Just a quick idea that spontaneously formed in the depths of my synaptic pathways: If weather becomes an issue in an environment where the weather is a valid variable, make luck rolls for both sides, the PCs and the NPC side. Whoever wins the luck roll, the weather is in their favor.

I have charts for most of Celtricia, based on the time of year.  My group's all play at a very slow pace, so the adventuring in the winter seems to go on forever.  My players are all really enjoying playing in weather that is getting warmer.  And since many of them use elemental magics, weather has a lot to do with casting.

One of the most memorable games I ever ran was a Solomon Kane session in which the players were stuck in a storm, fighting some undead. I assigned a fairly hefty penalty for muddy footing and low visibility, and it made what would normally have been a cakewalk into a tense slugfest that left one character seriously wounded for the rest of the game. It was awesome.

I used to have some stupid charts and graphs for weather and moon phases and stuff back in high school, but it's not important enough for me to put work into these days.

In my fantasy game, I use one of two systems.

The months of my Fantasy World Arth, have a dominant weather.  It makes the weather for a given month easier.

Storos has storms.  It rains every day for some of the day in the season.  
If I roll low, it is dry or only light rain.  If I roll high, it is quite wet/ stormy.  

When I am really being specific...  
I have a five slot track for temp and effect
The five basic points of temp: Freezing/Cold/Average/Warm/Hot
The Five basic for "weather":  Dry(calm)/ Dry (calm)/ sporatic/some/lot

Take yesterday's weather....
Roll 1d6 with a +/1 mod for season  (storos +0 for temp, +1 for rain)
Roll 0-2 slide down the track by one. Roll 5-7 slide up the track by one.
So if yesterday was average temp, sporatic rain.  I roll 2, 1d6 (3&4)
The temp would stay average, the weater would go to some (+1 MOD remember), thus raining mildly all day.  

Sometimes I keep serious track of this and have it effect crops months later or mess with trade routes... '

But really I use the first system more often.

Weather can drastically alter an encounter, whether it be combat or role-playing (watch a bunch of freezing characters out in the rain negotiate with a nightwatchman to open the town gates for them; you'll see), so it does factor into my games often.

You can find weather charts for every system (almost) and every setting (almost) on the internet. It also isn't hard to use GM fiat. And a lot of settings have official publications that contain weather charts and stuff. It depends on how serious the game is - the more serious it is, the less likely I am to use GM fiat. I do use calendars for my settings (both WFRP's Old World, Kingdoms of Kalamar and Hellfrost all have printable calendars), so I can keep track of what the weather has been like for the last few sessions. Thus my GM fiat is more like an educated guess.

But yes, weather is important in my games. Heck, in Hellfrost it is critical to the game!

-=Grim=-