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I'm trying to design a map for my new planet, but I'm realising I know very little of how planets come to be the way they are, especially when it comes to currents and climates.

To make things simple for everyone, let's take Earth as an example.

The new little planet Earth has formed, the bombardment phase is over, the crust has formed, water has been delivered to the planet's surface, The continents get reshaped by tectonic activities, and there is an atmosphere.

What I'm interested in is the following: what is it that makes winds and waters currents exist, and why are they the way they are?

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    $\begingroup$ Water currents are mostly driven by winds. Wind is generated by the difference in solar illumination on different parts of the planet. Hot air rises, cold air rushes from elsewhere to take its place. As the hot air rises up, it becomes cooler and will eventually come down again. See Hadley cells for the big picture. You may prefer to ask this question on Earth Science Stack Exchange. $\endgroup$
    – AlexP
    Jul 26, 2022 at 19:06
  • $\begingroup$ Not a direct answer, but this video and others on the same channel may be helpful. $\endgroup$ Jul 27, 2022 at 7:11

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That's a simple question with one of the most complicated answers on this Stack

We receive climate questions regularly and no wonder. Climate is complex. Unfortunately, most people aren't willing to simply define their climate and move on. They want it to be "realistic." I'm not a fan of "realistic." Too many really good stories get bogged down on trying to be godlike in the creation of the worlds. But that's another issue for another question. What I'm about to do is a horrific over-simplification that makes angels weep, demons laugh, and most dead scientists become zombies seeking my life in the name of scientific jihad. Fair warning. Reading this might give you cancer and put your name on the zombies' list. But remember, climate is complex.

1. Imagine if you will a ball, covered in water, and a hair dryer.

This first step sets up the basics. The hair dryer heats the ocean water on its side of the non-rotating, non-orbiting ball. As the water heats, its expands. That pushes water to the cooler backside of the ball where pressure increases. Eventually water starts forcing its way back toward the hot side of the ball. Chaos theory abounds, so you end up with basic currents across the "terminator" of the ball. Hot-to-cold-to-hot and back again.

Dry your tears, angels of doom! Staunch your laughter, demons of delight! Care not that the hair dryer would eventually evaporate all the water and that a real sun would burn such a world to a crisp! Sit back and enjoy the orgy of scientific debauchery! Avanti!

2. OK, now let's start the ball rotating.

As our water-covered ball (with water that magically can't evaporate, dontchaknow) begins rotating we start seeing some wonderful things. First, the currents get longer and start following latitudinal lines because the hair dryer's effect is lower at the poles than it is at the equator. That will cause currents of different lengths at different latitudes and start some longitudinal mixing. Second, the rotation of the ball will start dragging the water with it, so there are now two forces at play: heat from the hair dryer and the drag of the ball. Thus, you'll begin to see some smaller counter-currents form as as the two forces get in the way of one another. If you think about it, this is kinda a little bit how the Great Eye of Jupiter works (Get... off... my... foot... you... stinking...)

3. Now let's think about the atmosphere.

A bit... just a bit... let's imagine the water can evaporate into an atmosphere, that may or may not have existed in steps #1 and #2. As the world turns (hah.) it's going to exert a drag on both the water (already mentioned) and the atmosphere. This is mostly/kinda/at-least-part-of where Earth gets its Prevailing Westerlies. (Back angels and demons! Back!) But the funny thing is, atmosphere and water retain heat at different rates. Meaning you're not only going to get storms, you're going to get cold water dumping into hot water (you know... rain...) causing even more chaos. This is the primary problem with figuring out climate in your world (have you seen the resources in our List of Worldbuilding Resources? You should...). There's a LOT of chaos. It has order and purpose when you look at it closely enough, but the more pieces of the puzzle you put together, the more that chaos looks incredibly random. (Bobby, would you and your girlfriend go out on a double date with me and that demon over there?)

4. Finally, let's add some mountains, valleys, rivers, lakes, deserts, jungles...

This last step is where all the pain comes from. The ultra-simplification of the first three steps has hopefully made some sense as to the basics of currents and wind. But once you add landmasses, everything gets really ugly. Every flora type has an effect on weather. Every shift in land altitude, every cliff, every other body of water... it's unbelievable how complex this gets.

But (Have you met my girlfriend, Beelzebub? What do you mean you want to go out with Al Gore? He's not even a zombie! You didn't know that?), you can look at this through simplified eyes. When those currents in the first three steps hit a landmass, they turn. It's not like there's anything else they can do. It'll happily move in the new direction (an object in motion...) until something (landmass, change of temperature, another current...) gets in its way and it turns again. Add islands and the little bounders start turning all over the place I SAID BACK ANGELS AND DEMONS OF THE ABYSS!

Wind, of course, can go over the top of most landmasses... kinda. Once you add that third dimension things get really ugly. Let's simplify it this way. Wind can get up and over a shallow-sloped but really tall mountain. But will have a much harder time getting over the top of a sharp-sloped really tall mountain. Mountain ranges tend to channel low-altitude wind.

