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Mastering the Wild / Weather in the Wilderness / Temperature Variations and Precipitation

Precipitation

Source Ultimate Wilderness pg. 167
The next step in setting the local weather is to determine whether precipitation occurs and the intensity of that precipitation, if any.

Precipitation Frequency: Precipitation frequency is organized into five levels: drought, rare, intermittent, common, and constant. A region’s baseline precipitation frequency is set by the season, but it can be modified by the climate and other factors (for example, see the Deserts sidebar on page 168). A precipitation frequency can’t be reduced lower than drought or increased higher than constant. Check each day to determine whether precipitation occurs for that day; the percentage chance of precipitation occurring is summarized on Table 4–18: Daily

Table 4-18: Daily Precipitation Chances

Frequency	Chance of Precipitation
Drought	5% (decrease precipitation intensity by 2 steps
Rare	15%
Intermittent	30%
Common	60%
Constant	95%

Precipitation Intensity: The baseline precipitation intensity is dependent on the elevation and can be modified by the climate. Intensity has four categories. Light precipitation is the lowest level of intensity and generally consists of fog, a faint drizzle of rain or a few isolated flakes of snow. Medium precipitation represents a noticeable but not distracting fall of rain or snow. Heavy precipitation typically consists of a driving rainstorm or significant snowfall. Torrential precipitation is the highest level of intensity and consists of a deluge of rain or snow with conditions that can approach whiteout levels. Precipitation intensity can never be reduced below light or increased above torrential.

Once you establish the intensity and the temperature, you’ll determine how the precipitation manifests.

Precipitation Form: Precipitation can result in more than just rain. Depending upon the intensity and temperature, precipitation can range from a light fog or a faint drizzle of rain to a blizzard or thunderstorm. Once you know precipitation of a specific intensity is occurring, set the time of day the precipitation event occurs by rolling 1d12 to find the starting hour of the day and 1d6 to determine whether the hour is a.m. or p.m. (1–3 = a.m., 4–6 = p.m.).

Next, use the appropriate table below for the baseline precipitation intensity and whether the temperature is above or below freezing (32° F) to generate the specific type of precipitation and its duration. (Remember that the temperature is lower at night!) See the Cloud Cover section for the effects of precipitation on visibility for flying creatures.

Table 4-19: Light Unfrozen Precipitation

d%	Precipitation	Duration
1-20	Light fog	1d8 hours
21-40	Medium fog	1d6 hours
41-50	Drizzle	1d4 hours
51-75	Drizzle	2d12 hours
76-90	Light rain	1d4 hours
91-100	Light rain (sleet if below 40° F)	1 hour

Table 4-20: Light Frozen Precipitation

d%	Precipitation	Duration
1-20	Light fog	1d6 hours
21-40	Light fog	1d8 hours
41-50	Medium fog	1d4 hours
51-60	Light snow	1 hour
61-75	Light snow	1d4 hours
76-100	Light snow	2d12 hours

Table 4-21: Medium Unfrozen Precipitation

d%	Precipitation	Duration
01-10	Medium fog	1d8 hours
11-20	Medium fog	1d12 hours
21-30	Heavy fog	1d4 hours
31-35	Rain	1d4 hours
36-70	Rain	1d8 hours
71-90	Rain	2d12 hours
91-100	Rain (sleet if below 40° F)	1d4 hours

Table 4-22: Medium Frozen Precipitation

d%	Precipitation	Duration
1-10	Medium fog	1d6 hours
11-20	Medium fog	1d8 hours
21-30	Heavy fog	1d4 hours
31-50	Medium snow	1d4 hours
51-90	Medium snow	1d8 hours
91-100	Medium snow	2d12 hours

Table 4-23: Heavy Unfrozen Precipitation

d%	Precipitation	Duration
1-10	Heavy fog	1d8 hours
11-20	Heavy fog	2d6 hours
21-50	Heavy rain	1d12 hours
51-70	Heavy rain	2d12 hours
71-85	Heavy rain (sleet if below 40° F)	1d8 hours
86-90	Thunderstorm	1 hour
91-100	Thunderstorm	1d3 hours

Table 4-24: Heavy Frozen Precipitation

d%	Precipitation	Duration
1-10	Medium fog	1d8 hours
11-20	Heavy fog	2d6 hours
21-60	Light snow	2d12 hours
61-90	Medium snow	1d8 hours
91-100	Heavy snow	1d6 hours

