One of the convenient things about using the earth as a template for a world is that a vast resource of climate data is readily available for temperature, precipitation, humidity, winds and so on. This climate data, further, continues to increase in its detail and availability online. It would be impossible to create a world from scratch which would include, as part of its structure, the sort of detail available for earth—the best that one could do would be to estimate regions of an invented world based upon earth models…and therefore it is still useful to have at least a beginner’s understanding of meteorology.
Climate does not, however, completely describe the picture, as weather is turbulent and unpredictable. The easiest way to devise a system for the game would be to pick a date in the modern era (say, 1981), and then have exact weather reports for places on earth for that year, so that on March 23 as a DM you could say the weather in Lisbon was “this.” It rained all day, the temperature was 9 Celsius, the humidity was 64% and so on.
I’ve never liked that idea. I have wanted a system that could be random, something that would surprise the DM as well as the player…because it would be more fun that way.
I’ve already said that the benefits of having weather in a campaign help make the experience real. There are other reasons. How does weather affect a party that chooses to travel in various environments? How does heat or cold affect the equipment a party can carry or wear? What are the effects of wet weather on health and disease?
Admittedly, weather is such a pain. Virtually every system that has ever been advanced is cumbersome or useless. There are so many variables to weather that a detailed, random system is virtually impossible without dozens of die rolls being made for each day in order to provide results which aren’t irrational or worse, blandly repetitive. DMs can’t be bothered to memorize the tables involved because they can’t see the benefits to their campaigns.
Shelter is one of the three basic necessities to the human condition, but D&D by and large ignores it completely. A typical campaign has a party moving along through the environment, even through a driving rain, with all the concern of someone crossing a living room. It is always presumed that the characters are somehow brave, hearty souls who are unaffected by the wind in their face…but I argue that a wise character has the sense to get out of the rain. Weather is, I believe, another obstacle to be overcome, like traps, lairs or monsters. An untapped obstacle for many campaigns.
A simple system would seem impossible—but I want to make the attempt anyway. For me, if not a simple system, then no system at all.
I have been concentrating, to date, on the turbulent zone between 30 and 60 degrees latitude, where on earth the prevailing westerlies of the horse latitudes are turned northwards by the turning of the earth, coming into conflict with the polar easterlies turned southward. The former manifest as warm fronts, the latter as cold fronts…the conflict between the two creates storms. This conflict creates most of the weather of the United States, Europe, China and Japan. It is the weather with which we are most familiar and it is the weather most difficult to define through a system of dice.
Rather than creating a temperature based system that determines if the weather is warmer or colder than the average with a 50% chance of each, I’ve decided to base the system on the arrival of cold fronts. This is because, inevitably, a cold front will arrive at some point…the question is how long between fronts, how quickly the cold front will be moving and how warm and moist has the warm front become when the cold front arrives. It is the latter two conditions that determine the violence of weather conditions, including blizzards, thunderstorms and tornadoes. The exact moment of interaction between a warm and cold front is the most significant weather event in the turbulent zone.
Typically, a continental region experiences between 50 and 70 cold fronts a year. An intermontane region, such as Tibet or the southeastern American desert, can experience up to 180 cold fronts in a year—most of which will “break up” upon crossing over the large mountain systems of the Rockies or the Himalayas. The number therefore can fluctuate greatly depending on the region.
Cold fronts may be weak or strong; slow-moving or fast moving; massive or small. These things are determined by the season, by the topography of a region and by chance. Cold fronts lose their power as they move southward or as the air mass loses its integrity.
Cold fronts may also be moist or dry. Winter cold fronts moving over icy terrain from the frozen Arctic Ocean are typically dry. In winter or summer, cold fronts originating over the north Atlantic or Pacific are typically moist.
When interacting with warm fronts within this system, it is important to know the origin of the warm front to determine its humidity. Most large warm fronts of Earth are moist, as most originate from the Indian, Pacific or Atlantic Oceans. Warm fronts originating in the Sahara are dry, however; and many warm fronts, passing over mountains such as those of Iran or the Rocky Mountains, lose their moisture and are also dry.
Desert areas experience both cold and warm fronts which are consistently dry, either because of the origin of the fronts or because they are surrounded by mountains which leech the moisture from the air. Turkestan, Western China and southeast America are good examples of this.
Rainforests experience both cold and warm fronts that are consistently moist. Many jungles experience only warm fronts, as they are located at the equator. For places like Japan, Britain, Norway or British Columbia, moist cold fronts from the central Pacific and Atlantic consistently provide a great deal of rain with cool—not cold—temperatures.
It is a simplification, but for the sake of sanity, I’m going to argue that warm fronts move in this system only when the cold front has fully moved past a location, leaving a “vacuum.” This means that only cold front movement must be determined.
The following applies to slow-moving cold fronts:
Wherever a moist cold front pushes out a dry warm front, there is cloudy weather and gentle precipitation may result. Whenever a moist cold front pushes out a moist warm front, heavy precipitation results. Whenever a dry cold front pushes out a dry warm front, cold or cool clear skies result—potentially very cold, in winter. Whenever a dry cold front pushes out a moist warm front, a steady rain will result, increasing as the fronts push against one another.
In each of the examples above, a fast-moving cold front increases the intensity of the storm.
A moist cold front rushing at a dry warm front will produce cool, light rain over a long period. A moist cold front rushing at a moist warm front will produce a highly violent series of thunderstorms, known as a squall line, often three or four in one day, with considerable amounts of rain. A dry cold front rushing at a dry warm front will produce very brief thunderstorms (often without rain) associated with windstorms, funnel clouds or tornadoes. A dry cold front rushing at a moist warm front will produce a large, lasting and very violent thunderstorm, associated with hail and tornadoes.
So, the task would be to A) determine the chance that today a cold front is arriving; B) determine the humidity (moist or dry) of both the cold front and the dry front—both having specific likelihoods depending on the season; C) the strength of the front (weak or strong…I don’t want to be more specific than that) and D) the speed with which the front is moving, determining the violence of the results.
I haven’t actually crafted the table that will designate the speed with which a cold front is moving; I believe I’ll sit down tonight and do so. The table would have to include the expected effects associated with the speed—should be interesting. I’ll try to post have it done and post it tomorrow. Should be interesting.
I’ll also want to begin discussing the after effects of the cold front moving through. The day after, its presumed that the resulting weather would reflect the humidity of the front that’s passed, clear skies or cloudy…which would then lead to steadily clearing weather as the cold front moved farther away, replaced by the warm front bringing either clear skies or humid clouds, which would then be hit by the next cold front.