What would the weather be like after the Apocalypse? Or in a Hell-like setting? Using Markov chains to make fantastical RPG weather feel more realistic (Markov Chain Weather Part 2)

Assuming the Post-Apocalypse looks like a Mad Max film or a Fallout video game then we’d expect the weather to acquire an attitude - oppressive, arid heat punctuated with hazardous, radiation storms. In my second roleplaying game weather post I’ll expand on using a simple Markov chain to generate Post-Apocalyptic weather that nonetheless feels like it has “realistic” variations. As a bonus, I’ll describe an additional Markov Chain for a Hell-like setting (for example, the D&D 5e Descent into Avernus campaign).

 Albert Goodwin: Apocalypse (1903)
In a recent post I described a random RPG weather generator using a simple Markov Chain method for a fantasy Southern England setting. The idea was to mimic the way that mid-latitude weather flips between regimes of unsettled weather and settled weather. When the weather is unsettled it changes rapidly, associated with the passage of storms and fronts bringing rain, snow, and strong winds. But when the weather is settled we can get the same conditions day after day, for example, during a summer heatwave or a winter cold spell (both of which are often associated with anticyclonic blocking conditions). Most random RPG weather methods don’t include these realistic variations to the weather.

Of course, there is no reason why you should be playing a game with realistic mid-latitude Earth weather. However, Markov Chains are flexible and here I’ll describe a slightly more complicated method to generate weather for Post-Apocalyptic and Hell-like settings.

Post-Apocalyptic Weather 

So the first question is, what would the weather be like after the Apocalypse? Assuming the Post-Apocalypse looks like a Mad Max film or a Fallout video game, then we would expect long periods of oppressive heat punctuated with hazardous weather, such as radiation fog and acid rain. To mimic this we can use a Markov Chain, so the weather flips between longer periods of Oppressive weather and shorter periods of Hazardous weather. Roll 1d20 each day on the following table see if the regime flips or remains the same:


Note that the Markov Chain above is asymmetrical, the method will produce longer Oppressive regimes (lasting 6.7 days on average) and shorter Hazardous regimes (lasting 2.8 days on average). To keep the Oppressive weather interesting, the Markov Chain is also recursive. During an Oppressive regime, there’s a chance that the regime remains Oppressive, but that a new Oppressive weather type is rolled for.

Oppressive and Hazardous Weather Types for Post-Apocalyptic Settings

Similar to the previous method, there is a second roll to determine the weather type, which depends on the regime. During an Oppressive regime the weather should be unpleasant for the PCs. Long periods of arid, extreme heat, or days of humid, muggy weather, or episodes of foul, polluted air. At the start of the regime, roll 1d20 to determine the type of Oppressive weather. The type of Oppressive weather persists until the weather becomes Hazardous or another roll on the Oppressive weather type table is required.


During the Hazardous regime, the weather changes between violent lightning, terrible dust storms, and extreme rainfall events. There is also the chance of unnatural hazards, such as Radiation Fog or Acid Rain. Roll 1d20 each day to determine the type of Hazardous weather that befalls the PCs.

The Weather in Hell

And what would the weather be like in an Avernus or Hell-like setting? In some respects the weather should be thematically similar to the Post-Apocalyptic setting described above, with the weather flipping between Oppressive and Hazardous regimes. The Oppressive weather regime would be the same as for the Post-Apocalyptic setting. However, the Hazardous weather types would be more appropriate to a Hell-like setting. For example, Blood rain falling from the sky, magical tempests that raise the undead, and winds that strip the flesh from your bones. Roll 1d20 each day to determine the type of Hazardous weather that befalls the PCs.

A complete description of the Post-Apocalyptic and Avernus Random RPG weather methods can be found here.

Next post: Localised Weather using Markov Chains

As it seems there’s a bit of interest in these types of random weather methods, in the next post I’ll describe  how to randomly generate special weather for specific locations. I’m using this extended method for the weather tables in different regions of a D&D 5e hexcrawl setting I’m currently developing.



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