hot, and the other is wet and very hot. The culprit here is ocean currents. Cold water means cold air which means less evaporation, less clouds, and less rain. Whenever cold currents flow along landmasses, cooler and drier coastal conditions are created. The reverse is true of warm currents. Warm water means more evaporation, so expect wetter, warmer coastal conditions. It is worth noting these areas on the map.
In addition, expect cool coastal waters to be very nutrient-rich. These will be the planet's fishing hotspots (if there are fish). Also, expect coral reefs to form anywhere in the tropics where warm currents flow.
Lastly, there is an interesting phenomenon called El Niño Southern Oscillation events. These are irregularly periodic variations in winds and sea temperatures that occur in the pacific ocean. It has three main phases: Neutral, La Niña, and El Niño. Both of the latter can last several months, vary in intensity, and typically occur every few years.
In the neutral phase, or normal phase, the trade winds blow across the pacific. These winds push warm water across the ocean, piling it up in the west and drawing cool water to the surface in the east via a mechanism called upwelling. The temperature differences cause air to rise in the west and sink in the east, creating a huge circulation system known as Walker Circulation. 
In the La Niña phase, everything gets turned up a lot. Trade winds blow harder, warm water piles up in the west, cool water piles up in the east, and Walker Circulation strengthens, causing trade winds to blow even harder and boom look at this feedback loop. La Niña brings storms and flooding to the west and heat waves and droughts to the east.
In the El Niño phase, everything reverses. The trade winds weaken, allowing warm water to drift back towards the east. This breaks down Walker Circulation, causing the trade winds to blow even weaker, allowing more water to gather in the east, and oh look its another feedback loop, except everything is flipped. El Niño does exactly the same thing as La Niña, except reverse the direction.
If your world has a large open ocean akin to the pacific, expect equatorial landmasses on its periphery to be subject to ENSO events. Bear in mind that directions will flip if your planet spins retrograde. Oceans - done!
== Tides ==
=== What are they? ===
Lets take the simple earth-moon system as an example. Every point on earth is gravitationally attracted to the moon. The closer the point is, the stronger the attraction. Also, because the earth-moon system orbits around its barycenter, every point would be subject to centrifugal forces as well. 
Combine these two vectors, and you get a tidal bulge as the ocean deforms along them. As Earth rotates through the bulges, it experiences tides as you probably know them - High tide, 6hrs later low tide, 6hrs later high tide, and so on. These are called Semidiurnal Tides - two high and low tides per day. 
The sun also produces a tidal bulge, just weaker. When the two bulges are pointing the same direction, you end up with a "spring tide", as the high tide becomes really high. When the two are pointing perpendicular to each other, you get a "neap tide", as all the tides are more boring. No idea why they are called this.
This sounds simple, right? Nope. If it was really this simple, then all areas on earth would experience equal semidiurnal tides. Sure, most areas do experience such tides, but there's also "mixed tides" where everything's kind of wonky, and weirdly, "diurnal tides", one high tide and one low tide per day. There are also wildly varying tidal ranges across all the coastlines.
Imagine the ocean as a big bucket, and the coastlines as the bucket's rim. As the bucket rotates through tidal bulges, we get alternating high and low tides on either side of the bucket, and because of the coriolis effect the whole system would rotate. Tides are essentially standing waves centered on Amphidromic Points, a point which is unaffected by tides. In reality earth has several of these amphidromic systems, and they interact with one another to give rise to really complicated tides.
Why is this? Imagine waves as having a speed limit. If they exceed the speed limit, they split into smaller waves, creating more amphidromic points. The shallower the ocean, amongst other factors, the lower the speed limit. Therefore, big deep oceans like the pacific will have relatively few amphidromic points, whereas shallower areas like the North Sea will have many. Also, the wider the bucket, the more extreme the tidal range can be, which is why all but the biggest lakes do not experience noticeable tides. Even the mediterranean isn't big enough!
Landforms like the Bay of Fundy are great at generating huge tides. It is shaped like a funnel and gets gradually narrower and shallower towards the shore. This amplifies the effect of the tides. In addition to this already contrived system, the natural back-and-forth sloshing of water in the bay is aligned near-perfectly to the tides, further amplifying them. The tidal range in the mediterranean is a few pathetic centimeters, but in the bay of fundy it can be 116 meters! That's like 14 small cars stacked on top of each other. That's just vertical, if you consider the slope it can look like a hell of a lot more than that.
