Random Generation: Weather

Random Generation: Weather

As I mentioned in some of my previous posts, I was positively impressed by the weather generation rules from ACKS II. Thus, here is my adaptation of these rules to GURPS. It may look intimidating and complex, but it's actually pretty damn simple. However, I probably will make an automation program to make it even easier in the future.

At the start of each game day of a wilderness expedition (or any other day, if weather effects are important), you can determine the weather for the adventurers’ current 24-mile hex with three rolls of 2d: a temperature roll, a precipitation roll, and a wind roll. Each roll is adjusted by a modifier between -11 and +9 depending on the climate of the hex. After making the roll, use the Daily Weather table below to determine the prevailing weather for the day.
Day/Night Cycle: In most climates, the temperature modifier will be 1 or more points lower during the night. Keep the same roll you made for the day temperature and simply apply the new modifier, or you can re-roll.
Fast Travel: If the adventurers move at a speed that will carry them through two or more 24-mile hexes each day, you can either use the same roll or re-roll. If the hexes have the same climate, we recommend keeping the same roll, but if the climate has changed then re-roll.
Weather Fronts: To simulate the patterns of weather fronts, shift each day’s result by 1 point in the direction of the prior day’s result. Do not shift unmodified 2 or 12 results.
Temperature: If the daytime temperature modifier is +0 or less, use the first column of the table to determine the temperature for both day and night. If the daytime temperature modifier is +1 or more, use the second column. In most situations, you may use the average temperature given in the table, but if exact temperature is required, you may roll an additional die listed in there.
Wind Chill: As the temperature falls and wind speed increases, the effective temperature can drop off drastically. When wind speed is 5 mph or higher, and when temperature is 35 F or lower, cross-index the actual temperature with the maximum wind speed for the day in question on the Wind Chill table to find the effective temperature with the wind chill taken into consideration. Use this effective temperature to determine all temperature-based effects on living creatures.
 
Optional Rule: Simplified Wind Chill
If cross-referencing values to determine the effective temperature is too cumbersome, simply subtract the wind speed from the actual temperature. This should produce results close enough to the ones in the table in most cases.
 
Wind Direction
In certain situations, wind direction plays an important role. If this is the case, roll 1d12 on the following table every 12 hours. If wind direction isn’t important or if there’s no wind, ignore this step.

D12	Wind Direction
1	Northerly
2	Northeasterly
3	Easterly
4	Southeasterly
5	Southerly
6	Southwesterly
7	Westerly
8	Northwesterly
9-12	Prevailing wind direction for this locale

   If you do not know the prevailing wind direction for this locale, roll d8 instead of d12. 
             This table works best for overland travel. For convenience in small scale combat situations, determine the wind direction by rolling Scatter (p. B414).
 
Determining Weather Modifiers
The temperature, precipitation, and wind modifiers are determined by the season of the year cross-referenced against the climate of the region. Each climate is assigned a two- or three-digit climate code based on the Köppen Climate Classification system. If you are a worldbuilder by hobby, you’ll probably have worked out the climate for your entire continent and region. If not, you can use the Climate by Terrain table to decide which climate code to use based on how you’ve imagined the setting.
Continental vs Temperate: These climates are similar in that both occur at middle latitudes and both have four distinct seasons. However, continental climates occur in the interior of continents at middle latitudes, while temperate climates occur on the coasts of continents at middle latitudes. Once you have the climate code for the 24-mile hex, consult the Weather Modifiers by Climate and Season table, opposite, to find the temperature, precipitation, and wind modifiers.
Sea Weather: To determine weather at sea, use the closest land climate code and add +2 to the Wind modifier.
 

