Good morning interweb, let's world build.
A planet's lower atmosphere is essentially a giant circulation mechanism.
Its purpose? Transfer heat from the equator to the poles. Simple idea, but it gives rise to a pretty complex system
thanks, in no small part, to the Coriolis effect. To explain, imagine
I shot a super long-range gun from the equator directly northward.
It should hit a target directly to the north, right?
But, Earth spins counterclockwise on its axis (west-to-east) and because the Earth is a sphere,
points on the equator spin faster around its axis than points at the poles
So, as the bullet leaves the gun, the greater spinning momentum is conserved,
causing the bullet to veer to the right. The opposite occurs in the southern hemisphere.
The bullet would, from its perspective, be deflected to the left. That is the Coriolis effect in a nutshell. More in the "doobly-doo..."
Anyways, warm air rises at the equator. At about 12 to 15 kilometers up, it hits the tropopause, the upper limit of the lower atmosphere.
It then splits and begins to move pole-ward with the Coriolis effect
deflecting it to the east. At a latitude of about 30 degrees,
the air is effectively moving due east, so it can never reach the poles.
It cools, sinks, and travels back along the surface towards the equator and westward,
the deflection, again, being caused by the Coriolis effect.
We call this structure "the Hadley cell," and we call the prevailing winds within the Hadley cell "the trade winds."
Something similar happens at a latitude of about 60 degrees.
Relatively warm air rises at a height of about 8 kilometers, splits, moves pole-ward and eastward, sinks, and returns along the surface westward.
This is the Polar cell, and these are the Polar Easterlies; so called because they blow from the east.
Connecting these two cells is the Ferrel cell.
The prevailing winds in this region are called "The Westerlies."
The same structures occur in the southern hemisphere,
except everything is flipped; again a consequence of the Coriolis effect.
Finally the zones where each of the cells meet are called the Intertropical Convergence Zone,
the subtropical ridges, and the Polar Fronts.
Where warm air rises, we get a low-pressure zone, and where cool air sinks, we get a high pressure zone.
So, to apply this to your fantasy world, simply mark in the Intertropical
Convergent Zone on the equator, the subtropical ridges at 30 degrees north and south, and the Polar Fronts at 60 degrees north and south and
marking the required wind patterns... atmospheric circulation... Done!
Well, sorta. This pattern only holds for planets exactly like Earth.
Like if your planet rotates in the opposite direction,
clockwise (sun rising in the West and setting in the east), the pattern would look like
this:
same deal as before, except everything is flipped, which is cool,
but we can really start to mess with the structures if we change the rotation rate.
Rule-of-thumb here is that the slower a planet rotates, the less circulation cells it will have.
To give specific examples, planets with rotation rates 1/2, 1/4, 1/8, and
1/16 that of Earth's will have one cell per hemisphere
Basically, this, if the rotation is counterclockwise, and this, if it's clockwise
Planets with one to two times Earth's rotation rate have three cells per hemisphere.
Like before: counterclockwise, clockwise
Planets rotating four times as fast as Earth will have seven.
This, or this.
Note the latitudinal values that mark where various cells begin and end.
And planets rotating eight times as fast as Earth will have five cells per hemisphere:
This, or this.
Again note the rough latitudes.
Now, I have no idea why planets with a 4x rotation rate
buck the otherwise nice linear trend, but either way, decide which rotation regime is closest to your planet
and mark it in like we did earlier.
And before you ask, tidally-locked planets would look something like this:
Day side,
Night side.
Warm air rises at the equator on the day side. From the subsolar point,
powerful winds would blow eastward while less powerful winds would blow westward
From the north and south, cool winds would blow from the poles towards the equator.
Where all these winds meet, thick bands of precipitation will occur.
The hottest temperatures will be north and south of this zone due to the lack of moderating cloud cover.
The east side will be warm and wet, while the west side will be cooler and drier.
And on the night side, the region around the anti-subsolar point will be very cold and very dry,
whilst the rest of the hemisphere will be a little less extreme.
Here it is in 3d if you're having problems visualizing it.
Now, to be clear, all these circulation regimes are oversimplified.
Planets have pressure centers change over time and are influenced by the configuration of continents.
If you want to achieve something this accurate, check out the "doobly-do."
Personally, I think roughly knowing what's going on in your world is more than sufficient. Regardless, what's the point?
