RSSRandomized Weather – Work in Progress
Oct 27
I’ve been sorting through my notes and found these charts that allow you to roll up random weather for your campaign. I’m pretty sure I adapted this from Greyhawk’s original box set as Gygax had a real love for detailed charts like these. I decided to clean up the tables, remove things that were rules specific, and expand upon the material so that it would be (somewhat) realistic when applied to an Earth-like world. Of course, this isn’t comprehensive 100% accurate data based on meteorological fact but I did do a little research to make sure things add up so you don’t get 300+mph tornadoes or snow in the tropics.
This is a work-in-progress right now. I’ve left it rules ambiguous for now but will probably adapt it later into FG&G or something. I want to keep expanding it from Gygax’s writing to create something comprehensive and unique. I’ll start by generating weather over an entire year in a fictional world.
Everything below the dotted line is open game content.
—-
The base temperature is measured at the equator and prime meridian of an Earth-like planet. The distance between latitude and longitude is approximately 60 miles. The planet is assumed to have an Earth-like orbit around a similar sun (12 months in a year, 30-31 days in a month, approximately 365 days in a year).
Step 1: Find monthly base temperature. Determine the season to find the base temperature for the month. For every 3 degrees latitude north or south (up to 30 degrees north/south) adjust the temperature down by 1 degree. Adjust the temperature down by one for every 2 degrees of latitude north/south beyond 30 (up to 60) and adjust for every degree beyond 60. For example, at 30N, 60N, and 90N the temperature adjustment would be -10, -30, and -90 respectively.
Modify base temperature by the general terrain of the given area.
| Spring | Month 1 | Month 2 | Month 3 |
| Base temp. (F) | 54 | 62 | 68 |
| Daily high | +1d6+4 | +1d8+4 | +1d10+6 |
| Daily low | -1d10+4 | -1d10+4 | -1d8+4 |
| Weather (d%): | 40% | 44% | 42% |
| Sunrise (a.m.) | 0815 | 0732 | 0644 |
| Sunset (p.m.) | 1856 | 1929 | 1859 |
| Summer | Month 1 | Month 2 | Month 3 |
| Base temp. (F) | 74 | 82 | 90 |
| Daily high | +1d10+6 | +1d8+8 | +1d6+4 |
| Daily low | -1d10+6 | -1d6+6 | -1d6+6 |
| Weather (d%): | 42% | 36% | 33% |
| Sunrise | 0605 | 0552 | 0605 |
| Sunset | 2030 | 2052 | 2059 |
| Autumn | Month 1 | Month 2 | Month 3 |
| Base temp. (F) | 82 | 74 | 68 |
| Daily high | +1d4+6 | +1d8+6 | +1d10+5 |
| Daily low | -1d6+6 | -1d8+6 | -1d10+5 |
| Weather (d%): | 33% | 33% | 36% |
| Sunrise (a.m.) | 0633 | 0702 | 0732 |
| Sunset (p.m.) | 2017 | 1930 | 1841 |
| Winter | Month 1 | Month 2 | Month 3 |
| Base temp. (F) | 64 | 56 | 52 |
| Daily high | +1d10+6 | +1d8+5 | +1d10 |
| Daily low | -1d10+4 | -1d20 | -1d20 |
| Weather (d%) | 40% | 43% | 46% |
| Sunrise (a.m.) | 0806 | 0839 | 0841 |
| Sunset (p.m.) | 1805 | 1756 | 1731 |
| Terrain | Precipitation | Temp. | Wind | Extreme Weather (d%) |
| Hill | – | – | (1d2)+5mph (1)
-5mph (2) |
01-80 Windstorm81-100 Earthquake |
| Forest | – | -5 | -5mph | 1-80 Quicksand81-100 Earthquake |
| Jungle | +10% | +5 | -10mph | 1-5 Volcano6-60 Downpour
61-80 Quicksand 81-100 Earthquake |
| *Swamp/Marsh | +5% | +5 | -5mph | 1-25 Quicksand26-80 Sunshower
81-100 Earthquake |
| Wastes | -25% | +10 (day)-10 (night) | – | 1-40 Flash flood41-70 Dust storm
71-85 Tornado 86-100 Earthquake |
| Plains | – | – | +5mph | 1-50 Tornado51-100 Earthquake |
| Desert | -30% | +10 (day)-10 (night) | +5mph | 1-25 Flash flood26-65 Sandstorm
66-100 Earthquake |
| Mountain | – | -3 degrees Per 1,000’ | +5mph per1,000’ | 1-20 Wind storm21-50 Mud/rock slide
51-75 Avalanche 76-80 Volcano 81-100 Earthquake |
| Coastal | +5% | -5 (winter)+5 (summer) | +5mph | 1-80 Earthquake81-94 Tsunami
95-100 Underwater volcano |
| Sea/small island | +15% | -10 (winter)+5 (summer) | +10mph | 1-20 Tsunami21-40 Underwater volcano
41-100 Underwater earthquake |
* -5 temperature in arctic marshes
Step 2: Find extreme temperature shift. Roll d% to determine if a month’s base temperature will be modified by a period of extreme temperature shift. If an extreme temperature shift occurs, roll 1d20 to determine how long. Extreme temperature shifts occur on a random day: roll 3d10 to determine randomly. If an extreme shift overlaps into the next month, don’t check for an extreme temperature shift in that month until after the previous one has ended.