Daily Weather Table

2d result + modifier	Temperature (+0 or less)	Temperature (+1 or more)	Precipitation	Wind
-7 or less	-74 F -1d10 F (avg. -79 F)	-	Sunbaked	Still
-6	-64 -1d10 F (avg. -69 F)	-	Sunbaked	Still
-5	-54 - 1d10 F (avg. -59 F)	-	Sunbaked	Still
-4	-49 - 1d6 F (avg. -52 F)	-	Sunbaked	Still
-3	-39 - 1d10 F (avg. -44 F)	-	Sunbaked	Still
-2	-23 - 2d8 F (avg. -32 F)	-	Sunbaked	Still
-1	-11 - 1d12 F (avg. -17 F)	-	Clear	Still
0	-5 - 1d6 F (avg. -8 F)	32 + 1d4 F (avg. 34 F)	Clear	Still
1	-6 + 1d6 F (avg. -3 F)	36 + 1d4 F (avg. 38 F)	Clear	Still
2	5 + 1d10 F (avg. 10 F)	40 + 1d10 F (avg. 45 F)	Clear	Still
3	15 + 1d10 F (avg. 20 F)	50 + 1d6 F (avg. 53 F)	Clear	Still
4	25 + 1d6 F (avg. 28 F)	56 + 1d6 F (avg. 59 F)	Partly Cloudy	Still
5	32 + 1d4 F (avg. 34 F)	62 + 1d6 F (avg. 65 F)	Mostly Cloudy	2 mph
6	36 + 1d4 F (avg. 38 F)	67 + 1d4 F (avg. 69 F)	Overcast	4 mph
7	40 + 1d10 F (avg. 45 F)	70 + 1d4 F (avg. 72 F)	Drizzly/Flurry/Foggy	5 mph
8	50 + 1d6 F (avg. 53 F)	75 + 1d4 F (avg. 77 F)	Drizzly/Flurry/Foggy	10 mph
9	55 + 1d6 F (avg. 58 F)	79 + 1d3 F (avg. 81 F)	Drizzly/Flurry/Foggy	15 mph
10	60 + 1d6 F (avg. 63 F)	82 + 1d4 F (avg. 84 F)	Rainy/Snowy/Foggy	20 mph
11	65 + 1d4 F (avg. 67 F)	86 + 1d4 F (avg. 88 F)	Rainy/Snowy/Foggy	25 mph
12	68 + 1d4 F (avg. 70 F)	90 + 1d4 F (avg. 92 F)	Rainy/Snowy/Foggy	35 mph
13	-	94 + 1d4 F (avg. 96 F)	Rainy/Snowy/Foggy	40 mph
14	-	98 + 1d6 F (avg. 101 F)	Rainy/Snowy/Foggy	50 mph
15	-	103 + 1d6 F (avg. 106 F)	Rainy/Snowy/Foggy	55 mph
16	-	108 + 1d4 F (avg. 110 F)	Rainy/Snowy/Foggy	60 mph
17	-	112 + 1d4 F (avg. 114 F)	Rainy/Snowy/Foggy	65 mph
18	-	115 + 1d6 F (avg. 118 F)	Rainy/Snowy/Foggy	70 mph
19 or more	-	120 + 1d10 F (avg. 125 F)	Rainy/Snowy/Foggy	70+1dx5 mph

 
Wind Chill Table
This is a table from AD&D Wilderness Survival Guide, but there apparently is a similar one in GURPS Basic Set 3e.

Temp., F	Wind velocity, mph
	5	10	15	20	25	30	35	40+
35	35	20	15	10	5	5	0	0
30	25	15	10	5	0	0	-5	-5
25	20	10	0	-5	-5	-10	-15	-15
20	15	0	-5	-10	-15	-20	-20	-25
15	10	-5	-10	-15	-20	-25	-30	-30
10	5	-10	-20	-25	-30	-35	-35	-40
5	0	-15	-25	-30	-35	-40	-45	-45
0	-5	-20	-35	-40	-45	-50	-50	-55
-5	-10	-25	-40	-45	-50	-55	-60	-65
-10	-15	-30	-45	-50	-60	-60	-65	-70
-15	-20	-40	-50	-60	-70	-75	-80	-80
-20	-25	-45	-60	-70	-75	-80	-85	-90
-25	-30	-50	-65	-75	-85	-90	-90	-95
-30	-35	-60	-70	-80	-90	-95	-100	-110
-35	-40	-65	-80	-90	-95	-100	-105	-110
-40	-45	-70	-85	-95	-105	-110	-115	-115
-45	-50	-75	-90	-100	-110	-115	-120	-120
extra -5	extra -5	extra -5	extra -5	extra -5	extra -5	extra -5	extra -5	extra -5

Weather Modifiers by Climate and Season Table

Unfortunately, I don't think I'm allowed to provide the modifier table. See page 41 of ACKS II Judges Journal or make one yourself.

