If you’re running a wilderness campaign, you know that at some point you have to think about the weather. Weather can create interesting challenges for a party of explorers, or even just lend to the mood of a session. Referees can always just arbitrarily determine the weather based on their needs, but for long treks across the wilderness seem to call for randomized weather.

I’ve tried a few different schemes for randomly determining weather in my years of writing hexcrawls, but for the last couple of years have used a system that I think is relatively easy, and provides something usable, rather than trivial.

In each of my later hexcrawls, I begin my section on regional weather with this:

“You can use the following tables to determine the overall weather conditions during a hex crawl. The table is divided into the four seasons. Temperature is determined by rolling 1d6 and comparing the roll to the chances of temperature being freezing (below 30°), cold (31-60°), mild (61-85°), warm (86-95°) or hot (96° or higher). Freezing, cold and hot temperatures might require the adventurers to take steps to avoid negative consequences. Precipitation is a percentage chance. If the temperature is below freezing, the precipitation is snow (10% chance of hail). The TK can decide how much rain or snow falls during the day and its duration based on how much she wishes the weather to hinder the players.”

This is followed by a table like this:

Western Wood

	Winter	Spring	Summer	Fall
Freeze	1-2	1	–	1
Cold	3-5	2	1	2
Mild	6	3-5	2-3	3-5
Warm	–	6	4-5	6
Hot	–	–	6	–
Rain	55%	45%	45%	40%

The table provides a bare-bones account of the weather on any given day, which the TK can flesh out as much or as little as he likes.

The upper portion determines the general range of temperature based on the season, rolled on D6, while the last line is the percent chance of precipitation that day rolled on D%. If the weather is freezing, any precipitation that comes up is snow or maybe hail. Otherwise, precipitation is rain. How much rain? That’s up to the TK. If the TK wants the rain/snow to be a real problem for the PC’s, then it is heavy. Otherwise, it’s a moderate or light rain that provides mood and interest, without becoming a major pain in the butt.

Making the Tables

To make the tables, I could just make up the numbers willy-nilly. There’s nothing wrong with that approach, but I usually like to keep things more realistic. To that end, I choose a city in an environment like the one I’m simulating, and look it up on Wikipedia.

If I’m lucky, the Wikipedia page has a table like this one for Rio de Janeiro:

To figure out the percent chance of precipitation, I just take the total of the average days of rain for the three months that make up a “season”, such as December, January and February for Summer (I almost wrote “winter” until I remembered I was working south of the equator), and divide by 90. In this example, Rio would have a 32% chance of rain during the summer season.

I then take the average high, daily mean and average low for each of those three months, and rate it using the scale mentioned above and repeated here: Freezing (below 30°), cold (31-60°), mild (61-85°), warm (86-95°) or hot (96° or higher).

That gives me 9 temperature readings for each season – I use those to determine the chance on a D6 of a day falling into one of those temperature ratings. Using Rio in the summer again, we get the following temperature ratings:

	DEC	JAN	FEB
Average High	Mild	Warm	Warm
Mean	Mild	Mild	Mild
Average Low	Mild	Mild	Mild

So, we have 7 milds and 2 warms. Seven divided by nine is 78%. Multiply that by 6 (i.e. D6) and you get 5. That means a 5 in 6 chance of mild weather. We don’t need to do the calculation for warm, in this case – it would be 1 in 6, but if we had more temperature ranges, we would use the same procedue for each. Naturally, the Referee can intervene a bit in these figures. Because Rio can get quite hot in the summer, I decide to go 1-4 = mild, 5 = warm and 6 = hot.

You do this same process for the other seasons, and you end up with a table like this:

Rio de Janeiro

	Winter	Spring	Summer	Fall
Freeze	–	–	–	–
Cold	–	–	–	–
Mild	1-6	1-5	1-4	1-5
Warm	–	6	5	6
Hot	–	–	6	–
Rain	17%	29%	32%	26%

So, if I’m running some adventurer in the region around Rio during the summer – maybe they’re searching for some ancient ruins or a satellite that crashed in the region – I roll 1d6 and d%, On the d6, I get a “6”, meaning it’s a hot day. On the % I get a 53, meaning no rain – just humidity.