I am dissatisfied with existing weather-generation systems for RPGs of D&D's ilk. So I built one! Note that I am not a meteorologist; this is intended to be a simple system which generates 'close enough' results for gaming purposes, not a high-fidelity weather simulator.
In any given day, the weather is in one of three states: high pressure, low pressure, or in flux. High pressure is clear, unseasonably cold, and dry; low pressure is unseasonably warm, wet, and cloudy, sometimes with light rain or snow, or afternoon thunderstorms in the summer. Flux occurs during the transition between the two and is stormy and windy, with heavy clouds.
On day 0, roll a d6. One through three, the weather begins as high pressure; four through six, it begins as low pressure.
Each day of normal weather, roll a d8. On a 2-8, the state remains the same; if it was high pressure today, it is high pressure tomorrow, and if low pressure today, low pressure tomorrow. On a 1, a front rolls in tomorrow and the state changes to flux. The front is detectable some hours in advance, as winds pick up and clouds are visible moving rapidly from the horizon. After 1d3 days of stormy flux, the pressure switches; if it was high pressure when the one was rolled, it is low pressure after the flux, and if it was low pressure, it becomes high pressure.
Weather typically blows in from the west on worlds where the sun rises in the east (at least in my experience), unless there are mountains in the way.
This satisfies several design goals. First, it is more-or-less consistent with real-world experience in several ways. If the weather was cool and dry today, it probably will be tomorrow, but sometimes you get violent storms and things change. Weather is also in expectation stable for about a week, which is more-or-less accurate for the microclimate in which I find myself living. Things average out to moderate weather, but the result is never just "lukewarm and boring" ("Climate is what you expect, weather is what you get."). Second, it does not require a table lookup or percentile roll, nevermind several. Third, it does not go into unnecessary detail; three states are easy enough to remember and sufficiently different for gameplay purposes. Fourth, it is easy to tweak to suit local conditions. You could vary the die size rolled based on stability of various types of air masses (ie, maybe during a high pressure you roll a d10 but during a low pressure you roll a d6, so warm air is less common and marked by more frequent storms). You could make a 2 through 4 on the d8 "precipitation and no change in dominant pressure" during a low pressure. You could vary the pressure change probability seasonally, so during summer you use a d12 instead of a d8, leading to less frequent storms and better adventuring weather. You could change flux duration, add minimum times between fluxes, or any number of other minor tweaks. Much as one might build a random encounter table for the Valley of Storms, so one might alter the weather parameters. None of that extra complexity is necessary to using the core of the system, however.
By contrast, 3.x's weather system fails in all of these regards. It fails to make weather today relevant to weather tomorrow; the only consistency comes from the fact that "normal" (ie, no-op) weather is probabilistically dominant. It requires a percentile roll and table lookup, sometimes followed by another percentile roll. It expresses its temperatures and wind speeds in terms of degrees and miles per hour, which are hardly practical units for adventurers to measure in the field - one might argue that the intent is to use the temperature categories, but when you start adding things like Heat Wave and "temperature drops x degrees at night", it becomes fairly clear that you're supposed to be tracking it numerically. Finally, its ultimate sin (and a telling reflection on 3.x's design philosophy) is that the effects of the weather are expressed purely in terms of combat (and perceptiony skill checks, but those are almost inevitably just a precursor to combat).
Sure, weather does effect combat, sometimes in unexpected ways. The English won the Battle of Crecy in part because the French (well, Genoese mercenary in service to the French) crossbowmen had wet crossbowstrings due to an earlier downpour; while the English longbowmen had unstrung their bows, the crossbowmen had not (perhaps were not able to due to the construction of their crossbows), and so were unable to offer more then desultory fire. Alright, in lousy or just humid weather crossbows take a -4 to hit, bows a -2, and thrown no penalty (assuming visible targets otherwise within range). Shorten encounter distance during precipitation or fog by a small constant integer factor. If the ground's muddy or snowy, speed is reduced and attempts to knock people down are easier. Alright, done with effects of weather on combat.
On to the fun stuff! Effects of weather on exploration.
During a high pressure, seeing distant smoke is relatively easy, while during a low pressure it is more difficult because the humidity keeps the smoke down. During flux, spotting smoke is nearly impossible due to wind dispersion and precipitation, and building a fire in the first place is going to be difficult. Cloudy or stormy weather may make identifying one's mountaintop reference points more difficult if the peaks are above the clouds. The party's mercenaries are probably none too happy about the extra maintenance they will need to do on their gear, and cold, soggy (or worse, moldy) rations add insult to injury - while hardly a calamity, a penalty to any morale roll provoked during wet weather may be in order. Speaking of which, wet weather might render some rations inedible, forcing a reevaluation of the supply situation (likewise in hot weather, the water supply becomes less stable). The effects of heavy rain on the wizard's spellbook are potentially punitive; also true for the party mapper. Heavily-burdened mounts, pack animals, and especially carts may become hopelessly mired in mud or deep snow, and in general overland movement rate will suffer. Precipitation can wash away or obscure tracks while it is falling, but after it stops the mud and snow take and hold tracks, making tracking easier. If you like, say that mud persists for one day per day of rain. If some sort of endurance system is in place (next post, perhaps), wet weather makes it harder to keep warm, both because you get soaked and because there is little dry fuel for fires.
In other words, adventuring in persistent rain is not a good time. Probably the only good reason to do so with all other things being equal is a situation in which you are operating at a disadvantage in an area occupied by strong and coordinated resistance, where the shortened encounter distance means you can close with and defeat in detail enemy detachments before help can arrive. Flying predators (eg wyverns) might also be effectively grounded by a powerful storm, or impeded by low-visibility conditions like fog. Ambush predators might have their lairs flooded out or otherwise damaged by storms and be forced out into the open (conversely, flooded streams and rivers might be effectively impassable; sufficiently powerful floods might alter the flows for some time, or carve new watercourses, rendering maps out of date). Humanoids are likely to try to stay in shelter during storms, while some monsters may prefer to hunt in the rain.
And this is all saying nothing of freak weather like lightning storms or tornados or blizzards! This is bog-standard mundane weather that anyone hiking in the woods (or to and from class) in temperate climes will get to deal with sooner or later, and yet it already introduces quite a few things for PCs to consider during their forays into the wilderness (especially if the wandering monster table varies with weather).
And now I'm off to go bicycling in the cold drizzle...