Effects of Weather

Weather may have the following game mechanical effects:
 
Temperature
Cold temperatures outside the character’s comfort zone (below 35 F for humans) may cause FP loss (p. B430). Hot temperatures outside the character’s comfort zone (above 90 F) and even in the top 10 degrees of the comfort zone may cause FP loss, increase FP loss from exertion and dehydration, and increase water consumption (p. B434).
 
Sunburn
Spending at least 4 hours outdoors in the sun with unprotected skin during Sunbaked condition deals 1d-3 damage due to sunburn.
 
Wind
                The most comprehensive rules for wind effects can be found on page 14 of GURPS Disasters: Hurricane. For each level of the Beaufort Scale, you get values for penalized Move, attribute and skill penalties, and damage to artifacts. However, that’s far from everything.
                Climbing: Under hurricane conditions, Climbing rolls are at -2 in addition to all the normal penalties.
                Drifting Effects: Airborne persistent effects with Drifting (p. B105) move with the wind. Divine wind speed by 2 to convert miles per hour to yards per second.
                Flight: Those with Flight (Lighter Than Air) subtract 1 yard/second from air Move per 5 mph of wind speed if moving against the wind, or add 1 yard/second to air Move per 5 mph of wind speed if moving with it. Those with Flight (Lighter Than Air, Passive) use the same method to derive horizontal air Move. Those with Flight without Lighter Than Air subtract or add 1 yard/second per 10 mph of wind speed. Those with Walk on Air must roll against DX once per minute in hurricane-force winds (73+ mph) to avoid falling.
                Hearing: Hearing and Vibration Sense (Air) rolls are negatively affected by wind. The penalty is listed in the Beaufort Wind Scale table.
                Sandstorm: Wind creates dust clouds and sandstorms in deserts. The Hearing penalty listed in the table also applies to Vision, Infravision, and Ultravision rolls.
                Shooting: Crosswinds may push a projectile off course. This comes into play when shooting at extended ranges. The exact range where the penalties begin to accumulate depends on the type of projectile. The base penalty is equal to the IQ roll penalty listed in the Character Effects column in the Beaufort Wind Scale table. It doubles per step on the Size and Speed/Range Table (p. B550) beyond the base range for the projectile type.
                For arrows, bolts, and sling bullets, wind deflection starts at 150 yards. For firearm bullets, wind deflection starts at 300 yards. For thrown weapons, wind deflection starts at 500 yards. For blowpipe darts, wind deflection starts at 30 yards.
                For example, an arrow fired in 35 mph crosswind (-1 base penalty) at a target 200 yards away (one step on the table from the base range of 150 yards) would take a -2 penalty.
                Unprotected Flames: Wind speeds of 50 mph or higher automatically extinguish unprotected flames, such as candles and torches.
                Vehicles: Water and air vehicle control rolls made by the operator take a penalty equal to the DX penalty imposed on the characters. For air vehicles, this penalty is doubled when close to the ground or maneuvering around mountains or tall buildings. The effect of wind speed and direction on sailing speed is described on pages 158-159 of GURPS Vehicles.
                Water is considered rough for the purpose of attacking from a moving vehicle and bad footing penalties when wind speed is 10 mph or higher.
 

Beaufort Wind Scale

These speeds and effects are valid for Earth near sea level. Effects at each level are not cumulative with previous levels. The wave height is the average wave height at sea. Note that the intensity of effects is also related to the atmospheric density; at extreme elevations, such as on top of a mountain, use the effects of one level lower for every 5,000’ above sea level.

Beaufort Scale	Description	Wind Speed, mph	Wave Height, ft.	Move Effect	Character Effects	Artifact Effects	Hearing
0	Calm	<1	0	-	-	-	-
1	Light Air	1-3	0-1	-	-	-	-
2	Light Breeze	4-7	1-2	-	-	-	-
3	Gentle Breeze	8-12	2-3.5	-	-1 to all DX rolls	-	-1
4	Moderate Breeze	13-18	3.5-6	-	-1 to all DX rolls	-	-1
5	Fresh Breeze	19-24	6-9	-	-1 to all DX rolls	-	-2
6	Strong Breeze	25-31	9-13	-	-1 to all DX rolls	-	-3
7	High Wind	32-38	13-19	Move/2	-2 to all DX rolls; -1 to all IQ, ST, skill, and self-control rolls	-	-4
8	Gale	39-46	18-25	Move/2	-2 to all DX rolls; -1 to all IQ, ST, skill, and self-control rolls	-	-5
9	Strong Gale	47-54	23-32	Move/2	-2 to all DX rolls; -1 to all IQ, ST, skill, and self-control rolls	-	-6
10	Storm	55-63	29-41	Move/4	-3 to all DX rolls; -2 to all IQ, ST, skill, and self-control rolls	2 points of damage per minute of continuous wind	-7
11	Violent Storm	64-72	37-52	Move/4	-3 to all DX rolls; -2 to all IQ, ST, skill, and self-control rolls	10 points of damage per minute of continuous wind	-8
12	Hurricane	>73	>46	Move/10	-4 to all DX rolls; -3 to all IQ, ST, skill, and self-control rolls	50 points of damage per minute of continuous wind	-9

