Have you noticed that 95% of all RPG wilderness adventures are carried out in what we might call ‘moderate’ weather conditions? The temperature is average and it almost never rains, snows, gets windy or misty/foggy? Some GMs do run games in the desert, or snows of winter but the majority never do and I feel that they are missing a trick by not doing so.
I think that this is a shame as quite normal weather conditions such as rain, wind, snow and mist/fog can bring all sorts of interesting tactical and strategic factors to the adventure.
Let’s start with visibility. If you have spent any time outdoors you know that even fairly light rain or snow can reduce visibility drastically in moments. This reduces encounter distances from hundreds to tens of feet and also gives concealment to enemies and the adventurers. On exercise I have had hidden from experienced men in heavy rain just by squatting down on my haunches. They have walked by me just feet away in broad daylight.
In one battle in the War of the Roses that was fought in a snow storm, one army advanced its bowmen fired one volley of arrows then retreated a hundred yards. The other army fired off most of its arrows in repeated volleys at an enemy that was no longer in range.
In mist or fog enemies that are completely visible one moment are invisible the next. This is because mist and fog are not constant, they have variable density across the field. It can also move and roll across you in thin and thick waves. Mist and fog also limits your ability to navigate. If you don’t stay still you can easily get very lost very quickly. It is a regular cause of death amongst hill walkers and mountaineers many of whom simply walk off cliffs and precipices. Mist and fog also deaden sounds and can make it difficult to get any idea of which direction they are coming from.
In hotter climes marching men and animals produce dust that can also limit visibility as well as betraying your position to enemies miles away. Experienced commanders can determine the size, composition and speed of your forces by the dust they kick up. Imagine a desert orc scout seeing you approach hours before you arrive. And you wonder why they are always ready with an ambush.
In the medieval period rain could seriously limit an army’s firepower by the simple fact that bow and crossbow strings do not like to be soaking wet.
Rain quickly turns earth to mud. This not only reduces movement speed, but sucks at feet or produces a slippery surface that can make walking difficult and running all but impossible. This is why in the old days the marching season for armies was defined as those few summer months when the ground was hard enough to support marching men. In close combat the fighting men will quickly churn up the ground into a deadly quagmire. Even good fighters may fall and then be at the mercy of lesser men. In many medieval battles heavily armoured men who fell over found themselves unable to get up and some even drowned in the mud.
Deep snow can make fast movement impossible and any movement very, very tiring. A fit man who can walk thirty miles in a day across good ground may be reduced to just five across a snowy landscape, even if they have made snow shoes. In many northern lands villages and even towns could be cut off for months in winter.
If you have been a walker or a cyclist you know the sapping effect that even a light wind can have. It cools you down and even if it isn’t a head wind the buffeting effect tires you quickly. A strong wind can stop you dead as it is just too much effort to keep going. Add rain, sand or snow and a strong wind turns from being an impedance to deadly in moments.
Then there is the issue of projectile weapons. A strong wind, regardless of direction, should apply a serious negative modifier to shooting. Even a light wind can cause projectiles to go off course or to land short or long.
In a fantasy RPG you may wish to consider which creatures grew up and are adapted to these conditions. Goblins are seldom seen as a dangerous foe. However, if you say they live on the high moors and have adapted to those conditions then have them attack the party in heavy rain they may become a lot more of a challenge. The characters won’t be able to use their long-ranged firepower, encounter distances will be short and suit the goblins. The goblins will have wide taloned feet that allow them to move quickly over mud, while the characters will be slipping and sliding all over the place.
How about fighting ogres in thick fog, where the ogres have excellent hearing and can pinpoint the position of their foes? Or fighting Yeti in amongst snow where their white fur blends with the background?
So by simply throwing a turn in the weather at the characters you can change a dull encounter into an interesting and possibly deadly battle for survival.