Rain, Rain, Don’t Go Away – Climate and Weather in World-Building, Part III

So ….

Man, it's been a month.  A long hiatus.  Now that I am somewhat stable over here in the desert, I have a little time to continue my series, do some other blogging, and in general just get back to writing, as much as I am able to.

In Part I, I talked about getting your planet set-up.  In Part II, we discussed the placement of major land masses.  So now let's talk turkey – or more precisely, water.

Water, that lifeblood of all things (or, at least the living things we know about), has a few magical weather properties.  One is the ability to exist in all three states – solid, liquid, vapor – at the same temperature and pressure .  More importantly, water has an enormous heat capacity compared to earth or air, which will presently become important.  But we'll come back to that in Part IV.  First, let's discuss how our water moves around.  I am going to go on with the assumptions we made about our fantasy world being Earth-like in character.

I linked to the Hadley Cells and how they work.  Because of near-constant heating at the Earth's equator, you almost always have heated air at the equator (scientifically known as the Inter-Tropical Convergence Zone).  When air is heated, it rises.  As air rises, it begins to cool.  Air can only hold so much water vapor at a given temperature, and it is less at colder temperatures.  So if air can hold X water at a temp, it might hold 1/2X at a lower temperature.  The excess water precipitates out, first in the form of clouds, then rain and other phenomena.  That constant rising air over the equator means a near-constant rain over the equator and tropical regions.  This is all a fancy way of explaining why all the world's rainforests are in a thin equatorial band around the planet.

Further north and south, those same Hadley Cells produce a band of high pressure in the sub-tropics, about 20-30 degrees north and south of the equator.  High pressure means the air is subsiding, or settling from above to the Earth.  This is the antithesis of rain, meaning the sub-tropics tend to be very dry – hence the bands of deserts and desert-like environments around the 20-ish N/S of the equator.

Now this is where it gets complicated, because there are exceptions.

The American Southeast, by rights, should be a desert.  It is about the same latitude as the Sahara.  So what gives?  Well, the band of high pressure sits directly over the Gulf of Mexico.  High pressure (the big blue "H" you see on a weather map) rotates clockwise.  So if you imagine a spiral pattern from that "H" over the Gulf, what do you see?  You see air laden with Gulf moisture blowing up over the land, watering it ..,. whereas over the Sahara, a big "H" spinning draws up … dry air from the south.  You can even see the difference between Georgia and Arizona.  If the high pressure is some degrees south, winds from the southwest wrap moisture into Georgia.  Winds from the southwest of Arizona bring dry air from the Mexican mountains.

These bands are not fixed.  They migrate with the seasons – southward during the northern hemisphere's winter and northward during it's summer.  This accounts for how during the summer, India enjoys the monsoon, receiving tropical levels of moisture during the summer but becoming drier under the high pressure in the winter.

So what do these things mean for your world?  Well, how are your continents placed?  You can expect a band of rain forest across the interior and deserts in the sub-tropics, unless you have some mitigating body of water as a source of moisture.  We haven't even discussed the effects of topography; imagine there was a massive mountain range along the Gulf Coast of the United States.  If there was, much of the southeast US would be very arid.  The mountains would simply squeeze the moisture out, as happens in the Cascades in the Pacific Northwest.  Coastal Washington and Oregon are quite green and lush.  The eastern parts of the state?  Not so much.  This is something to think of when placing deciduous forests and lush green farmland.  Without a steady supply of rain, especially if you are writing about a medieval-ish agrarian society (with primitive irrigation), these things don't exist.  Pine forests are a little different matter.  If you have had millennia of steady patterns of rainfall over some area, you would have the forests of eastern North America, western Europe and southern Brazil.

Remember, nothing about this is supposed to be ironclad, just to get you thinking about how you set your world up.

Next time, we'll tackle that heat capacity of water and how that affects things on a big and small scale, fronts, and some other details that shape your land.