 
Clouds
Clear, Partly Cloudy: These results are purely descriptive.
Mostly Cloudy: Clouds are heavy enough to worsen the darkness penalty on moonless nights from -7 to -8.
Overcast, Drizzly, Rainy, Flurry, Snowy: Clouds are heavy enough to worsen the darkness penalty by -1 at twilight or night when moon is present, or by -2 on moonless nights. Thus, twilight hours with no clouds impose a darkness penalty of -2, and it becomes -3 when Overcast. Cloudless moonless nights impose a darkness penalty of -7, while Overcast moonless nights decease it to -9. Stars are not visible at night under such cloud cover, imposing a -5 penalty on Navigation (Air or Sea).
For more information on illumination levels see Light and Darkness in My Games.
 
Precipitation
If the Temperature is 32 F or lower (i.e. below freezing), then Drizzly precipitation creates Flurry conditions and Rainy precipitation creates Snowy conditions. If the Wind is Still, then Drizzly or Rainy precipitation creates Foggy. Such conditions impede Vision rolls based on normal vision and Infravision, but not Ultravision. The severity of the impairment is modeled with Range Increments. Each multiple of a Range Increment results in ‑1 to Vision. The listed increments are for open terrain; double them while beneath heavy vegetation.
 
                Drizzly: Drizzly conditions represent either short rain showers periodically through the day or light but continuous rainfall of about 1/4” per day. The Range Increment for Vision impediment is 50 yards. Hearing and Vibration Sense (Air) rolls take a -2 penalty. Alternatively, if you’d like more variety, roll 1d4x25 yards to find the Range Increment for Vision rolls and 1d3 to find the Hearing penalty. Mud forms after three days of Drizzly conditions.
                Rainy: Rainy conditions bring heavy or violent precipitation of at least 1” per day. The Range Increment for Vision impediment is 10 yards. Hearing and Vibration Sense (Air) rolls take a -5 penalty. Alternatively, if you’d like more variety, roll 4d6 yards to find the Range Increment for Vision rolls and 1d4+3 to find the Hearing penalty. Mud forms after one day of Rainy conditions. An adventurer who endures rainy conditions for seven consecutive days must make a HT roll to avoid catching a disease. Halve overland travel speed.
 
                Flurry: Flurry conditions represent either periodic snow showers or light but continuous snow flurries at a rate of about 1/2” per day. The Range Increment for Vision impediment is 50 yards. Hearing and Vibration Sense (Air) rolls take a -1 penalty. Alternatively, if you’d like more variety, roll 1d3x25 yards to find the Range Increment for Vision rolls. Snow forms after three days of Flurry conditions.
                Snowy: Snowy conditions bring heavy snowfall of at least 10” per day. The Range Increment for Vision impediment is 10 yards. Hearing and Vibration Sense (Air) rolls take a -5 penalty. Alternatively, if you’d like more variety, roll 4d6 yards to find the Range Increment for Vision rolls and 1d4+3 to find the Hearing penalty. Snow accumulates after a day of snowy weather. An adventurer who endures snowy conditions for seven consecutive days must make a HT roll to avoid catching a disease. Halve overland travel speed.
 
                Foggy: Foggy conditions can arise from hazy mist, or dense fog. The Range Increment for Vision impediment is 100 yards. Alternatively, if you’d like more variety, roll 1d20x10 yards to find the Range Increment for Vision rolls. Halve overland travel speed.
 