| d% | Temperature (F) |
| 01 | Extreme low (-15) |
| 02 | Severe low (-10) |
| 03-04 | Low (-5) |
| 05-96 | Normal |
| 97-98 | High (+5) |
| 99 | Severe high (+10) |
| 100 | Extreme high (+15) |
|
1d20 |
Days |
|
1 |
1 |
|
2-3 |
2 |
|
4-10 |
3 |
|
11-14 |
4 |
|
15-17 |
5 |
|
18-19 |
6 |
|
20 |
7 |
Step 3: Determine daily temperature. Roll for the highest and lowest temperatures for each individual day. High occurs at 1300, low occurs one hour before sunrise.
Step 4: Determine weather. Roll d% to determine if adverse weather will happen on a given day. If weather conditions don’t occur, skip to step 5.
4a: Weather type. Roll d% to determine weather conditions then roll for length, any accumulation, and wind speed. If 100 is rolled, roll d% again for special weather conditions based on terrain. If weather can’t occur on a particular day because conditions won’t allow it, discard the roll: there is no temperature for that day.
| d% | Weather | Min.Temp | Max.Temp | Cont.(d%) | ExcludedTerrain |
| 1-2 | Blizzard, heavy | – | 10 | 5% | Desert |
| 3-5 | Blizzard | – | 20 | 10% | Desert |
| 6-10 | Snow, heavy | – | 25 | 20% | – |
| 11-20 | Snow, light | – | 35 | 25% | – |
| 21-25 | Sleet | – | 35 | 20% | – |
| 26-27 | Hail | – | 65 | 10% | Desert, waste |
| 28-30 | Fog, heavy | 20 | 60 | 25% | Desert, waste |
| 31-38 | Fog, light | 30 | 70 | 30% | Desert |
| 39-40 | Mist | 30 | – | 15% | – |
| 41-45 | Drizzle | 25 | – | 20% | – |
| 46-60 | Rain, light | 25 | – | 45% | – |
| 61-70 | Rain, heavy | 25 | – | 30% | – |
| 71-84 | Thunderstorm | 30 | – | 15% | – |
| 85-89 | Tropical storm | 40 | – | 20% | Desert, plains |
| 90-94 | Monsoon | 55 | – | 30% | Desert, waste, plains |
| 95-97 | Gale | 40 | – | 15% | Desert |
| 98-99 | Hurricane/typhoon | 55 | – | 20% | Desert, waste |
| 100 | Extreme weather | – | – | 1% | – |
| Weather | Accumulation(inches) | Length(hours) | WindSpeed(mph) |
| Blizzard, heavy | 2d10+10 | 3d8 | 6d8+40 |
| Blizzard | 2d8+8 | 3d10 | 3d8+36 |
| Snow, heavy | 2d8+2 | 4d6 | 3d10 |
| Snow, light | 1d8 | 2d6 | 4d6 |
| Sleet | 1d2 | 1d6 | 3d10 |
| Hail | None | 1d4 | 4d10 |
| Fog, heavy | None | 1d12 | 1d20 |
| Fog, light | None | 2d4 | 1d10 |
| Mist | None | 2d6 | 1d10 |
| Drizzle | 1d2 | 1d10 | 1d20 |
| Rain, light | 1d3 | 1d12 | 1d20 |
| Rain, heavy | 1d4+3 | 1d12 | 2d12+10 |
| Thunderstorm | 1d8 | 1d4 | 4d10 |
| Tropical storm | 1d6/per day | 1d3 days | 3d12+30 |
| Monsoon | 1d8/per day | 1d6+6 days | 6d10 |
| Gale | 1d8/per day | 1d4 days | 6d8+40 |
| Hurricane/typhoon | 1d10/per day | 1d4 days | 7d10+70 |
| Extreme Weather | Accumulation(inches) | Length | Wind Speed(mph) |
| Sand/dust storm | – | 1d8 hours | 5d10 |
| Wind storm | – | 1d10 hours | 8d10+20 |
| Earthquake | – | 2d12 hours | 1d20 |
| Avalanche/mud/rockslide | 5d10 | 1d10 minutes | 1d20 |
| Volcano | 1d8 | 1d10 days | 1d20 |
| Tsunami | – | 1d2 hours | 1d20 |
| Quicksand | – | – | 1d20 |
| Flash flood | – | 1d6+2 hours | 1d20 |
| Downpour | 1/per hour | 3d4 hours | 1d6-1 |
| Sunshower | .5 | 6d10 minutes | 1d20 |
| Tornado/cyclone | 1/per hour | 5d10 hours | 20d12+70 |
| Oasis | – | – | 1d20 |
4b: Determine weather length. Roll d% to see if weather will continue after its duration. If conditions continue, roll 1d10 to determine if it changes.
