Guest Post: Using Weather in Your Game


Sarah Darkmagic - Posted on 18 May 2011

Weather can be a tricky topic in games. Try to model it too closely, and the DM and players quickly get lost in details that don't really help the plot. Make it too random and players might call foul and say the DM uses only to his or her advantage. Fortunately, Scott, @ScottyMet on Twitter and a meteorologist, came to rescue. Below is his guest post on how to bring weather into your game. Besides Twitter, you can find him online on his blog, Always a Gamer.

Patterning Weather

Real-world weather is based on the complex interaction of many factors, all combining
at the same time, in a seemingly completely random fashion. So complicated are all the
processes that go into weather that real-world forecasts are still only reliable out to about
24 hours, after which the uncertainty grows progressively larger, and the accuracy of
predictions gets worse. However, rather than random, weather does happen in a straight-
forward cause-and-effect way. The behavior of a specific “parcel” of air can be tracked,
and how that parcel will react to changes in temperature, humidity, pressure, etc, can be
accurately predicted. However, the reason weather prediction is so difficult is simply
the fact that there are billions of these “parcels” surrounding the planet, all interacting
with their neighbors, influencing them and being influenced by them, while at the same
time moving around the planet, interacting with different terrain, and rising and falling
between different heights. Computers simply aren’t powerful enough to work with all this
information, all at the same time, and still produce predictions in a timely manner.

In gaming, especially running the game as a Dungeon Master, we have the benefit of
being in complete control over the environment that our players’ characters live in,
including the weather. We decide if it is raining or snowing or a bright sunny day, and we
decide when that weather will change and what it will change too. Furthermore, since it is
a world with magic, the weather can change to anything we want on a whim, going from
a bright, sunny, warm day to a raging snowstorm burying the countryside in a matter of
moments. Such drastic changes are typically to drive the story, but there are times when
you simply want weather to progress along more natural path.

In the past, there have been several attempts to design weather systems for games. They
all involve rolling dice to determine what happens, and then more dice to determine
what happens next. Those systems never felt right, and the results were, ultimately,
unsatisfying. Dealing with weather progression doesn’t need tables and dice, though. It
doesn’t require the DM to keep complex records. If you want this kind of detail in your
game, it is as simple as picking a starting point, and following some fairly simple rules
after that.

For your starting point, simply pick your weather conditions. Think about what time of
year it is, what climate your locale is in, and if there are any particular conditions you
need to start. Many adventures start with inclement weather, to drive adventurers indoors,
to where the action occurs. It isn’t necessary, of course, but it happens enough for it to be
a cliché. Don’t let that deter you, though. It’s a cliché because it works!


Once you have your starting conditions, you just need to progress things from there.

Rainy or snowy conditions are due to a region of low pressure. Winds generally blow
counterclockwise around this region, spiral inwards towards the center of the region, and
are stronger the closer to the center of the region you are. This region, if you could see
the boundaries between the air masses that make up the system, would look like a notch-
shaped valley cut into the side of a hill. The base of the western side of the valley would
be the cold front, the slope on that side would be tall, steep, and rounded. The base of
the eastern side of the valley would be the warm front, and the slope on that side would
be long and shallow. Precipitation occurs as warm, humid air blows from the south and
southwest, up these slopes, cooling as it rises, and produces clouds and rain or snow.
Because the slope of the warm front is shallow and gradual, precipitation in this area
tends to be steady and wide spread. With the steep slope of the cold front, air rises very
quickly ahead of it, and the weather you get along this front is more energetic, producing
showers or flurries, thunderstorms or snowstorms. Behind the cold front, winds swing
around to the northwest, the precipitation tapers off, and the skies clear.

Warm, sunny weather is typically due to a region of high pressure. Winds blow clockwise
around this region, spiral outwards from the middle, and are weaker the closer to the
center you are. This region would look like a wide dome. Temperatures are generally
cooler on the east side of this region, where the winds blow generally from the north, and
warmer on the west side of the region, where the winds blow generally from the south.
Heat waves and droughts can be associated with these regions, depending on the time of
year and how persistent they are in an area.

There is some variation in the cycle, of course, but generally an area will go through a
cycle from high pressure to low pressure to high pressure to low pressure and so on. How
long it takes to go from one to the next is really up to you and what you need for your
adventures.

