We all know that England is famous for raining, an awful lot. The Dark Albion setting, being a fantasy version of England, is no different!
Now, I know what you're thinking, a little rain never hurt anyone, right? True, but a lot of rain can be positively deadly for a medieval party on the road.
First though, let's figure out how often it rains, and where.
This is an important image to check out, although its contemporary and not medieval, but even if the numbers would be slightly off the general pattern would still be mostly correct:
Take note that some areas, specifically the mountainous areas, and the less populated regions, have heavier rainfall. There are two important things to note in this: first, it is not a coincidence that the area of heaviest population is also where there is less rain. Too much rain makes a place less livable.
Second, and important for Albion campaigning, the isolated wilderland places are the places that get the worst rainfall!
In the period where Dark Albion is set, Europe was at the start of a slow but steady cooling period that would end up having major climate effects on the population. One of the reasons for this was that with the cooling period came increased rainfall. The "little ice age" caused serious decrease in crop yields and increase in famine that kept Europe from being able to repopulate after the devastation of the Black Death, and there is ample historical/archaeological evidence of mass abandonment of villages in highland regions throughout the British Isles in this period. Entire communities ceased to exist because of this.
Now, we can't get specific information on medieval weather patterns at the level we're used to today, but this being an RPG and not a history lesson, we can wing it with an approximation. See the brown areas on the map above? Let's say that during the summer period there's a chance of 2/6 that on any given spring/summer day it'll be raining. If you wanted to be more precise you could maybe switch that to 4/10 in springtime, and 2/10 in summer or early autumn.
The non-brown areas would get rain on a 3/6 on any given day; or if you prefer, 5/10 in springtime, 3/10 in summer/autumn.
In either case, if you generate a rainy day, there's a subsequent chance of 4/6 that it's only a partially rainy day (it only rains lightly, for only a few hours); otherwise, it's heavy rain, that lasts throughout most of the day.
Why does all of this matter?
When it's raining, especially if it's been raining heavily, or over several days without ceasing to give time for things to dry, there are several problematic results for adventurers on the go. First, the dampness can affect equipment: under heavy rain, bowstrings (regular and crossbow) and other equipment can become unusable. Even arrows can be wrecked by rain as they absorb the damp and become heavier. Now, most bowstrings would be treated with beeswax to make them more resistant to rain, and quivers were made to be closed and sealed as waterproof as was possible at the time. But this does not completely prevent the harm, only mitigate it. When it's raining, bowstrings have to be removed, and arrows sealed up. This means that if the PCs don't do this for some reason, there's a very high chance (4/6?) that their missile weapons will be wrecked. Adventurers in this period should know about this, though, and if they were smart would take precautions; but this means that in the case of a random encounter, they won't have their missile weapons ready for action.
And of course, if your PCs are of the lucky rich sort to have firearms, this is at least as big a problem when it comes to gunpowder. If your powder gets damp, it won't fire. In seriously wet conditions, even if you've taken care to store your powder and pistol/rifle carefully, there's a 4/6 chance that a gun will simply fail to fire.
Even if you manage to get a shot off, be it with bow or with powder weapon, firing in the rain should cut your range by half. Note that visibility is seriously decreased in the rain as well, by one-third in light rain to one-half in heavy rain, so the range reduction becomes partly academic.
Are your PCs travelling with a cart to take all their extra equipment there, and all their loot back home? Remember that only the most significant of main roads had any kind of protection from rain; if they're going on most medieval roads, rainfall will muddy up the whole place, and reduce travel times by half as well. There'll be an additional 1/6 chance every hour of travel that the cart will get stuck in the mud and require strength checks to push/pull it out.
Horses are elegant creatures, but if its raining heavily they won't be able to go on most terrain at more than a walk; and if you try to charge with a horse at an enemy in the rain, some type of check will be required to avoid disaster.
Are the PCs traveling in the mountains? Rains can cause mudslides. Any time that its been raining for at least two days in a row, there's a 1/10 chance every day of heavy rain of getting caught in one while in mountainous areas. A saving throw is required to avoid getting knocked down by one and taking 3d6 points of damage (if you're on a horse, there could be additional damage from falling off).
Are the PCs heading to a river? Rains can cause rivers to flood. Any time there's been rainfall for more than five days in a row, or heavy rainfall for three days in the past week, there's a 4/6 chance that the river will have flooded, making crossing almost impossible (except perhaps at major bridges).
Can your PCs manage to get to a town? That will sure be good for their survival in the rain. If not, they're stuck trying to build a fire and shelter in pretty sub-optimal conditions.
Wilderness survival skills must be rolled, at a penalty depending on light or heavy rain, to successfully build a fire. If you can't do it, then even in the summer night, heavy rain can cause bitter and harmful cold.
Any time that a PC sleeps outside in rain (light or heavy) without some kind of shelter and the warmth of a fire, there's a 1/20 chance of becoming ill. This would be some type of flu, most likely, but remember that back then this was a pretty dangerous thing to get, without adequate rest and medical attention.
Saving throws should be rolled to recover, with penalties if the PC is not able to get rest (in a warm and DRY environment!) and bonuses if they are getting attended by a doctor. Failing would mean that the disease evolves into life-threatening pneumonia, which would leave the PC flat on their back for at least a week or two, and would require further saves to avoid decline of Constitution and possibly death.
Now of course, snow is as dangerous as rain, at least, and even colder. And in this period it certainly snows in Albion during the winter (with the cold getting worse as the campaign period progresses). But there's a simple reason why we focus on rain and not snow: you'd have to be INSANE to go traveling through wilderness in the winter. That's something to remember in your campaign in itself: PCs should spend the winter close to home, and not out adventuring; travel would only be short and in safe areas (safe being defined in this period not as actually safe, but as places where you could get from one village/castle to another in under a day of winter travel). If your PCs insist on going out adventuring in mid-winter, feel free to hit them with everything they deserve.
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