I find that weather is extremely variable and difficult to represent with simple rules. With the exception of the side effects of weather (e.g. movement in mud), I have tended to rely on "GM calls"...
RainThis makes everyone wet, cold and miserable.
Proper weatherproof clothing would not be around in Legend, so prolonged exposure would lead to everything not protected by an oilskin, or sealed in a pot, getting wet. Lighting fires and torches (without sorcery) becomes that much more fun...
Visibility will be reduced - depending on how heavy the rain is. For drizzle there's not a great reduction, but a heavy downpour can see visibility reduced to 50m (even less for a real cloudburst).
Noise, wet and cold would affect PERCEPTION, which the GM could reduce by -1, or more (depending on how heavy the rain is).
Maximum missile ranges are limited by visibility... Although it should be noted that water is not kind to bow and crossbow strings - which will stretch if wet. A stretched bowstring reduces range and effectiveness; range could be reduced by 5m/10m/20m, while Armour Bypass and Damage would both be at -1... At least, until the archer finds a dry string.
(This affected the Genoese crossbowmen at the Battle of Crécy 1346)HailLike rain, but worse. Increase the penalties.
Note that hail can (very occasionally) be properly dangerous - with golf-ball sized hailstones that fall killing both men and beasts (as happened to Edward III's army in April 1360 -
https://en.wikipedia.org/wiki/Black_Monday_(1360) ). Characters should find shelter quickly, or the GM can start inflicting 0-3 (1d4-1) damage per round!
WindThis can affect
light missile weapons; daggers, rocks and javelins would not be affected. A penalty to ATTACK might be appropriate (see Acoma's suggestion).
Note that wind direction is also important; firing upwind would reduce range, while firing downwind would increase range.
(This had a major effect at the Battle of Towton in 1461)FogThis affects visibility (obviously) and, sometimes, muffles sound - making it hard to hear where the noise is coming from. Penalty to PERCEPTION is appropriate (-1 or more depending on the thickness of the fog). Unless it's a supernatural mist, it is rare that fog fully obscures vision below 20m (and even that's quite thick); so an Assassin would have to disengage, move away, then return (presumably from a different direction)
unnoticed to get another Shock Attack...
Fog effects would be worse at night, of course. Not to mention that fog is
wet, making torches harder to light.
As has been mentioned, fog is a physical effect (water in the air); so it would impede all kinds of vision. It might even (if thick enough) cause trouble for some spells (e.g. Eyes of Night) as the droplets reflect intense light... (That's why driving with full beam headlights in fog is a bad idea!)
Then, for those heading to the Crusades, there are things like dust and sandstorms - which can cause proper damage (even death)...
As for the longer-term effects of climate, page 10 of the Perils download from the Cobwebbed Forest provides some suggestions:
http://www.cobwebbedforest.co.uk/librar ... Perils.pdfUnless it's part of the story, I tend to keep weather effects relatively minor (penalties in the order of -1 to -3). It's not necessarily realistic; more a way to bring a bit of "real world" into the game without causing undue frustration and making the scenario unplayable. Really bad weather keeps everyone (man and beast)* under cover and combat is not too likely anyway.
* Ok, maybe not the Undead...