Book of War includes the following simple, optional rule for weather, rolled once at the start of a game (Open Game Content between the rules):
|2-7||Sunny||Orcs/goblins at -1 morale|
|10-11||Rainy||Missiles at -1 to hit|
|12||Stormy||No missiles, move cost x2, |
cavalry attack 1 die/figure
Now, most of the time this won't make much difference -- until the ability to Control Weather gets put into play, as by any Storm Giant or Wizard in the advanced game. Then it becomes highly likely that the special figure can severely disadvantage opposing goblinoids, or more generally any missile troops or cavalry (recall that modifiers are in terms of a d6 roll). We've found that when playing by advanced rules, both sides almost always pick at least one Storm Giant by default so as to access this powerful ability.
I've previously written a column on "Spells Through Ages -- Control Weather", as Book of War was being finalized last year. Among the observations there is that the OD&D spell is immensely powerful, permitting instant conjurations of tornadoes, heavy rain, heat waves, etc., without limit. AD&D did seem to have a clever modification of allowing only a "one step" modification to weather, which I included in BOW. You can't automatically switch the weather to "Stormy" and lock down all the opponent missiles; the natural weather already has to be at least "Rainy" before that's possible. (And multiple spells or giants won't help; only a one-step change from natural, maximum, is ever allowed.)
Even so, the ability has proven to be very powerful. Partly this is due to the way we've adjudicated the gray areas of the spell in-game: First, there isn't any 10-minute delay to the spell as specified in AD&D, so the effect is immediately usable by the caster (otherwise, you'd need a rule for casting it prior to a battle, unlike any other spell, which is the only time it would be useful). Second, you're not stuck with just one change; if desired, you can switch the weather up and down by one step per turn (common usage: make the weather "Rainy" and cripple the opponent's missiles on one turn; then switch back to "Cloudy" and let your own missiles fire effectively, etc.). Third, opposing weather-controllers don't simply negate each other; they're allowed to ping-pong the weather back and forth on opposing turns, according to taste.
Now on the one hand, you might find this overly fiddly (weather changing dramatically every minute?). But we've actually found this to make for a lot of exciting and dynamic play situations (fun, you might say). Control Weather is a top-level ability that makes an immediate and high-impact effect on the game; it's in your face every turn as the casters battle for the sky overhead turn-by-turn. The alternative option, perhaps saying that preferred weather is picked only once before the battle and that opposing casters simply cancel each other invisibly, seems not nearly so dramatic or interesting.
One other thing that we need to adjudicate, not being written in the book: What happens to weather when the controller is killed? Options here seem to be (1) Nothing, weather stays at current level for the entire battle; (2) Gradual reversion back to natural weather, by one step per turn; or (3) Immediate switch back to natural weather, cancelling the magic. (Consider also how this interacts with an opposing caster who was being partly cancelled out; does it switch multiple steps to their preference instantly?) We've been playing by option #3, which is perhaps mangling normal D&D rules a bit (spell effects generally don't end with the death of a caster), but it makes for interesting in-game situations, where opposing weather-controllers are critical targets if the opposition can chase them down and eliminate them.
So: what's your preference? Do you like dynamic weather changing turn-to-turn while a battle progresses, or do you prefer the AD&D-style delay, where one choice for weather would be maintained throughout the whole battle? Do you like the idea of opposing casters ping-ponging the weather every turn, or should they simply neutralize each other without visible effect?
[Picture courtesy smiteme under CC2.]