Finally there's the Tidal Bore, a rare phenomenon that only really effects regions with large tides. The incoming tide floats anywhere that feeds the effected bay, forming a wave that flows upstream, against the current. The largest tidal bores in the world happen in the Qiantang river in China. They travel upstream at speeds of up to 40 km/h, they get up to 9 meters high, you can SURF ON THEM, and they produce a low, trainlike rumble as they approach. Tides are awesome and also weird and complicated and hard to calculate.
=== How to Calculate Them ===
Artifexian made a very nice spreadsheet here, go ahead and make a copy.
In general, to calculate the tidal forces exerted by a major moon on your planet, use this equation:
T = (2230000 • M • D)/R3
T = measure of the tidal force on the planet
M = mass of satellite (in Earth masses)
D = diameter of planet (in earth diameters)
R = semi-major axis of satellite's orbit (in earth diameters).
If you want to calculate the forces exerted by a star on your planet use this one with these inputs:
T = (0.46 • M • D)/R3
T = measure of the tidal force on the planet
M = mass of star (in solar masses)
D = diameter of planet (in earth diameters)
R = semi-major axis of planet's orbit (in AU)
The constants are caused by unit conversion. Run the numbers, take the output of the lunar equation as your standard tide - positive, high tide, negative, low tide. Sum the lunar and solar tidal equations to get spring tides. Again, positive value is high tide and negative value is low tide. Subtract the solar from the lunar tidal equation to get the neap tide. Finally, multiply each by 0.54 to convert them to meters. Remember, these are tides in the deep open ocean. Actual tides will vary massively depending on local geography.
Everything we just did is massively, massively oversimplified. To get truly accurate tides, you'd need to run Laplace's Tidal Equations, which are
uh
involved?
So we fudge a bit. No big deal. If you want you can check if anything is tidally locked with this equation.
E = (T • A)/M
With these inputs:
*E = total tidal effect
*T = mass of all tidal forces acting on the world
*A = age of the star system (in billions of years)
*M = mass of the world (in earth masses)
Round the number you get to the nearest integer. If its 50 or above, the object is tidally locked. The object will be locked to whatever exerts the most tidal force on it. However - if you notice that the moon-star equation is producing larger numbers than the moon-planet equation, you should probably recheck the numbers or move the moon in by a lot. Hilariously oversimplified tidal calculations DONE.
What about habitable moons? same equations apply, just with slightly different inputs. M is the mass of the planet in the lunar tides equation, not the moon in this case. There is a catch. The tidal force exerted by a planet on its moon will be very large. However, to actually experience tides, the moon has to rotate through the tidal bulge. The moon is probably tidally locked, so that isn't happening. Double planet systems suffer from a similar issue, so you're stuck with just stellar tides. For those who want to handwave this, there's a double planet and habitable moon tide spreadsheet linked above.
What about multiple moons? Same equations as before just run them for each major moon. Ignore the pathetic asteroids though. Each of these will produce a different tidal bulge, and when they all intersect you'll get insane tides, and generally there'll be complicated relationships between them. When all the moons align in the sky, the tides will be seriously huge. If you make that happen when the planet is closest to its star, the tides will be bigger again. If you contrive a Bay of Fundy clone, they'll be bigger again. If you make it so that a storm hits there at high tide, there won't be enough measuring sticks in the universe to measure the absurdly insane tides you've just created.
What about multiple star systems? If you have a P-type orbit, just run the solar tides equation from earlier and simply combine your stellar masses. If you have an S-type orbit, ignore the other star and let it wallow in depression over not affecting the tides enough to be noticed.
=== The Intertidal Zone ===
The intertidal zone, the region above water at low tide but below water at high tide, are super productive ecosystems. There are so many life-forms that can live here that would be fun to write about. Animals that forage on these can be hunted as well. Therefore, settlements nearby the shore will do quite well. 
However, intertidal zones can affect life on a more fundamental level. No tides, no intertidal zone. No intertidal zone, no land animals! Intertidal zones act like a halfway house for creatures moving out of the water to colonize the land. The smaller it is, the harder it would be for creatures to make the initial leap onto land. If there is none, then life may take considerably longer to do so and may never do it at all. Who would make all the measuring sticks we need then? 
== Conclusion ==
In this part, we finished some things we didn't do in the last part, and are now officially done with the physical properties of the universe. Next, we will begin writing about life in the universe in Worldbuilding Guide Part 8: Life & Evolution.
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