Being in the Main the Mouth of Olde House Rules

Wednesday, September 20, 2017

Weather Tables From a Real Meteorologist (and a Simpler Alternative He Also Uses)...

Ok, so the stormy specter of weather arises from time to time in all of our games, and the more immersive the setting, the more likely we are to incorporate it.  Like the passage of time, weather is one of those things we all seem to agree matters but don't always get around to doing.  Well as it happens, yours truly is a retired meteorologist and happy to tackle it, so let's start by setting a few ground rules.  Just to make the whole thing easier.  If you're one of those daring souls who imagines a flat earth (well, as long as we're doing the whole magic thing), I suppose scientific accuracy doesn't really apply and, to be honest, it shouldn't have to...


But if you're a stickler or, like most of us (I'm guessing), then your world is spherical.  This stuff matters, but I'm guessing no one wants MET 101 and more importantly, I'm not much interested in rehashing it.  So here's a link if that's your thing.  No, I'll just get to the good parts and explain what probably should be happening; and this is the case even if you follow my link...


Oh, and let's make another little assumption.  Let's assume the campaign is set in the mid latitudes of our hypothetical sphere.  Good.  Now things get easier.  Basically, most weather most of the year is the result of migratory pressure systems.  Lows with their attendant frontal systems and highs with their characteristic stability.  As the low approaches, expect southerly winds with increasing (and lowering) clouds and precipitation (rain in late spring or summer and snow in winter and/or early spring).  Eventually, the front passes, the winds shift, and it gets both cooler and probably drier depending on the air masses involved.  Often, the winds pick up and become stronger with very cold air.  Rinse, lather, and repeat as necessary...

Or for greater simplicity, just consult the following weather table:

Clear/partly cloudy
Partly/mostly cloudy
Clear/partly cloudy

*Precipitation (rain/snow) varies by season

Now, here's a cool trick: Roll 1d10 to establish the starting day, assign the applicable weather (adjusted for season) and follow the progression down the chart, returning to the top and continuing.  There, (most) weather made easy!  It might be the only weather table you'll ever need...

The above assumes a retreating high pressure system replaced by an approaching low and frontal system to trigger clouds and precipitation.  Of course, the latter is seasonal except in the south, where applicable.  There's not a lot of snow in Florida for a reason.  Ditto for your world's equivalent (the bona fide tropics work differently).  And as a caveat, during the summer months, the jet stream moves north and a semi-permanent subtropical high dominates more of the mid-latitudes, producing a stabler pattern which makes the above less pronounced.  You know, summer for most of us.


Now, if you didn't follow the link (shame on you), just know that air flows clockwise around a high, counterclockwise around a low, and reversed in the southern hemisphere!  Lows suck.  All of their air spirals inward (surface convergence) and up, so you get instability, clouds, and precipitation.  Highs blow, meaning air sinks and spreads out (surface divergence).  Cold air masses are always more dense and the winds stronger.  Seriously, highs blow.  But there are exceptions to this.  And pressure changes across these systems.  Sometimes quickly, sometimes not so much.  This is referred to as the pressure gradient.  The stronger the gradient, the higher the winds - and with more intense the weather; well, in most cases...

But sometimes, the gradient is non-existent, especially under a high, and the winds are calm.  Here, moist conditions in the fall or winter can result in fog that only dissipates when the winds pick up or things heat enough to break the inversion (don't ask).  Just compare to your own experience.  


Good question.  What about it?  First off, let's assume a 1 in 1d6 chance of a severe weather event on days when precipitation is called for.  If something bad is indicated, just assign a seasonal hazard.  Thunderstorms can spawn hail, high winds, and tornadoes, while winter is prone to blizzards and ice storms, etc.  Pick what you like and make 'em hate you...   


Now, the equator and subtropics are a little different.  Here, you see a steady pattern (no fronts) influenced mainly by terrain effects and/or migratory cyclones.  Typically, afternoon thunderstorms and morning to midday stability with lots of humidity.  Tropical storms and hurricanes are pretty much a bogey you can throw in to stir things up.  And you can add whatever rules you want (or have handy) for environmental exposure and the like. 