Earth, Wind, and Surf – Climate and Weather in World-Building, Part II

Yikes, I got lost somewhere since last time.  I'm overdue for Part II, so here it is.

Previously, I discussed the sun-space parameters that set up the success for the world.  As before, it is usually just simpler to assume an earth-like world:  year-long revolution around the star, 24-hour day, 23.5 degree tilt for the seasons.

As an aside, you are correct:  this does involve a lot of assumptions.  But the world is supposed to flavor and add to your story, not be the driving factor (unless it is).  Assumptions are okay, as long you are internally consistent.  (If you think that's wrong, ask your favorite physicist if they have ever assumed a spherical cow.)

1) Start with the discussion of Hadley cells.  Hadley cells basically postulate a conveyer-belt effect of airflow in the atmosphere:  heating and rising at the equator (the spot of most consistent heating), settling around thirty degrees north and south of the equator, and rising again around sixy degrees north/south.  What this basically means is that you have continual low pressure at the equator – meaning lots of convection (i.e., rain) – and high pressure in the sub-tropics (+/- thirty degrees latitude form the equator).  More on that in a moment.  You can read more on Hadley cells here.

2) We move on to the Corilois force.  Again, you can read up on more of the physics here, but the net effect involves the rotation of the Earth resulting in a right-hand curvature of motion in the atmosphere.  This is the effect that gives us the different prevailing winds at different latitudes.  Between roughly thirty and sixty degrees (on the north side, that encompasses most of North America and Europe), the prevailing winds are westerly – meaning that weather systems move from west to east.

It's worth pausing here to revisit the first entry on the subject.  You can now see how some of those interactions might change.  Say you want your sun to rise in the west.  Spin your world in the opposite direction, the Corilois force moves in the opposite direction and BOOM, the prevailing wind will be the opposite of our world.

Okay, so now that we've done that, what next?  You need to place your major landmasses and determine the geographic features that aren't dependant on weather patterns.  In this case, I would be talking about mountain ranges and dry land versus ocean land.  In the large scale, mountains work independent of rain; plate techtonics drive the formation of mountains and either raise continents or drive them into the drink.  Barring supernatural/magic drivers or cataclysmic events (a la the movie 2012 or the mountain being thrown on the city of Istar in the Dragonlance series ), land mass formation is hyper-slow and once you place them, feel free that throughout your history, the land won't change that much.

Now, you've placed your land masses, determined prevailing winds.  What next?


In the absence of land masses, the currents would basically follow the winds.  You don't have to get hyper-anal about this, but just use real-world guidance to come up with something realistic.  (This will become important in part III.)  For example, around North America, the water tracking from the equator up to the northwest enters the warm Caribbean and Gulf of Mexico, flows up the east coast, then across the Atlantic, creating the … wait for it … Gulf Stream, the warm water that keeps western Europe somewhat warmer than other locations at that latitude.  Conversely, on the west coast, cold water flows down from northern Pacific, which is why the water off San Francisco is significantly colder than off Virginia Beach, even though they are at approximately the same latitude and receive about the same amount of solar radiation in a year.

(Caveat:  I am not an expert on current dynamics, so like I said, just looking for a common-sense answer.)

Okay, so we have the rock in space, with some parameters set.  Now we've added prevailing winds, land masses, and some water flow.  What's next?

That lifeblood known as water.  What does it do?

That's what we'll discuss next time.

3rd Rock From the Sun – Climate and Weather in World-Building, Part I

So ….

As I sit here this moment and watch the tree branches outside settle and groan under the weight of falling snow, I started thinking back on how many fantasy books I've read, and how the weather behaved in such books.  Surprisingly (or not), a lot of it was questionable.  Why is that?  Well, I 'spect most folks don't know the how/why behind how weather works and I figured – being a trained meteorologist and lacking another subject to talk about today – I'd offer some insight.

Muse:  Aren't you a precious peach?  And by 'precious,' I mean 'pretentious,' and by 'peach', I mean jackass.

