This post is entirely in reference to the post, “Timewasting” by my friend Alexis over on taodnd.com concerning weather patterns on fantasy worlds. Go catch up.
“If you’re inventing your own world, where do you start? What are the prevailing winds? Are you able to identify the principal zones of convection and subduction at the various latitudes of your world, to at least make an educated guess as to where said winds should originate?”
What’s the weather like on your fantasy world and why am I attempting to answer this question? To answer the second question first I am a D&D player these last few decades and I spent a good 4 years studying physical geography, including meteorology, in University. Meteorology is, unbeknown to many, a branch of the social “sciences” along with psychology and astrology and if you’re asking for my humble opinion – it absolutely belongs there as forecasting the weather is as much science as it is magic.
So, what’s the weather like in your magical realm of ? Aside from the fact that you can do whatever you want with a magical kingdom of your own imagination (it’s just a game, it’s magical, your players won’t give a rat’s ass anyway…) That didn’t work for me when I built my fantasy kingdom simply because as a geography major, I was chiefly concerned with how man interacts with their environment. The weather would be the big mathematical X everyone is trying to find in the real sciences. The weather affects everything. The weather is king. Which is why this has become a complete post and not a comment or addendum to the original post by Alexis.
The other obvious questions are, “who cares” and “why bother”. In answer to both I will admit that I am a nerd and created my own D&D world which includes a very detailed map with consistent weather patterns and very serious thought behind oh… the placement of mountains, earthquakes, and the like. “Why bother”, because that was my fetish in the 90′s – and frankly, I hadn’t discovered my other fetish as I had no game and couldn’t get laid.
Here is a primer of key points to consider when figuring out the weather of your fantasy world. I offer Wikipedia links not because they are a great source, but because they have free pictures and are a great resource for other links.
1. The Coriolis Effect

The Coriolis Effect
I’m willing to bet you’ve never heard of this and I hope you’ll understand that my explanation is limited because of time, not interest. Ever see that Simpsons episode where Bart calls Australia and asks which way the water spins? Unless your world is flat you will have to consider that it spins. If it spins then the Coriolis Effect will affect the oceans and the winds and that’s critical to understanding how the wind blows. The effect has some major consequences to weather.
The Coriolis Effect as it pertains to winds and oceans is most notable at the equator. Notice how the clockwise rotation of the Northern Hemisphere and the counter-clockwise rotation in the Souther Hemisphere meet and for the winds towards the West? This produces the “Horse Winds”, “Doldrums”, Trade Winds and Hurricanes.
They get the name Horse Winds because sailors stuck in them used to have to eat their horses to survive. I must admit that the Coriolis Effect isn’t the chief instigator of this weather pattern, but if you’re sailing out of Italy or England and down to South Africa you’re going to hit the Doldrums.
“The doldrums, usually located between 5° north and 5° south of the equator, are also known as the Intertropical Convergence Zone or ITCZ for short. The trade winds converge in the region of the ITCZ, producing convectional storms that produce some of the world’s heaviest precipitation regions”. – Trade Winds, Horse Latitudes, and the Doldrums [About.com]
Interesting how weather, history of sailing and a simple scientific principle can change your outlook on your fantasy world. As a side note, the grammatical differences between “effect” and “affect” have lost all meaning to me now.
2. Solar Radiation

Hadley Cell Cross Section
Like a microwave, solar radiation heats things up. Equatorial regions receive the most solar radiation and therefore the most heat. I hope that you already know what happens to hot air. (Hint: It expands and rises). This rising of the hot air produces absolutely crap winds for sailing (remember the Horse Winds earlier?) Combine this with that and you get murderous sailors.
Let’s look at the Earth from profile for a minute. At the equator the air is rising, the Coriolis Effect is bending it towards the poles and as the air moves towards the poles it begins to cool. As it cools it will begin to settle or “sink”. This is called a Hadley Cell. There are 3 such cells between the equator and the North Pole with 3 more as you approach the South Pole.
Because the air rises and moves to the poles, where do you think the new air comes from to replace the air that just left? That’s right, from the poles.
3. Oceanic Currents & Wind

Oceanic Currents
People never give the currents of the ocean enough credit for influencing the weather. It’s actually the currents that move the air more than any other effect out there and it’s the currents that create some of the most spectacular weather patterns such as my favorite, the Aleutian Low (the main drivers of Chinooks here in Southern Alberta, more commonly known as foehn winds) and El Niño. For the rest of North American readers consider the currents in the Caribbean or If you’re in India you’re very familiar with the Monsoon current in the Indian Ocean.
What I want you to notice, because this is important, is how the Coriolis Effect, Solar Radiation and the oceanic currents all seem to be moving in similar directions until you hit a huge land mass like the North America, Africa or Australia.
If you are at all familiar with how hurricanes track across the equator, towards the Caribbean and then North towards the U.S. consider the currents you see in the map to the right, solar radiation and the Coriolis Effect. Is it really any mystery any more?
Now imagine you’re Columbus and you’ve just set out for a 3 hour tour… He headed South towards Africa where he planned to make a hard… West. (Have you ever read about his first voyage and wondered how the man survived?) Horse meat, but I digress.
4. Mountains

adiabatic warming creates Chinooks.
If hot air rises, what happens if you raise air? More to the point, if a wind blows up against a mountain what happens? Remember my favorite weather pattern, the Aleutian Low? It blows very dense, very wet air up against the Rocky Mountains.
“Orographic lift occurs when an air mass is forced from a low elevation to a higher elevation as it moves over rising terrain. As the air mass gains altitude it expands and cools adiabatically, which can raise the relative humidity to 100% and create clouds and, under the right conditions, precipitation.” – Wikipedia
The Pacific North West and the Aleutian Low is an excellent example of this. High levels of precipitation on the West side of the Rockies, warm dry Chinook’s on the East side. There is actually a very neat current (notice how I didn’t say “cool”?) on the West side of New Zealand. Check the map above. Can you guess what kind of weather pattern is produced there? Can you guess what kind of forest exists on the West side of the island?
5. Pull it Together

Prevailing Winds
So what does this all mean for your fantasy world? How do you take this and create ecological diverse areas based on weather patterns? How do you determine what those weather patterns are?
I would suggest that you start at the equator of your world. Work up the Coriolis Effect (see diagram above). Work up the primary Hadley Cells (do a little more research and it’ll tell you where to draw them). Figure in the oceanic currents and then draw in the prevailing winds as a construct of the above. That’s when you take the mountains into consideration. Windward get’s the rain, leeward gets lovely weather from time to time during the winter months… or a raging hot desert depending on latitude.
If you really want to get into a bit further, go and check out jet streams, high pressure systems and low pressure systems, cold fronts and my favorite, occluded fronts.
Hope that gets you started on the right track because weather is a key component of the ecological environment. You don’t get Chinook’s blowing through rain forests in the high steppes.

