Where does the wind blow?

Want to figure out wind, weather, and ocean current patterns for an arbitrary world?

I'm not a metrologist, nor do I even play one on TV. I do have an insatiable appetite for odd topics, especially geographically-related ones. The level of expertise I try for is enough to make a plausible fictional climate. And remember while I present this, that I'm mostly talking climate, not weather. Weather systems will override these average winds often. This tutorial should help you figure out believable winds across your own world map. Since he was asking about how to ensure a certain area really *would* be desert, I've started with Slipguard's planet Megalaos.

The general surface winds across an idealised globe, whose axis is vertical, and whose surface is uniform, would be something like this. (Non-idealized Megalaos used as a background).

These surface winds are paired with opposite circulation aloft, like so:

But our planet is tilted. Even our supposed planet-sized cue ball would only have winds like that on the fall and spring equinoxes. Those cells of circulation are driven by both rotation and sunshine. So in the northern hemisphere's summer, the pattern shifts north, until at northern midsummer it would look like so:

And in the northern winter the 'solar equator' is down at the Tropic of Capricorn. Here that's 23.5 degrees south latitude, since earth's axial tilt is 23.5 degrees. That's why I asked what Megalaos' tilt is. The more pronounced the tilt, the bigger the swing would be from summer to winter and back. Less tilt, somewhat more placid weather and climate both. Hmm - or maybe the term should be *even*, not necessarily calm. Midwinter like so:

I'm assuming Megalaos' tilt is about like Earth's, for the sake of the argument. Ahhh, but neither Megalaos nor Earth have a uniform surface. Ocean and land soak up vastly different amounts of sunshine. Then there's ice.....

Our understanding of weather and climate is based thus far on ONE planet, so it's fair to figure for vastly differing surface conditions, the circulation would be different. I don't know how things would work on an ice world or a dried-up desert planet.

At any rate, what seems to happen is that in a given hemisphere's winter, there's a general high pressure concentrated over big land masses, at least those around the default high pressure peak of 30 degrees, shifted to follow this 'solar equator'. Smaller land masses, not so strong. In summer, things shift and the strongest highs wind up over ocean. In either case, if there's not much land under the belt of high pressure - what I labelled 'horse latitudes', then there are high pressure blobs over ocean. The pressure differences 'clump', so instead of a band of high, you get a string of ovoid regions of high. If you have a really big landmass the effect might be more pronounced - Earth's Eurasia, for instance. Megalaos' Boris continent isn't nearly as big as Eurasia.

High pressure, low pressure - who cares? Well, the pressure is relatively high or low based on what the air is doing. Look at the black-background globe above. See the air rising at the equator? Particularly if it's passed over ocean lately, that air is fairly wet, and rising, it cools. Cooling, it can hold less moisture, and it dumps a lot of it. There's a strong reason that equatorial areas tend to heavy rain. Coming back down, that's fairly dry air, hence the deserts on Earth that are around 30 degrees north and south. The other generally high pressure zone, with descending drier air, is at the poles. Antarctica may have kilometers of ice, but it's technically a desert.

OK, so straying even further into fantasy :-) ... in that I'm guessing even more, but it's all still plausible... we could figure Megalaos' seasonal highs and lows like so - winter first:

Did I mention I stretched your map to show 1 1/3 diameters? Makes it easier for me to visualize this stuff if I'm not having to mentally wrap vectors left edge to right edge. I'm also assuming the north and south edges of your world map don't run all the way to the poles.

Since I don't want to presume to repost someone else's map, here's Paidixira and Megalaos. Well, except for the fact I am using a simplified Megalaos map as the base for the first part of this tut :-).... I have been referring to the continent map further down that thread for climate preferences - the one where The Cusp is mostly desert.

Now, given that these highs and lows space themselves out apart from one another, there's multiple arrangements that would be about as plausible. Knowing that you want the midwest of Paidixira to be dry, we'll say that's where the main continental high lurks. I'll go further in a minute, but like you can guess from the way the wind bands slant, northern hemisphere highs have an outflow that spirals clockwise. So a high over western Paidixira leaves that dry-ish air curving down across the SE of the continent too - I see you have a desert across The Cusp. We try to accomodate :-). Air crossing land doesn't add much moisture, so there's not a lot of rain there in the SE. Those crisp Times Roman highs and lows are in reality very blobby, and shift around. Maybe some of the winter half of the year there's two weak highs, one on either end of the continent.

In the (northern) summer, you might get an arrangement like so:

The N hemisphere lows are counter-clockwise spiraling inflow. I can plausibly imagine a couple of weak lows bracketing Paidixira like shown, or one larger one more centered on it. In either case, the combination of clockwise outflow on a high and counterclockwise inflow on a low still conspire to drag wet air across those mountains to dry it out before it gets to the desired deserts. I'm thinking the pair get the SE dryer, so your depicted The Cusp desert is sensible.

I'm sitting here twisting imaginary airmasses with my hand above the screen. Son #3 is asking if I'm drawing doorknobs :-). It's time to do some more realistic wind than those idealized belts. Southern hemisphere highs and lows keep the same in vs out, only the spiraling is opposite-handed. Crossing the equator you can apparently get a shifting of 'handedness' of the curve to the winds, thanks to the coriolis force. Another rule of thumb I haven't mentioned is that equatorial belt labeled as the doldrums on the ideal globe is more of a trough than a string of pits, if you want to visualize pressure as height ("low", "high" - works for me). And the effective line of that 'solar equator' is called the intertropical convergence zone, and bends farther away from the real equator over land than it does over water. The 60-degree(-ish) low area can be a trough too, especially in a hemisphere's winter. Honest, I got all that stuff from climate and metrology books & sites... any wrongness is me, any rightness is luck, or somebody else's research.

Make some arrows. Hmm - The bigger ones will do for a crude set of vectors. I used smaller ones for a lot of detail on the Aurora Wind Maps (see far below), and it drove me nuts. You guys who can drop in a curve with arrowhead with a couple of clicks, please do so and make prettier pictures than this.

Done my clunky way, dupe a bunch of those - enough for each high and low. Or get one low about right and dupe THAT for the other lows.

By the way - I dunno if there's a convention for a software-agnostic tutorial. I'll note each time I have created a bunch of something I go back and collect all of those on a single layer.

I do the lows first, since the arrowheads are at known locations (next to the symbol):

Now - inflow to the lows has to come from one high or another. Just start filling in the map with flow, remembering highs source clockwise (in the northern hemisphere. Whoops - that thought reminds me: I have already loused up the southern Lows - they spiral the other way. Siiiigh. I illustrate this to make the point it's easy to get carried away and do all alike... :

At least fixing handedness is a simple horizontal flip.