So, if you wanted the weather to progress realistically in the area of your adventures, you
could have the PCs seeking refuge from miserable, rainy weather, taking refuge in some
caves, only to discover that it is the lair for a tribe of goblins. After defending themselves
from the attacks of their unwilling hosts, they discover that the goblins are allied with
tribes of hobgoblins and bugbears that are planning something big, and the goblins
were going to meet them soon after the rain stops. Setting out to find these other tribes,
the rains end, and some warmer weather sets in for the journey. As the PCs approach
their destination, they see storms gathering on the horizon, with flashes of lightning
illuminating a tower in the distance. These storms roll through as the PCs assault the
hobgoblin encampment, with driving rain, strong winds, and thunder and lightning
covering their movements and fighting. Moving on from the hobgoblin camp, they make
their way to the tower they saw. The storms have moved on now, still visible flashing
off to the east. As the PCs travel to the west, the winds grow lighter, turn to blow from
the south and the weather becomes sweltering hot, humid and stagnant by the time they
arrive at the tower and the encampment of the bugbears, possibly giving the PCs a chance
to sneak by weary, complaining guards, or perhaps catch them unawares.

The weather details in this scenario aren’t necessary, but they add some interesting
flavor to the adventure, and even open up some possible avenues for the PCs to approach
their encounters in a slightly different way or grant them some benefits if they use the
situations to their advantage.

Predicting Weather

PCs can predict the weather with a Nature skill check. The higher the roll, the farther out
the forecast can give details for.

DC Easy – next 3 hours
DC Moderate – next 6 hours
DC Hard – next 12 hours

The forecaster can get fairly general information about the longest period their roll
exceeds the DC for. For each successively shorter period, details increase.

What you can tell for the time period:

  • Where you likely are, in reference to high or low pressure, or warm or cold fronts.
  • Whether cloud cover will increase or decrease.
  • Whether wind direction will change and whether wind speeds will increase or decrease.
  • Whether temperature will increase or decrease, and generally by how much.
  • Whether humidity will increase or decrease, and generally by how much.
  • Potential for precipitation.

For the farthest period you have details for, you can know two of the above points (your
choice). For the next closest period, you can know four of the above points (your choice),
and for the next closest period, you can know all of them. Thus, exceeding the Hard
DC will give the forecaster two details about the forecast for 6-12 hours from now, four
details for 3-6 hours, and all of the details for the period for the first 3 hours.

Weather effects

There are some general effects that can be applied to encounters, based on the weather.

  • Strong winds can cause challenging terrain. Succeed at an Athletics check to move
    normally, otherwise experience difficult terrain when moving against the wind, or be
    pushed one square in the direction of the wind when traveling across the wind or with the
    wind.
  • Gusting winds act as Strong winds, above, but happen intermittently. At the beginning
    of each round, roll 1d6. On a 5 or 6, the winds gust until the end of the round. Increase
    the range on the die for more frequent gusting.
  • Fog, rain, or snow cause lightly obscured conditions, granting partial concealment when
    not adjacent to an attacker.
  • Thick Fog, heavy rain, or heavy snowfall cause heavily obscured conditions, granting
    partial concealment when adjacent to an attacker, and total concealment when not
    adjacent to an attacker.
  • Frigid cold or sweltering heat can be used as a zone, which attacks anyone in the area.
    Attack vs Fortitude, inflicting ongoing cold or fire damage, respectively (save ends).

Weather can even be the target of an encounter, or one of the targets. As an example,
here is the Stormrage Cyclone, a level 5 Solo. The design of this “creature” involves
three successive auras, each with their own attack, and an attack for when an opponent
ends their turn adjacent to the cyclone. Note that, although the Cyclone has hit points
and defenses, it is not a living creature, and does not have a will of its own. It makes its
way across the encounter map, possibly having some randomness in its movement, and
affecting every combatant on the battlefield. It can be attacked and destroyed, with force
and cold attacks having the greatest effect on it, but it is meant more as a hazard than a
direct opponent.
Stormrage CycloneStormrage Cyclone

tags

Great article. I always say that campaigns have perfect weather too much of the time. Just because it's a fantasy world doesn't mean it's always sunny. I wrote something related to this about working seasonal changes into your campaign a few months ago: http://www.rovingbandofmisfits.com/?p=581

Thanks!

It was always a running gag in my friend's campaign that there was never any weather. It was so unusual to have weather that when he put in some storms so that my character could test out this magical item that involved lightning, we thought the world was ending and urgently sought shelter, rather than have any thoughts of testing magic items! :)

After the group traveled into a swamp, and then emerged to find 1-foot of snow on the ground, I joked that it didn't actually snow... the layer simply fell straight out of the clear blue sky, all at once, to cover the land!

And so, it may as well be random.
http://donjon.bin.sh/d20/weather/

Very interesting post. I dig it ;)

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