                Mud and Snow
                Mud accumulates after one day of Rainy or three days of Drizzly conditions in grasslands or scrublands terrain. Once mud forms, adventurers have their speeds halved for all purposes unless traveling on paved roads. Earth and gravel roads provide no benefit in mud and wheeled vehicles cannot move in mud except on paved roads. Mud dries in one day of 95 F or hotter weather with no precipitation, or seven days of moderate weather with no precipitation and freezes in one day of 32 F or colder weather with no precipitation.
    Snow accumulates after one day of Snowy or three days of Flurry conditions. Once snow accumulates, adventurers have their speeds halved for all purposes. Roads provide no benefit and wheeled vehicles cannot move in snow. Snow melts in seven days of moderate temperatures or one day of 95 F or hotter temperatures, creating mud (as above). A combination of snow and Sunbaked conditions may cause snow-blindness (GURPS Tactical Shooting: Extreme Conditions, p. 9).
 

Climate by Terrain Table

Köppen Code	Name	Biome	Typical Terrain Type
Af	Tropical rainforest	Rainforest	Jungle
Am	Tropical monsoon	Rainforest	Jungle
Aw	Tropical savanna with a dry winter	Savanna	Grassland (savanna)
As	Tropical savanna with a dry summer	Savanna	Grassland (savanna)
BWh	Hot arid desert	Desert	Desert (sandy or rocky)
BWk	Cold arid desert	Desert	Desert (sandy or rocky)
BSh	Hot semi-arid steppe	Semi-Arid Desert	Barrens (rocky)
BSk	Cold semi-arid steppe	Steppe	Grassland (steppe)
Csa	Temperate with a dry hot summer	Scrub	Scrubland (low, sparse or high, dense)
Csb	Temperate with a dry warm summer	Scrub or Forest	Scrubland (high, dense) or Forest (deciduous)
Csc	Temperate with a dry cold summer	Forest	Forest (deciduous)
Cwa	Temperate with a dry winter and hot summer	Forest	Forest (deciduous)
Cwb	Temperate with a dry winter and warm summer	Forest	Forest (deciduous)
Cwc	Temperate with a dry winter and cold summer	Forest	Forest (deciduous)
Cfa	Temperate and damp year-round with a hot summer	Forest	Forest (deciduous)
Cfb	Temperate and damp year-round with a warm summer	Forest	Forest (deciduous)
Cfc	Temperate and damp year-round with a cold summer	Taiga	Forest (taiga)
Dsa	Continental with a hot dry summer	Forest	Forest (deciduous)
Dsb	Continental with a warm dry summer	Taiga	Forest (taiga)
Dsc	Continental with a cold dry summer	Taiga	Forest (taiga)
Dwa	Continental with a dry winter and hot summer	Steppe	Grassland (steppe)
Dwb	Continental with a dry winter and warm summer	Steppe	Grassland (steppe)
Dwc	Continental with a dry winter and cold summer	Steppe	Grassland (steppe)
Dwd	Continental with a dry winter and very cold winter	Tundra	Barrens (tundra)
Dfa	Continental and damp year-round with a hot summer	Prairie	Grassland (prairie)
Dfb	Continental and damp year-round with a warm summer	Taiga	Forest (taiga)
Dfc	Continental and damp year-round with a cold summer	Taiga	Forest (taiga)
Dfd	Continental and damp year-round with a very cold winter	Taiga or Tundra	Forest (taiga) or Barrens (tundra)
ET	Polar tundra	Tundra	Barrens (tundra)
EF	Polar ice cap	Tundra	Barrens (tundra)

 
                Supernatural Weather
                Optionally, roll 3d at the start of each week, then roll 1d8 to determine the day, rerolling the results of 8. A result of 17 or 18 indicates supernatural weather. The table of natural weather phenomena probably should be region-dependent, but you can see some examples in these posts.
 
Elemental Hazards - https://enragedeggplant.blogspot.com/2022/03/hazardsequipment-elemental-hazards.html
Horror Environments - https://enragedeggplant.blogspot.com/2018/12/hazards-horror-environments.html
Supernatural Desert Hazards - https://enragedeggplant.blogspot.com/2018/11/supernatural-desert-hazards.html
Supernatural Frostfell Hazards - https://enragedeggplant.blogspot.com/2018/09/supernatural-frostfell-hazards.html
Supernatural Sea Hazards - https://enragedeggplant.blogspot.com/2018/11/supernatural-sea-hazards.html
Vile Weather - https://enragedeggplant.blogspot.com/2018/04/vile-weather.html