Why do all this? So, as we saw with the tidally-locked examples, atmospheric circulation plays a role determining wind patterns,
precipitation, and ultimately biome placement, but for now, I'm gonna talk culture and weather.
Going back to a more Earth-like configuration, we can divide a world into a hot zone, temperate zones, and cold zones.
Anything that evolved in a given zone probably won't do so well in another.
A Peruvian crop planted in Canada will likely die,
but an Italian crop planted in Iran or Japan, say, will probably be okay.
Peru and Canada lie in different zones;
Italy, Iran, and Japan lie in the same zone.
Same goes for animals, hence no equatorial polar bears,
and people. Empires are more likely to "do empiring" in zones similar to their homeland.
For these reasons, Jared Diamond of "Guns, Germs, and Steel" infamy suggests that zonal landmasses are more likely to develop
advanced civilizations, because they provide a somewhat homogeneous space for people to share crops and animals and acclimatize to each other's diseases.
Now to be fair, these aren't hard limits; just ask corn, or the British, or any historian.
Nonetheless, atmospheric circulation can play a role in the distribution of flora and fauna and civilizations.
Also, where the cells meet, maritime barriers are created.
Sailors used to refer to the Intertropical Convergence zone as "the doldrums" because winds there tended to be fairly stagnant,
meaning that sailors could be stuck there for days, if not weeks, on end waiting for winds to change,
leading to the threat of scurvy, delirium, starvation, cabin fever, and death.
Same goes for the subtropical ridges, a.k.a. "The Horse Latitudes,"
so called because sailors stuck there would sometimes throw their horses overboard to conserve water.
These zones will impact how the seas are navigated on your world.
Similarly, the prevailing winds will also be a factor. Like back in the age of sail,
this
would not have been a great route to take. A "ye oldie" galleon will be sailing directly into the prevailing wind
Instead, getting from West Africa to South Africa meant riding the trade winds to Brazil and riding back across the Atlantic.
In terms of weather phenomena, expect tropical cyclones, think hurricanes,
to form over equatorial oceans between about 5 and 20 degrees north and south on your world,
and expect them to make landfall in the direction of the prevailing wind.
Thunderstorms can occur pretty much anywhere where you got cold air masses moving into regions of particularly warm, moist air.
The U.S. is a great example of this.
They can also occur where air moves upwards over a region with an immense supply of water, i.e.
areas around the Intertropical Convergence zone. And, they can occur in mountainous regions.
As for tornadoes, expect them to occur in the Ferrel Cell between about 30 and 50 degrees north and south
The Ferrel cell can be seen as a turbulent eddy that occurs as a result of the other two cells, and as such,
its winds are fickle and prone to variability,
perfect for getting those tornados spinning.
Added to that, tornado genisys most often occurs in
regions were warm, moist air flows at low levels and cool, dry air flows up high.
On Earth, this basically only happens in the States,
which is why the U.S. receives way, way, way more tornadoes than anywhere else.
And so, biomes aside, that is just about that.
But let me leave you with a cool weather-based bit of geofiction.
Imagine we took Mordor, scaled up the Sea of Nurnen, and made it an estuary spilling out into an equatorial ocean.
This basically hits all the thunder storm criteria from earlier.
Mountains (and lots of them)? Check.
Tropical location? Check. Lots of moisture? Check.
Cold air coming in from the mountains mingling with the warm air of the lake? Check.
Mix it all together and we get
"Mordor 2.0" can expect thunder and lightning for the vast majority of its year.
Each night, storms are raged for about ten hours or so, and at peak, lightning would strike about 30 times per minute.
Sounds badass, right? And super fantastical!
Yeah, it's not.
It happens on Earth
in Venezuela
at Lake Maracaibo.
It's amazing. Earth, sometimes you make the best fantasy planet!
I feel like this black top is gonna blend with the background a lot and I'll look like a floating white head. "The beacons are lit!"
Good morning interweb, heads-up. I just created a new Patreon tier that allows early access to the scripts,
so if that sounds like a thing you'd be interested in, head in to the "doobly-doo." Thank you so much for watching,
thanks to the patrons for supporting me in my nerdy endeavors, and in particular thanks to
and a massive thank you and welcome goes to new patreon:
whose name I probably butchered. I'm sorry. Until next time, Edgar, out!