Thank you, Muse, for that note of encouragement.

Just to preface:  this is just guidance, which is based on the physics and observed phenomena of our real-life environment.  It's not all-encompassing, nor is it absolute.  Tolkein's Middle-Earth, for example, existed as a plane in space before becoming a planet revolving about the sun.  As such, physics as we understand it would not apply.  In the Mistborn trilogy, there are some imposed differences on the physics of that world, which altered weather patterns.  Alien worlds can have any number of explanations and if the writer is in this situation and is having trouble coming up with plausible reasoning for why the world exists the way it does, I think they are better off employing some hand-waved version of, "A Wizard Did It," than some half-assed explanation that will be picked apart by the more discerning readers.

Okay.  I'm going to split this subject into several pieces, starting with the big planetary picture, then focusing down to more finite areas.  Hopefully, it will prove useful in some small way to any readers and I will answer questions as best I can.

All right, enough talk.  Let's make the following assumptions:

– We're dealing with a spherical planet.  If you have a flat earth, or a supernatural space, then this all may not be applicable.
– We're dealing with life-forms as we know them, based on carbon and water.  If your aliens are silica, again, this may not apply.
– There are no world-altering magical effects per above.  Straight, normal world.

If you make these assumptions, you are left with a ball of blue and green, hurling through space.  Pretty, but thus far, no answers on weather.  But back to the ball.  Now before you worry about weather, you have to answer some questions about your world.  This isn't too hard but sketch it down on a sheet of paper and keep it handy; the reference will help a lot.

1) Orbit.  How far is the world from the sun(s), and how fast is it whipping around said sun(s)?  This affects several things.  Astronomers speak of something called the "sweet spot" (In terms of orbital distance) in which liquid water can exist.  This is based on the solar output of the star(s) and the radial distance of the orbit.  But it's worth noting that if you go for the aesthetics of multiple suns, it will affect the orbit, and thus, the seasons.  Here's an an example of orbital tracks around a binary system.  Some of this can be ignored, I think, easier than the weather, as most folks are not up to the effects of binary stars on the orbits and the cascading effects.  As a note, orbital track also affects year length and seasonal cycles.

2) Tilt.  The Earth is tilted on it's axis approximately 23 degrees relative to its orbit.  This tilt is what gives us the seasons (simplified explanation here), and such effects as the varying lengths of the day over the seasons.  If your planet is tilted zero degrees relative to the orbit, every day consists of 12 hours of day and 12 hours of night (or half each of however long your day is).  A tilt of 90 degrees means the world's spin is such that it is flat relative to the orbit, meaning, among other things that over the course of the year, the sun would rise at every point on the compass, not just in the east, and that only the equator would get sunlight every day of the year.

3) Rotation.  How fast does your world spin on its axis?  Earth takes just a hair over 24 hours to complete one spin, hence the day length.  A faster or slower spin has affects wind currents, the daily heating/cooling cycle, cloud formation, and any number of other factors.  The Earth also spins counter-clockwise (as viewed from the north pole); if you flip that, your sun will rise in the west, not the east.  Also, your prevailing winds would reverse direction (which means, in bulk of the United States, big weather features would move from east to west, not vice versa).

This is all very academic and fascinating … but in the long run, I recommend sticking with an Earth-like setup for your world.  One, it's familiar.  Two, it's a lot less stress on you.  But I offer these items up for consideration because in the event you want things to be different, you need to consider these issues.  You may catch some flack ("Why does this book have the north pole in darkness in the winter?  Why does the sun always rise in the east?") but that will be minor.  Besides, unless these are major plot points in the book, the world-building is the flavor, not the meat of the story.

Next:  Part II, in which we discuss topography, prevailing winds and currents, which drive biome growth and sustainment, and why all those fancy terms are important when you want to discuss why Thrug the Barbarian is getting rained on in the desert.