Weather tables for RPGs

WEATHER PATTERN        DRY        AVERAGE    RAINY
=====================  ========   ========   ========
clear sky              01-65      01-25      01-15
cloudy                 66-80      26-50      16-35
light rain             81-90      51-70      36-60
drizzle/steady rain    91-95      71-85      61-75
downpour               96-98      86-95      76-90
thunderstorms          99-00      96-99      91-97
hurricane/monsoon/etc  N/A        00         98-00

Roll one or more weather tables for each climate type: roll 50x percentile and label them with odd numbers 1-99. even numbers represent "transition" days; in a dry climate, if you have 1=clear, 3=drizzle, and 5=clear, 2 and 4 might well be clear days too, with the third day being clear for some portion of the day, with just a passing storm dumping some rain for a few hours. In an average climate, days 2 and 4 might be cloudy, while in a rainy climate day two might be cloudy and day 4 might be a light rain or more drizzle. You can either fill in the transition days when building the table, or leave them open to define per-session.

Once you have a couple of tables for each climate (say 3 each), you just bring one of each table (e.g. dry1, average1, rainy1) to a session and cycle through them between sessions. Each time the party heads out, just roll a single percentile dice: this will determine where on the table the weather pattern starts; then you just move up through the table each day, cycling back to 1 if/when you hit 100.

EXAMPLE:
I am using the table "Rainy2", which I rolled up as follows (using the atmospheric noise RNG of Random.org -- I thought it was appropriate!):
01: 54 - light rain
03: 11 - clear sky
05: 24 - cloudy
07: 68 - steady rain
09: 75 - steady rain
11: 62 - steady rain
13: 19 - cloudy
15: 33 - cloudy
17: 56 - light rain
19: 70 - steady rain
21: 73 - steady rain
23: 68 - steady rain
25: 40 - light rain
27: 95 - thunderstorms
29: 35 - cloudy
31: 51 - steady rain
33: 76 - downpour
35: 72 - steady rain
37: 45 - light rain
39: 99 - monsoon or thunderstorms
41: 80 - downpour
43: 86 - downpour
45: 06 - clear sky
47: 17 - cloudy
49: 36 - light rain
51: 91 - thunderstorms
53: 32 - cloudy
55: 80 - downpour
57: 68 - steady rain
59: 40 - light rain
61: 43 - light rain
63: 74 - steady rain
65: 32 - cloudy
67: 83 - downpour
69: 35 - cloudy
71: 85 - downpour
73: 36 - light rain
75: 56 - light rain
77: 56 - light rain
79: 57 - light rain
81: 29 - cloudy
83: 76 - downpour
85: 48 - light rain
87: 59 - light rain
89: 93 - thunderstorms
91: 44 - light rain
93: 66 - steady rain
95: 04 - clear sky
97: 00 - monsoon or thunderstorms
99: 07 - clear sky

You'll note that 39 and 97 are labeled "monsoon or thunderstorms" - I've decided that I should only have a monsoon happen if it's "monsoon season", and that will be decided on a per-session basis (also, an inland rainy area probably wouldn't have monsoons).

EXAMPLE
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So my adventurers head out... I roll one percentile, and get 8.

That falls between 7 and 9 which are both steady rain, so on day one of their adventure, it's raining steadily. The ground's a bit muddy, but it's not causing much difficulty. Day 4 (which is 12 on the table) the rain finally starts letting up a little. As they're bedding down at the end of day 5 (13/cloudy), the clouds finally part and they see the stars for the first time on the road. The ground's still wet but they're thankful for the reprieve.

Since this is the first day in a few that it's stopped raining, there may be more animals out and about, offering a chance encounter of a pack of hungry wolves (which the party may just be able to shoo off) or perhaps some deer that have come out to forage that might provide some fresh meat for dinner.

Clouds start forming again the afternoon of day 6, promising more rain, but it stays dry through day 8. It starts raining lightly again on day 9 (17/light rain), but just as the rain starts picking up on day 10, they arrive at their destination.

Now the next time they head out in this session, I stick with the same table, and roll one percentile -- it comes up 93.

There's a steady rain, and they decide to wait a day or so to see if conditions improve. 95 (two days out) is clear sky so I tell them that on the next day the rain lets up and it's just cloudy with intermittent sprinkles. They figure this is good enough and hit the road -- so day 1 of their travel is 94.

They're rudely awakened the morning of the third day (97) with a sudden storm! The rain is coming down in sheets, and the ground trembles with the thunderclaps. By the time they've hastily broken camp, small rivulets are already running along the ground where they slept scant minutes before. They (wisely) decide to make for higher ground, and as they huddle under an outcropping on a large rock formation, they hear an increasing rumble that builds out of the background noise of the rain, and the ranger spots a flash flood in the distance that obliterates the remains of their campsite. They end up losing two days of travel while they wait out the storm and let the ground dry a bit.

The clouds finally break on the fifth day so they head out to make up for lost time; the sixth day is 100, meaning day 7 will be 1 (light rain), so I give them a little break and keep the sky clear for the sixth day. They're able to make up some time, and on the seventh day, under a light rain, they reach the city.
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As you can see, a single weather table can easily provide "random" weather for a full session. Obviously you don't have to narrate the weather day-by-day, but you can use it to add flavor to their travel and "encounters" both actor-related (e.g. wolves/deer/etc) and (like the flash flood in my example) that provide a feel that something's happening without the same repetitive "oh grab your dice, here comes the 'random' encounter".

With different tables for different climates, and multiple tables to use from session to session, this can paint a picture of a diverse and changing environment without having to stop and roll "random" weather every day or so of travel. You could make it even more interesting by adding a "wind" table to use in conjunction, so perhaps the hurricanes/monsoons/etc only happen when you get strong winds+strong rains, and strong winds in a dry open environment might mean a tornado or a dust storm!