Weather and mechanics
Now we step into the crunchy snow. Now we feel the mechanical clockwork roll about beneath our feet. Now we talk about how the weather affects our mechanics, our combat, our challenges.
First and foremost, weather is almost universally a penalty. This makes sense because the default rules for any game always assume that nothing is aiding or hindering you. Once another factor is added in, it's going to make almost anything harder. From climbing a rope slicked with sleet, to firing an arrow through howling wind, to listening for invaders amidst a rainstorm. But let's think about that last example and consider this: Someone else' penalty is your bonus.
Because bad weather is generally going to penalize a lot of actions it's important to consider how it can be used most effectively, and how different systems are going to be affected by this. Consider a gritty realistic game where failure is common and perilous. Having a rain slicked precipice can quickly turn an encounter into a desperate struggle to hold on while battling an equally hindered foe, and there's a very real risk that everyone is going to fall to their death. Conversely in a more high action, high power game a slippery floor might be something you can easily negate with a power in order to gain a small advantage. Likewise in that high powered game maybe a fall from the roof to the back yard is just a good way to tack on a bit of extra damage before the duel continues on ground level.
Introducing volatile weather also allows a GM to have an extra bit of say in an encounter and a chance to add a little fudge to a fight. A sword fight in an open field on a sunny day between a player and their foe is pretty likely to be a test of skill with little else involved. But on a mountainside during a blizzard? A player at risk of having a boring fight may find them self slipping and clinging to the stone for dear life. A villain catching the upper hand may find a small, or very very large blob of snow suddenly dropping upon them. And a defeated enemy who should ideally re-occur can take a quick tumble off a mountain to "certain" death only to return later, frostbitten and vengeful.
Weathercan also pull double duty with combat as a great way to make a switcheroo. After a few rounds of battling Orcs on a mountainside, the ferocious sounds of battle trigger the real encounter, a deadly avalanche! Or perhaps the player on duty finds them self scanning pitch blackness during a torrential storm only for a single bolt of lightning to suddenly illuminate the dozens of werewolves lurking in the blackness. That second one is an especially good surprise if a player takes off their metal armor for fear of being struck by lightning.
And of course some powers and abilities directly rely on or affect weather themselves. Powerful solar weapons, crackling storm magic, the power to bend the elements, all of these are great ways to showcase the scope and ability of a character, PC and NPC alike.