Oh, yeah, wind has layers. Those layers have a lot to do with how thin the atmosphere is and how well the atmosphere can hold moisture. Oh, and wind affects currents and currents affect wind and the sun affects them both (the hairdryer) and the moon gets in the way, too. And if you haven't noticed, we're now surrounded by tens of thousands of weeping angels and laughing demons. And they look hungry.

Did you actually answer my question

Probably not. I didn't even mention orbits, axial tilts, polar caps, glaciers, volcanos, and quite literally a thousand other variables (did you see a zombie behind the couch? DON'T JOKE ABOUT THINGS LIKE THAT!) But with a little luck I gave you some insight into where to start thinking this very, very, very complicated problem through (did I mention the List of Worldbuilding Resources? I did? OK!) and gave you an idea about how you need to simplify it to avoid being drawn into a Cthulhu-sized orgy of madness. I've seen worldbuilders become absolutely vapor-locked over the details of climate all so they can figure out if their raiding party is passing through a believable rainstorm.

The angels and demons would be much more passive if you just said it was raining and let the details go. But, it's up to you whether or not you want to start giving the little honkers names. I'm just sayin'.

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    $\begingroup$ For some reason I half expected this to end: THE PONY HE COMES :-) $\endgroup$
    – mcalex
    Jul 27, 2022 at 8:06
  • $\begingroup$ Don't forget that air currents from the drag at the equator move faster than similar currents further north and south. Since they run at different speeds, they then interfere with each other creating even more chaos. Ditto for surface air currents that rise due to warming - they run into slower moving air masses higher up. $\endgroup$
    – user53931
    Jul 28, 2022 at 12:07
  • $\begingroup$ @CTeegarden "...quite literally a thousand other variables..." I might have been optimistic in my use of "thousand." $\endgroup$
    – JBH
    Jul 28, 2022 at 15:18
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Winds and water currents exist because there is a temperature imbalance between the equator and poles. If Earth did not rotate these would both flow basically due north (within the limits of geography), that is warm water currents on the surface and cold water in the depths flowing back south. But because the Earth does rotate the Coriolis force causes them to flow in great cyclic paths instead.

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    $\begingroup$ This could be expanded a bit more: Hadley cells, water and wind flowing around certain features, links out to colleges with degrees in meteorology... $\endgroup$
    – PipperChip
    Jul 26, 2022 at 19:19
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The Suns job is to heat the earth. The heat tries to distribute itself evenly across the whole planet.

Heat moves from high density to low density. From warm to cool.

Factors getting in the way or total even heat distribution:

  • Earths is a sphere and rotating.
  • varying density of air, water, soil.
  • reflectivity (absorption of heat) of the different substances that earth is made of ie: Albedo
  • geography of the terrain, whether on land or subsurface.
  • the interactions between the atmosphere lithosphere and hydrosphere

There is more, like volcanism, but these are the major ones. All of these contribute to and interact with one another. In literally incomprehensible ways. Thus we spend millions to predict the temperature for tomorrow within a few degrees. Uncountable hours of computer processor time at relatively feeble attempts to predict future climate.

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Fluid movement in a gravitational field happens because of differences in density. Differences in density are caused either by difference in materials (e.g. water and air, water goes down, air goes up) or difference in temperature when the material is the same (e.g. hot water goes up and cold water goes down).

What causes difference in temperature can be differential heating/cooling caused by insolation or exposure to other heat sources like magma.

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Hadley Cells for beginners

Until you do get the full story, here's the short version. Warm air rises on the equator and flows north until it finds a spot where the updrafts don't put up too much resistance, then drops it. Those places tend to be over bodies of water around 30 degrees north or south. This makes those areas warmer and wetter than their latitude would normally suggest. On our planet, it's the Mediterranean Sea, the Black Sea, and off the cost of Washington/Oregon.

From there, winds will blow inland and head east in the mid latitudes. Around the equator, winds tend to blow west.

Note that this is the inverse of how tropical storms spin. For non-global air masses, the Coriolis effect will dominate air movement, making it spin counterclockwise in the northern hemisphere and counterclockwise in the southern.

You'll also get the inverse happening in the temperate regions. Hadley Cell air also pulls air down from the northern regions, but it tends to drop in different areas, like around the Great Lakes. Once again, you get a lot of wind blowing away from those areas, but cold wind. These are called Ferrel Cells, but they're not as significant as Hadley cells.

But what does this mean for water?

Air blows water around. (shocker!) I'm sure you've heard of El Niño and its sister, La Niña. This is caused by the westerly equatorial winds blowing the waves westward, which causes the water to bunch up against Asia and Micronesia. Every two to seven years, the water sloshes back, causing a very wet time all over the western coast of the Americas.

You now know more about how planetary air movements than the vast majority of readers. Most of the rest of it involves random probabilities and specific examples that may or may not apply to a different geography.

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