Friday, November 11, 2016

Weather Generator, Part 1: Introduction

Illustration from "Weather in the World of Greyhawk" from The Dragon #68

In writing Book of Fiends, I decided to add some "additional" spells - many of them originally Druid spells from Eldritch Wizardry. Since some of these spells involve weather and have little meaning without some sort of campaign weather system, I started to think about designing or at least adapting a weather generation system for Seven Voyages of Zylarthen/OD&D.

The systems I found fell into three basic categories:
  1. Simple. I would put, say, the Delving Deeper system into this category. Saying it's simple is not a criticism. Indeed, I think the Delving Deeper system is the best out there. For a system generated with dice, it must be simple in order to be playable. The problem, of course, is if you think simple is, well, too simple.
  2. Complex. The Dragon 68 and The Dragon 137 put forward realistic and satisfying weather generation systems. As I understand it, the scheme in issue 68 was later incorporated into the Greyhawk Boxed Set campaign setting. The problem here is that I just can't see how these systems are really playable. Just to calculate the weather for one day in one climate zone requires many die rolls. Doing so would either slow down play or require a huge amount of boring prep work for the referee. An the thing about weather is while it might be very relevant to play, it's more often not very relevant. You don't want to take ten die rolls to discover that it's partly cloudy with a high of 72 degrees and a gentle breeze.
  3. Computer generated. There are a number of sites where you can ask for one day's weather (or in one case a week's worth of weather). Sometimes you can change the climate parameters and sometimes you can't. But I find the computer dependent snapshot approach unsatisfying. It's usually only a snapshot, and frankly, I don't want a system where you have to keep visiting a website. It's not very 1974.   
In response to these alternatives, I decided to design an Excel workbook for instantly calculating 365 days worth of weather for as many different climate zones as you want.

I'm not computer programmer but I've always liked Excel and do a fair amount of work with it in my profession.  

The idea is, once you do a small amount of prep-work - basically deciding what your climate zones are going to be and whether there will be any variation from the basic templates - you can calculate all of your world's weather at the touch of a button. You can then save it a PDF or set of PDF's and either print them out or save them on your tablet or phone.

It's complex, but the complexity is already baked into the system for you.

The worksheet incorporates ideas from those two issues of The Dragon plus some ideas and tweaks of my own.

In the next few posts, I'll be explaining how to use the workbook. You can download the BETA here.

It's a BETA because I want to put more things into it. Among other things I want it to display movement rates. (You want to keep marching cross country through a blizzard? Fine, but you move at 3 instead of 12, or whatever) But before I do that, I need to finalize my thoughts on Zylarthen/OD&D wilderness movement.


CAMPAIGN CALENDAR is where you get your results. I've already set it up with 19 pre-set climate zones that track this Zylarthen/OD&D campaign map. So you literally can use it as is.

But if you want to tailor it to your world, you can pretty quickly do so by making adjustments in the first three sheets.

The workbook makes heavy use of random functions. If you're familiar with Excel, you know that by simply changing 1 cell (typing 1 into any blank cell or whatever), every random function will recalculate. This is how you generate weather for different zones. Or you can simply recalculate a year's worth of weather if you don't like what you got. Or you can sit there like a weather nerd and waste an hour seeing all the different sorts of weather patterns that are possible for an equatorial monsoon zone, or whatever.

You can also turn off the recalculation function or, of course, save one of the results to a separate workbook (saving values not formulas) or by saving to a PDF.

The cells in red are what you might want to change. You don't want to touch the cells in black unless you want to experiment with changing the entire workbook. Most of the red cells are framed by a black "default" cell, giving you a "recommended" setting or range of settings.

There are 3 worksheets in non-caps: Calendar Data, Random and Movement. You don't want to touch these, although you can.

It's all up to you. I haven't locked anything.  However, keep in mind that some of the formulas are extremely complex. If you alter something in black, especially in the Calendar Data sheet, you might screw everything up and not know how to get it back.

I suppose if you do want to fool around with the "guts," you want to save an original copy first.

See, you can play God but playing Meta-God is problematic.

Again, the worksheet may be downloaded here. I'll be explaining how to use it over the next few posts, but you're halfway familiar with Excel, you might be able to figure it out on your own.

For now, here's the map that I based the climate zones on. It's from a post I wrote two years ago:

And here are some sample months from four of the zones.

  • # means there's lightning.
  • "x in rain" means the entire storm, yields, yes, x inches of rain.
  • Stars and crosses are rainbows. You can get anything from a normal "single" rainbow to a bridge to Valhalla. But to get that rare bridge you might have sit there, generating new zones for a while.

1. A stormy August in the Great Coastal City:

2. It doesn't rain much in the desert, but sometimes lightning strikes from a clear sky. Also, the temperatures drop precipitously at night:

3. The rainy season in the Great Jungle:

4. Winter in the Tundra:

I hope you find the Weather Generator interesting and useful. And I look forward to improving it. As always, feedback is welcome and appreciated.



  1. This is freaking awesome...and as I am running a Kingmaker Campaign currently it should come in oh so handy. I just need to Figure out how to change the calendar to Kulthea and the 5 70 day months...sigh.

  2. wow...This...Is...Awesome!

    And when I tweak it for Kulthea (The Shadow World) and it's 5 70 day months. It will be even more useful.

  3. Thanks!

    Just thinking aloud about it, I assume you could just change the labels on the days. So if it's Day 71, the labeled date goes from 3/9 to 2/1 or whatever. the players need not know that the underlying mechanic is based on a 12 month cycle.

    The other thing you could do would be to insert 40 extra days onto the end of each of the first 5 months and then drag/paste all the rows down from 30/31 to 70 for those months. Then in Climate Templates you could fill in the info for 5 of the months and just ignore the others.

    What do you think? Does that make sense?