| 1d10 | Change |
| 1 | Move up one space on weather chart |
| 2-9 | No change. Roll for duration |
| 10 | Move down one space on weather table |
Step 5: Wind speed, wind chill, and humidity. If no weather conditions occur, roll 1d20-1 to determine the wind speed for the day and modify it based on terrain (otherwise, roll based on the weather conditions). If the temperature for the day is 35 degrees or below, subtract the wind speed from the temperature to find the wind chill. If the temperature is 75 or below, roll d% to determine the relative humidity. Subtract the “precipitation chance” by terrain from the humidity.
—-Description—-
Avalanche/rock/mudslide: An avalanche or rock/mudslide continues down the entire length of a mountain or hill from the point it begins. It slides down in a corn formation, becoming wider at an exponential rate. For every mile a slide travels, an additional 5d10 inches of accumulation occurs. Slides inflict massive structural damage, uprooting trees and toppling buildings.
Earthquake: Earthquakes occur all the time but only rarely are they felt. When an earthquake occurs, it’s of magnitude 1d8+2. An earthquake lasts about one minute. An aftershock will occur at the end of the earthquake’s “duration.” The aftershock is the same magnitude as the main quake minus 1d3. If the aftershock is greater than magnitude 2, roll for duration: another aftershock will occur afterward. Earthquakes with magnitude 2 or fewer aren’t felt.
There’s a 10% chance of a foreshock occurring: a weaker quake before the main one. If a foreshock occurs, roll 1d8+2 then subtract 1d3. At the end of the duration, the main shock occurs at the foreshock’s unmodified strength.
An underwater earthquake creates a tsunami up to 500 miles to the nearest coast.
Flash flood: Flash floods occur over a short period of time, accumulating 1d4+1 inches of water every hour. Up to 2 feet of water is enough to carry away large sized creatures and small structures such as shacks.
Frostbite: Exposed limbs (particularly the fingers and toes) have a 1% chance of becoming frostbitten for every degree below 32. For every 10 minutes exposed, a cumulative 1% chance of becoming frostbitten occurs. 1st degree frostbite in the arms and legs makes movement and combat difficult and are healed after an hour of rest and care in temperatures above 32 degrees. Further exposure leads to stage 2 frostbite which requires 1d20+10 days to heal. Further exposure leads to stage 3 frostbite which can’t be healed: the nerve tissue is dead.
Hail: Hail is usually a few centimeters in diameter. 10% of hailstorms will have hail 1d8 inches in diameter. These massive hail stones inflict 1 point of damage per inch against unarmored creatures creatures or creatures with exposed heads. Structures made from material such as straw, glass, and old wood suffer full structural damage. Metal and sturdy wooden structures subtract 4 points from hail damage. Hardened stone is unaffected by hail of all sizes.
Humidity: When the combined temperature and humidity index are greater than 140 degrees, the requirements for water are doubled. Furthermore, characters with human-like bodily functions must rest for an hour after every 3 hours of travel or risk passing out. Physical exertion, such as running or combat, is difficult and risks exhaustion or heatstroke.
Oasis: Travelers stumble upon a desert oasis. There may be other travelers or creatures present.
Quicksand: Travelers stumble upon a pool of quicksand 1d12 feet deep. Dense creatures, such as those wearing armor, and panicking creatures sink instantly. Quicksand generally isn’t deadly: an unarmored, non-burdened human can crawl on the surface at a rate of 5′ per minute.
Snowy Terrain: On a sunny day, characters without eye protection have a 5% chance per hour to become snowblind in snowy or icy terrain. Snowblind characters are blinded for 1d6 x 10 minutes.
Sunrise/Sunset: Indicates the time the sun crests the horizon. This is not the exact time when light can or can’t be seen. Depending on terrain, light may appear or disappear sooner depending on if the horizon is visible or not. For every degree of latitude north/south of the equator, subtract 2 minutes from the sunrise/sunset. For every degree of longitude east/west of the prime meridian, add 4 minutes. At 60-90 degrees north or south, the sun remains visible during the longest days of summer (mid-summer) and isn’t visible during the longest nights of winter (mid-winter). For every degree north or south of 60 latitude, the sun doesn’t set or doesn’t rise for 2 additional days. At either pole, there are 180 days of perpetual light and 180 days of night.
Tsunami: Tsunamis are a wave of water 10d10 feet in height. 1d4+1 waves occur throughout the duration of the event. The waves move rapidly and destroy structures 1 mile off the coast before they gradually decrease and function as a flood.
Volcano: Lava flows downhill in a random direction until it settles into a depression or reaches water. Ash blankets everything within 10d10 miles of the volcano, causing exposed people to choke.
Underwater volcanoes eventually form whole islands over thousands of years.