And once again, here's an easy little chart to break it down:

Late night/early morning
Clear/partly cloudy
Clear/partly cloudy
Partly cloudy
Early afternoon
Partly cloudy
Early evening/overnight
Partly/mostly cloudy

*Light/steady winds (any direction); variable/strong with thunder

As indicated above, wind direction varies but is usually light outside of thunderstorms.  Often, terrain helps channel air, causing winds to flow along natural features, whether valleys or any adjacent mountains, etc.


Of course, weather magic is doubtless an element, and this defies all of the above.  You don't need anything here to know that a storm out of clear skies or snow in the summer (or tropics) are signs of something amiss.  You piss of the local shaman at your peril, so don't do that.  Really. 

There, that's not too much, and we offer it mainly as a quick-reference guide for GMs and as a guard against "daily weather tables" that might produce unrealistic progressions.  Of course, such charts are great for establishing starting weather, after which the first table applies.  But for time-strapped (or ingenious, because despite being a meteorologist, I do it this way), you can simply begin your campaign at the "real world" time of year and at a similar latitude.  Then just follow the actual pattern.  This saves a lot of time and let's your players worry about the weather in two worlds!  All fantasy mirrors real life, so give 'em hell if you've got it, and go where the wild winds blow... 


  1. This is brilliant and I like that it's based on SCIENCE! I will be adjusting my weather charts shortly.

    One thing to ask - sometimes, some places get socked in with good weather or bad weather for a long time - like a few days of rain, or 3 weeks of sunshine/very little precip.

    Do you find that different areas of the world are prone to that sort of thing? (I'm thinking Seattle/Great Britain for the rain) If so, I'm thinking I should model it with some sort of dice roll to progress from precip to non-precip, or vice versa, so that there might be three or four days of rain.

    1. Thanks! Yes, sometimes the jet stream, which guides the aforementioned pressure centers gets "kinked up", causing systems to become stationary (or nearly so). This is especially the case with high pressure, which can result in heat waves and drought over the affected area...

    2. Western coasts are moderated by the ocean, with generally milder temperatures. And without any mountainous terrain (as with California), lows off the Pacific can introduce rain year-round in Seattle...

      As for England, this is a large island surrounded by water (lots of moisture) and influenced by the Gulf Stream current, which carries warm water from the southern Atlantic northward. This also accounts for the fact that Britain is warmer than its latitude might otherwise suggest. So, lots going on here...

    3. Hey, thanks for that! So... what would you be looking for in trade, if I wanted you to look at my map and give me a rough idea of what kind of weather I can expect in the various areas of my continent? It's a landmass the size of Russia...

    4. I sent you a message on g+. Glad to help!

    5. Haven't seen it! You can email me at chgowiz at gmail, if you prefer. Thanks!

  2. 1-in-4 chance each day that it progresses?

    1. Sure, you could do that on a d4. In fact, this might indicate a blocking high or an occluded (and decaying) low depending in what you get. Sometimes the pattern moves faster than that, so I built some leeway in the table...

  3. This is pretty similar to my own efforts. I always find myself wanting to come up with weighted climate tables for every region... but in practice it's not really necessary.

    Anyway, good stuff. Give 'em hail.

  4. thank you for this your broadcast provided bright clear concept..


  5. Hey! I know I am late, but is there a short tool like this for temperatures too?

    1. Just some rules-of-thumb. On clear periods (around the clock) with light winds to no winds, there's typically a 20-30 degree spread between the daily high and low temperature. If cloudy around the clock, temps vary 10-15 degrees. Clear nights get colder, cloudy ones stay warmer. Strong southerly winds bring warmer temperatures, strong northerly winds bring colder air. Hope this helps...

  6. Also late, but I have a deeper question.

    During worldbuilding, sometimes we run into fantastical geographies. Right now I'm envisioning a sort of "lost valley" very similar to Marvel's Savage Land. A tropical jungle in the heart of what is essentially Antarctica. Any thoughts on what kind of weather & climate this would produce inside the valley?