Thursday, May 18, 2017

OED Wilderness Rules Draft: Weather Events

The fourth and final part of a draft of modified wilderness adventuring rules for OED-style games. Something like this was playtested at Helgacon this year. The principal feature it that it exchanges the daily Lost die for a simple Weather mechanic. Whereas in the past I've struggled in Sisyphean style with super-complicated weather rules (e.g.: Dragon #68 "Weather in the World of Greyhawk", AD&D Wilderness Survival Guide), this simple d6 roll is making me perfectly happy in play now. Thoughts?

Weather Events

In wilderness adventures we replace the daily Lost die roll (see Vol-3, p. 18) for a Weather roll. Thus two dice a rolled each day: a red die (for Encounters) and a blue die (for Weather). On a roll of 5-6 some weather Event occurs; consult the table to the right. The default situation assumes a temperate environment in the summer, with a base daily temperature of Warm (clo 0).

Heat Wave: Daily temperature is Hot (clo –1). No travel or work can occur without magical protection.

Light Rain: Travel may be attempted normally, but there is a 2-in-6 chance of becoming lost (mostly travel in circles, end move 1 hex in random direction).

Heavy Rain: Travel is impossible (except possibly for large creatures like giants, ents, balrogs, etc.)

Thunderstorm: Travel is impossible, and parties not in shelter have a 1% chance of a random character being struck for 6d6 lightning damage (save for half).

Travel prohibitions generally mean that no encounters can occur (except as indicated). We assume that no long-range movement occurs in the winter season (due to snow, cold weather, muddy roads, blocked mountain passes, etc.), including sea travel. The DM is encouraged to create Event tables for other climactic zone as desired.

Notes and References

  • The specific frequencies of events above were modeled on northern Europe; specifically the Paris Le Bourget airport in June to August. We downloaded daily historical weather data for these months 2001-2009 and computed statistics as above. Paris was chosen as central to battles in the Hundred Years War. Technically, a d8 Event die would be more realistic (1-in-8 heat wave, 2-in-8 thunderstorm), but the d6 was used above for simplicity in play.
  • Frequency of rainy days may be 25-50% in Atlantic region, northern Europe. Heavy rain (> 10mm per day) occurs in eastern United States about 15% of days (1 in 6), Europe somewhat less. Rainy days evidence clustering (suggests roll for multi-day duration, or increased chance next day?).
  • This short table of middle ages battles shows battles occurring from March to October, with the largest cluster from June to August, which we used as the assumed “adventuring season”:
  • No winter sea travel as given in Unger, Richard W., The Ship in the Medieval Economy 600-1600. (p. 128, 131; but compare to later era on p. 175). If winter sea travel permitted, then weather should also be modified in that season.
  • For a translation of a year-long medieval weather record, possibly by Roger Bacon, see the link below (cites C. Long, “The Oldest European Weather Diary?”, Weather, Vol.29:6 (1974) pages 233-237):
  • For more exotic weather options reported in medieval times (such as cooked wheat, barley, beans, fruit, fish, snakes, stones, ash, rocks, people, slabs of ice, and blood), see Paul Edward Dutton, “Observations on Early Medieval Weather in General, Bloody Rain in Particular”, The Long Morning of Medieval Europe: New Directions in Early Medieval Studies (Chapter 9) (currently available on Google Books search).


  1. For me, the notes of why you chose those numbers is prime as it not seems as you had pulled it out of the hat.

    Also thanks for the links about why we walk in circles when lost.

    1. Glad you appreciated that! I was glad to find those articles recently.

  2. Not as up on the wilderness survival rules so I can not speak to what you are trying to emulate by replacing the lost die with this.
    I am curious though on your thoughts about using one check to see if an event occurs, and a 2nd to determine the event v. using one chart. (ex: 2d6: 2-3 = Heat Wave, 4-8 = no event, 9 = light rain, 10 = heavy rain, 11-12 = T Storm)

    1. Looking at the Outdoor Survival/OD&D rule, I feel like getting lost happens too much (in O.S. for experienced explorers it's much less than in OD&D), and the mechanic of being forced into straight-line travel has always irked me as unbelievable.

