## Wednesday, 23 January 2013

### A Chance of Snow

Every day in the frozen north brings a chance for snow. By leaving the relative safety of Rovaniemi and the surrounding villages behind, Külk and Joonas are taking a chance on the weather and the terrain. The wilds are not to be travelled without careful thought...

I had been fast and loose with the environment and weather up until now. I had always assumed that the PCs had some kind of provisions, and that they were wrapping up warm. I kept dropping hints that the weather was going to get worse though, and that heading north would lead to severe snowfall. Precautions must be taken etc.

However, I didn't want to railroad any blizzards or consequences. I wanted the weather to be unpredictable. So before the session on Tuesday I thrashed out a kind of probability-based "Fibonacci-like weather generator". A Fibonacci sequence is dependent on the numbers that came before. So the third number is dependent on the second and first numbers, the fourth number is dependent on the third and second and so on. I decided that a simple way to include this was to say that "today's weather is dependent on the previous two days, with some randomness."

In Somewhere North there are three kinds of weather: Clear, Snow and Blizzard. If the weather is Clear then there is no real cloud in the sky; the odd spot of snow maybe, but in general, travel is easy. If there is Snow then there is a generally gentle fall, 2d4 inches across a day at most. Tracks made at the start of the day will be covered hours later and will take a difficult tracking or search roll to find. Travel could be hard-going at times, but is not impossible. Maps are easily followed.

If there is a Blizzard things are much worse. You can see d2 times 10 metres in front of you at best. The terrain is difficult to pass. 2d3 inches of snow will fall in an hour for 3d8 hours of the day, with the wind constantly whipping it up in front of you. If you're on a sled and stop, a drift of 2d3 inches will accumulate along the edge facing the wind, and you might have to dig yourself out. You have to take care of animals especially. In a Blizzard, every hour of travel the party leader must make a WIS check. Failure means that the party is lead 5 times d4 degrees off course.

Today's weather is dependent on the previous two days, with some randomness, means that the previous two days of Clear, Snow or Blizzard conditions influence the probability distribution (checked with a d10 roll) of what today's weather is:

 Day Before Yesterday Yesterday Today (d10 roll) Clear Clear Clear: 1 Snow: 2-8 Blizzard: 9-10 Clear Snow Clear: 1-3 Snow: 4-9 Blizzard: 10 Clear Blizzard Clear: 1-2 Snow: 3-7 Blizzard: 8-10 Snow Clear Clear: 1-3 Snow: 4-9 Blizzard: 10 Snow Snow Clear: 1 Snow: 2-7 Blizzard: 8-10 Snow Blizzard Clear: 1 Snow: 2-6 Blizzard: 7-10 Blizzard Clear Clear: 1-3 Snow: 4-8 Blizzard: 9-10 Blizzard Snow Clear: 1-2 Snow: 3-7 Blizzard: 8-10 Blizzard Blizzard Clear: 1 Snow: 2-5 Blizzard: 6-10

Interesting? How do you do weather in your games?

It would also be neat to hack together some kind of weather pattern simulator for, say, a month. Given the nine initial two day condition pairs and the corresponding distributions, how easy is it to forecast the week ahead? If it has been clear for the last two days, how confident can you be about the next week? And if you've been barely hanging on for the last two days in a blizzard, what are the odds that you're going to stay stuck?