It would be too hard to codify every single weather effect, so I'd go with two general rules of thumb:
- Weather that is merely Hostile mostly has an environmental effect; if it's raining, you have to take steps to keep things dry; if it's snowing, you should bundle up; if it's broiling hot, you need to drink more water. Failure to act appropriate to the weather over the long term has risks.
- Weather that is Very Hostile can have immediate effects (saving throws and damage rolls for every exposure, no matter how short.)
Stuff Falling from Sky: if it's hailing, or if something else is falling out of the sky, every exposed character must roll a 5 or less on d6 (Avoid Danger Roll) every turn; a result of 6 means that character was hit for 1d6 damage. Small or fragile objects like hail/ice doesn't always do damage, though; if wearing armor, including a helmet, maximum damage equals (descending) AC. Ignore shields, unless players specifically mention covering their heads with their shield (in which case, they don't get benefits of a shield in ordinary combat until they lower their shields.) Stuff like burning cinders from a volcano does full damage. Some lighter material like sleet might only stun or cause lingering pain for 1d6 turns.
Stuff Rolling across the Ground: If there's a lava flow, flash flood, wild fire or low cloud of poisonous gas, it can change direction on a whim. Roll another reaction roll: on Hostile or worse, the stuff heads towards the explorers, while Friendly indicates it moves away. It changes direction randomly thereafter; the party rolls a d6 every turn thereafter (Avoid Danger) to see if it changes course towards them (6+) or continues in its current direction (1-5). If the party splits or if there are monsters exposed to the stuff, too, every group or individual rolls a d6. If the stuff can split, it moves towards everyone who rolled a 6+. Most of this stuff does full, real damage, and fire or lava can catch other things on fire, causing additional damage.
Tornadoes: These move randomly, like Stuff Rolling across the Ground. If a tornado catches you, it attacks like an air elemental. If it's just near you and happens to pass over debris or something breakable, it throws material 3" in all directions as if it were sling stones or darts: everyone makes a d6 roll as for hail, except the stuff isn't falling from above, so shields limit damage if held normally. Naturally, if a tornado passes over a heap of actual weapons (the local javelin shop,) all damage is full and real.
Lightning: Also like hail, except armor doesn't reduce damage, metal armor adds +1 to your d6 roll to be struck, any cloth or leather covering can catch fire, and you have to make a save vs. paralyzation. If paralyzed, make a system shock or adversity roll; failure means death.