      The thing with the 2 dice idea is that it's in the same fistful as the encounter die and I feel it might be confusing to parse 1d6 and 2d6 in the same throw. The encounter die itself triggers a check of other tables and I wanted to emulate that for symmetry and recall.

  3. I might change the term 'heat wave' to just 'hot day' instead; usually 'heat wave' refers to an extended period of high temperatures. A lot of places use a minimum of three days, I believe. Much longer heat waves are also possible, which might argue for some sort of monthly weather roll in addition to the day-to-day rolls. One of the 'small' heat waves of three days or so would just be a result of rolling up multiple 'hot day' results on the day-to-day weather table.

    As for rainy day, clustering, maybe rearrange the table a bit like so:

    1: Light Rain
    2: Light Rain
    3: Heavy Rain
    4: Heavy Rain
    5: Thunderstorm
    6: Heat Wave

    After any result of rain or thunderstorm, increase the chance of a weather event on the next day to 50% (4, 5, or 6) and apply a -1 to the die roll for selecting the event (eliminating the possibility of an extra-hot day the next day; this is intentional, as evaporating rainwater cools the air, making a temperature spike less likely the day after a storm... certainly not impossible, but a d6 only offers so much granularity)

    1. Not bad -- it's just that at this point I have a constitutional aversion to needing to record any state or cases from day-to-day. Philosophically I'm pretty dedicated now to the daily die rolls being totally state-free (even if that admittedly shaves off some realism; I've been burnt so bad by that in the past!).

    2. I had an idea now.

      Roll for the week. The result does not mean it is statically in that weather but it is marked by that.

      The weather becomes a little bit more permissive than the table says but the DM can enforce a meaningful case of the weather.

      Example: It is Heavy Rain. Long distance travel is highly risk (it is up to the players to try) but go out for a couple hour is possible.

      Lets say they go out into the dungeon... They will take more time to get there and will arrive soaked in water and mud. Sometime later the DM can say the dungeons is flooding and some places have running dirty water to the knees. If not warm and dry in the inn with a good meal at the end of the day, they make a Save vs. Poison or become sick for the rest of the week.

    3. Leaning with Delta, the only way I'm going to deal with a stateful algorithm is if I've implemented it with a stable psuedo-random generator that reproduces the campaign calendar on-demand.

    4. I do agree with Daniel's point that "Heat Wave" by definition means numerous days (3+) of hot days back-to-back. I just changed that to "Heat Spike" above.

      A bit idiomatic and not absolutely perfect... but I'm embarrassed to say how much research I did looking for a better phrase (note last two references added today). Also considered: Hot spell, high temp, warm front, record high.

    5. GAH! It just looked so ugly to my eye. I switched it back to "Heat Wave" even though that's admittedly an abuse of terminology. I just couldn't find another phrase I was happy with. I tried, I really did. :-)

  4. I think that I'll stick with the middle position, Lisa Cabala's article, "Weathering the Storms", from Dragon 137. If I ever become ambitious, I might similarly download historical weather data and adjust the charts accordingly. I like the fact that it has a range of climate zones but broad temperature and precipitation categories.

    1. Thanks for citing that, because my knowledge of Dragons after 100 is sketchy/hazy. On a quick scan of it, my critiques are, like other TSR published weather systems, (a) it's too complex for my purposes (8 pages), and (b) it doesn't have any in-game effects on PCs! That is: for all the work it's almost purely descriptive (several tables I think just lifted from real meteorology sources) and has no in-game effect unless the DM adds more detail.

    2. Quick read, I might have missed in-game effects (but it would be in line with other TSR systems to leave them out).

    3. There's another Dragon article that I intended to use for the effects of weather ("The Role of Nature", #108). I just think that Cabala's system is a good, solid middle ground between the high-detail "Weather in the World of Greyhawk" approach and the quick-and-dirty approach that you're proposing here. It also lends itself well to a spreadsheet, which is an idea I grabbed from some OSR blogger, though I forget who.

    4. Also, if you were to expand the system here to a variety of climate zones, I expect that it would end up nearly as long as Cabala's system. If you were to restrict her method to a single climate zone, it would be nearly as short as the one you lay out here.

      Also, you could incorporate your effects to her descriptions with little effort, though perhaps some finer granularity would be appropriate - which is, as I say above, the middle ground.