Weather Generator, Part 2: Weather Parameters

Illustration from "Weathering the Storms" in The Dragon 137

I made some general comments in Part 1, yesterday.

If you haven't downloaded the Weather Generator Excel Workbook and would like to, you can find it here.

Okay, let's look at the first tab on the left, WEATHER PARAMETERS.

The red values can be changed. The black values aren't locked, but you don't want to change them. Or, rather, in most cases nothing would really happen if you did - they're just default reminders.

However, for the reds, in every case, you don't need to change anything, but may simply leave the red values untouched. I would actually recommend this.

I. Choose Game System

This is self-explanatory. For the moment it has no effect. It's in there for when I build movement rates for different game systems into the Calendar.

II. Choose Hex Size and Base Move (if applicable)

Ditto.

III. Choose Precipitation Event Name

I'm not sure why anyone would want to change these, but if you want to change "HEAVY BLIZZARD" into "STRONG BLIZZARD" or whatever, you can.

Now that I think of it, I suppose one reason might be to change the names into those of another language.

IV. Choose Relative Chance of Events

Two things to note:

1. These are the general defaults. You can (and almost certainly will want to) change them within particular climate climate zones. But if for some reason you think the default for DRIZZLE should always be twice as likely as LIGHT RAINSTORM, go ahead and increase the DRIZZLE % or decrease the LIGHT RAINSTORM %.  Again, they don't have to add up to 100%.

2. These apply only within possible temperature ranges. For example, if the temperature is -10 degrees, whatever value you have put in for DOWNPOUR, it's not going to happen.

V. Minimum and Maximum Temperatures for Events

You can't change these (if you do edit the black, nothing will happen). This section is there in part to illustrate how the algorithm works.

VI. Choose Duration of Events

These cannot be changed within the particular climate zones. So if you want to change them, you're only chance is now. The durations are intended to be "realistic," but I might have been wrong about them, and in the end it's your call. If you want sleet storms to last an average of two weeks in your world, go for it.

VII. Choose Chance of Events Continuing

This is similar to, but not quite the same as VI. Sleet storms last 1d6 hours but then have a 20% chance of continuing for another 1d6 hours (and then have a 20% chance of continuing for another 1d6 hours...). You can change that.

VIII. Choose Wind Speed of Events

Here also, this is your only chance, as these are not adjustable within the climate zones. If you think I've overestimated the power of Gales, change it.

IX. Choose Precipitation Amount (per hour)

You can change the average precipitation amounts and/or the average variance of them. The variance mechanic adds or subtracts a percentage up to 100%. So "+/-90%" means the inches of rain per hour could be anything from 10% to 190% of the average. 

X. Choose Chance of Lightning

The Dragon 68 article has a mechanic where whenever there is lightning, you have a small chance of being hit by it, and if you are, you take some ridiculous amount of damage. Though the chance is small, given enough chances, you will be hit.

In my view, that mechanic is insane. It implies that any low-level party that attempts a long-term wilderness trek will almost certainly be electrocuted.

I would dispense with that mechanic.

For me, the main purpose of lightning is to decide when spell casters can cast a Call Lightning spell. It's an extremely powerful spell, especially for its level, so you want lightning to be somewhat rare, but not so rare such that the spell will be meaningless.

Also, lightning adds atmosphere.

You can also change the lightning frequencies within particular climate zones. This only changes the recommended default. It actually makes perfect sense that some zones - those nearer to the equator, "evil" zones and so on - would have much more lightning than others.

Keep in mind that the chances are per 12 hour period. So, for example, if there's a 25% chance of lightning during each half-day of TORRENTIAL RAINS, it's highly likely you'll get a number of lightning episodes in, say, a two-week storm. 

XI. Choose Chance of Rainbow

Okay, it sounds silly, but rainbows can be fun (I know, I know). Single, or especially double or triple rainbows might be omens. In extreme cases, you can actually see storm gods in he sky. Each precipitation event has a separate chance of ending in some type of rainbow. Change these if you want. 

XII. Choose Chances for Type of Rainbow

If you want every rainbow to be a literal physical bridge to Valhalla (a Bifrost bridge), go for it.

XIII. Choose names and Chances for Cloud Cover

All the sources I've seen have percentages that track the given defaults. But if you think, say, that Very Cloudy days are too common, change it. You should know, however, that I have modified these chances in my algorithm for each climate template. For example, in areas of low precipitation, there's more than a 25% chance of it being Clear. But if you want to change the chances, generally, or if you want to change the name "Very Cloudy" to "Overcast" or whatever, go for it. 

XIV. Year begins on which Phase of the Moon?

Moon phases can be important. Or not. It's obviously up to you. But if you want to change the phase your year begins on, you can. Each phase will last 30.44 days, so the phases will gradually diverge from the months.

This assumes one "earth-type" moon with the same orbit period as our moon. Unfortunately, the workbook isn't set up to change that. I might try to do so in the future.

By the way, a friend reminded me that no version of Excel includes the "Moon Phases" font. But it's easy to download and add it. (It takes about a minute.) You can find the font here.  

XV. Choose Other Modifiers

Most of these you can also change within particular climate zones, and some will also be affected by terrain, but you can also change them generally here.

Base wind variance/maximum: If there is no precipitation event, the default wind speed will vary from 0 to 15 mph. You can change that. Increasing or decreasing it will also increase or decrease the variance for when there is a precipitation events.

Overall wind speed modifier: If you simply want your entire world to have a windier (or less windy) default, go for it.

Overall temperature modifier: Ditto for temperature.

Overall precipitation modifier: Ditto for precipitation.

Rainbow chance multiple: Change those general rainbow chances, you rainbow unicorn. Or not.

Precipitation duration multiple: Maybe you think every rainstorm, snowstorm and hailstorm should last longer.

Temperature consistency: This is an interesting one. The algorithm assumes that temperatures will vary within a range (given by the climate zone and season) from day to day, but the temperature consistency percentage represents the chance per day that it won't vary by that much (it will still vary by a little bit). Thus, you get longer lasting "heat waves" or "cold waves." You can adjust this.

Cloud cover consistency: same idea.

By the way, I'm not a weather nerd. I did a fair amount of research for this but some questions I still don't really know the answer to. For example, do clear days cluster?

Temperature drop variances: In almost every climate and in every season, the temperature drops in the evening. You can modify the possibilities here. It's probably more important to modify which possibility you get in a particular climate zone. But you can change the general parameters here if you like.

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That's all for now. Tomorrow, Climate Templates!