In most worlds, this is a question players never have to consider. If there are tables for the weather, they are very simple - a few possibilities based on a simple random die roll at best. Chances are, the table hasn't been memorized by the DM, isn't consulted regularly and may come up only because of the druid's call lightning spell or some other detail that needs addressing.
That is because weather is ineffably hard to manage; there simply are no words. Everything about weather is ridiculously chaotic and imprecise, while at the same time adhering to rigid principles of geography and physics. Offering tables for weather is like trying to predict the position of one of two feral cats, fighting violently inside an iron cage, a minute from now.
The only possible solution is to simplify. It needs to be understood, however, that the more a given system simplifies the weather, the less relevant it becomes. That is why role-playing games do not include rules for weather - because any weather system that can be detailed in the space of two pages inside a book will be inherently useless where it comes to contributing to the player experience.
Anyone who has set out to make a weather system for their world discovers very quickly how much work it takes to lift that system to the point where it is relevant. Be it known that it is a massive amount of work. More than an ordinary DM, anywhere, wants to put into their campaign, just so the party can get wet once in a while. For the amount of work it requires, even an elaborate weather system's value will defy the DM's persistence.
Yet I and others find ourselves fighting to get such a system built. Why?
The most obvious reason is that weather is important in all our lives. In a culture without technology, even a culture of fifty years ago, weather was even more important. For a pre-modern culture, for a technology that existed four hundred years ago, weather was the greatest single factor in everyone's life. There's no question about that. All culture revolved around it.
For some of us, who want to reproduce the experience of living in a world where the elements are fundamental to the outlook of the inhabitants, weather remains the fatal flaw in the design. Without it, the world is a fake, a sham, artificial . . . an inauthentic, cheesy swindle, like the moment in a film when it's realized the 'metal armor' worn by the actors is made of painted plastic.
It just won't do.
And we would solve it - except that a weather system is such a phenomenal frustration to build, we're literally forced to accept our impotence there. It isn't just that we haven't got a weather system . . . it is that there seems no possible way to create one.
Those of us who try recognize the wall we're bashing our heads against. The reality is, we're going to kill ourselves before that wall comes down.
Okay, so there's the motivation. Let's pause for a moment and talk about what we'd like a weather table to do.
Well, temperature for a start. Knowing the temperature would allow us to measure how the characters are dressed, how much water they will need to drink, whether the rivers or ponds they mean to cross are frozen over, how important a fire is, whether they're better off being in an inn instead of camping out, the practicality of armor in both very hot and very cold environments, etcetera. There's a lot of character actions tied to temperature.
Next, knowing whether it is raining or not would be helpful. Rain makes a big difference in travel and combat - as anyone knows who remembers when professional football had to contend with outdoor stadiums. Rain increases hypothermia, makes life much more slippery and unpleasant, is a critical factor in whether rivers and streams can be crossed and in general speaks to many details about setting up camp, riding a horse, crossing through wilderness and the onset of disease. What's more, comparing precipitation to temperature tells us whether we can expect a downpour or a blizzard.
Because it matters, the magnitude of the rain is also something we'd like to know. Is it going to rain a lot? Is the snow deep? There's a wide gap between a few spattering raindrops and a deluge. We need to know how much as well as if it rains.
Next comes wind. Wind is where it gets tricky. We can throw a dart at a board to get temperature; we can make rain/sunny a 50/50 proposition if we want and roll a d4 to know how much rain . . . but wind is a problem. It isn't enough to simply roll a die to determine which direction the wind is coming from - if wind direction were fully random, how would ships travel from one side of the world to the other? It isn't enough to roll a die to see how strong the wind is, because a complete calm will devastate ship travel while a little too much wind will destroy whole towns. Making a wind chart means creating a bell curve of some kind . . . but when three or four random gales turn up in a month, pretty soon that bell gets taller and taller. Whereupon, well, we get a situation where the wind is the same every day.
Moreover, so many of the other features we want out of wind are both important and hard to make universal. We want a generation system for storms. Rain doesn't always fall from a thundercloud, but we want to know when it does. We want a generation system for specific types of storms: hurricanes, tornadoes, ice storms, hail storms, dust storms, sand storms, freezing rain, fog, mist and all the lesser versions of that list that produce a 'feel' for the weather without actually being the full-on manifestation.
Which brings up another order of magnitude for everything discussed so far: how long? How long does the temperature last? How long does the rain fall? How long does the wind blow? How long must the characters endure a thunderstorm, a sand storm, a fog? Are the characters in the direct path of the hurricane, or are they on the fringe? How close is the tornado? When it occurs, are the characters even in danger? Does the freezing rain fall all morning, or just for a minute or so? Does the hail beat against the ground for a half a minute or does it hammer down for a quarter of an hour, creating a hail fog?
See, the real interesting thing about the weather are all the deviations that turn up. Temperature, precipitation and wind are not enough; these things combine with the hydrography and topography of an area to produce these strange, fascinating spin-off results that can't be managed with a simple table, for two fundamental reasons:
- Everywhere in the world is different, and thus requires a unique table. Fogs turn up in my part of the world rarely. They usually don't persist. A table that will produce a 'fog' result that applies where I live would be useless for the Eastern Seaboard or the North Sea or the Bay of Bengal. What's more, the difference between these places is quintessentially important to the pleasure of one part of the world having different weather than another part; so we are talking hundreds of tables to produce a meaningful variety.
- Interconnectivity between different elements of the weather is hopelessly imprecise; so we know the temperature, the rainfall and the amount of wind - so what? Who's to say that these three things, in the magnitude they presently have, will produce a specific, given result? Does a wind shear always produce a tornado? Do thunderclouds always produce rain? Does lightning always strike at the ground? Is a certain place on the coast always hit by a hurricane in the right season?
There's another consideration that I hesitate to bring up, since it isn't a problem my world has. Just to put it out there, however, to emphasize the difficulties most worldbuilders have where weather is concerned: what is the data for temperature and precipitation for a world like, say, Greyhawk? Where are the hurricane tracks? Where does 'tornado alley' occur, due to consistent wind shears due to masses of air between the pole and the subtropics? Where do the doldrums start? If we were going to map out sea currents, where would we put them? For sea currents, as we know, are very important to places in the world like Alaska or Norway, places so far north that if it were not for the Japan Current and the Gulf Stream, they would be virtually uninhabitable.
I hope this starts to suggest the enormity of the problem. If the reader truly wants to know the full scope of what could be realized, spend a few days with wikipedia, following every link on every page like the one I used to give 'types of fog.' Start here. Stay in the general subject of meteorology and diligently keep track of every phenomenon that could make a significant change to a normal, everyday event in your present campaign.
You'll run out of energy before you run out of phenomenon.