Table 4-25: Torrential Unfrozen Precipitation

d%	Precipitation	Duration
1-5	Heavy fog	1d8 hours
6-10	Heavy fog	2d6 hours
11-30	Heavy rain	2d6 hours
31-60	Heavy rain	2d12 hours
61-80	Heavy rain (sleet if below 40° F)	2d6 hours
81-95	Thunderstorm	1d3 hours
96-100	Thunderstorm	1d6 hours

Table 4-26: Torrential Frozen Precipitation

d%	Precipitation	Duration
1-5	Heavy fog	1d8 hours
6-10	Heavy fog	2d6 hours
11-50	Heavy snow	1d4 hours
51-90	Heavy snow	1d8 hours
91-100	Heavy snow	2d12 hours

Drizzle: Drizzle reduces visibility to three-quarters of the normal range, imposing a –2 penalty on Perception checks. It automatically extinguishes tiny unprotected flames (candles and the like, but not torches).

Fog, Heavy: Heavy fog obscures all vision beyond 5 feet, including darkvision. Creatures 5 feet away have concealment. Heavy fog typically occurs early in the day, late in the day, or sometimes at night, but the heat of the midday usually burns it away. Heavy fog occurs only when there is no or light wind.

Fog, Light: Light fog reduces visibility to three-quarters of the normal ranges, resulting in a –2 penalty on Perception checks and a –2 penalty on ranged attacks. Light fog typically occurs early in the day, late in the day, or sometimes at night, but the heat of the midday usually burns it away. Light fog occurs only when there is no or light wind.

Fog, Medium: Medium fog reduces visibility ranges by half, resulting in a –4 penalty on Perception checks and a –4 penalty on ranged attacks. Medium fog typically occurs early in the day, late in the day, or sometimes at night, but the heat of the midday usually burns it away. Medium fog occurs only when there is no or light wind.

Rain: Rain reduces visibility ranges by half, resulting in a –4 penalty on Perception checks. Rain automatically extinguishes unprotected flames (candles, torches, and the like) and imposes a –4 penalty on ranged attacks.

Rain, Heavy: Heavy rain reduces visibility to one-quarter of the normal range, resulting in a –6 penalty on Perception checks. Heavy rain automatically extinguishes unprotected flames and imposes a –6 penalty on ranged attacks.

Sleet: Essentially frozen rain, sleet has the same effect as light snow, but any accumulation typically doesn’t last longer than 1–2 hours after the storm.

Snow, Heavy: Heavy snow reduces visibility ranges to onequarter of the normal range, resulting in a –6 penalty on Perception checks. It extinguishes unprotected flames and imposes a –6 penalty on ranged attacks. Heavy snow impedes movement even before it begins to stick. Moving into a square during a heavy snowstorm requires 1 extra 5-foot square of movement (this stacks with difficult terrain). Every hour of heavy snow leaves 1d4 inches of snow on the ground. As long as at least 2 inches of snow remain on the ground, the requirement of an extra square of movement to enter a square of snow persists. If at least 1 foot of snow remains on the ground, 2 extra squares of movement are required to enter a snow-filled square instead. A heavy snowstorm has a 10% chance of generating thundersnow and has a 40% chance of becoming a blizzard if the wind speed is severe or stronger.

Snow, Light: Light snow reduces visibility to three-quarters of the normal range, resulting in a –2 penalty on Perception checks. Light snow has a 75% chance each hour of extinguishing unprotected flames and imposes a –2 penalty on ranged attacks. Light snow does not impede movement unless it continues for 2 or more hours, at which point moving into a square of such snow requires 1 extra 5-foot square of movement (this stacks with difficult terrain). Every 2 hours of light snow leaves 1 inch of snow on the ground. As long as at least 2 inches of snow remain on the ground, the requirement of an extra square of movement to enter a square of snow persists. If at least 1 foot of snow remains on the ground, entering a snow-filled square instead requires 2 extra squares of movement.

Snow, Medium: Medium snow reduces visibility ranges by half, resulting in a –4 penalty on Perception checks. Medium snow extinguishes unprotected flames and imposes a –4 penalty on ranged attacks. Medium snow does not impede movement unless it continues for 1 hour, at which point moving into a square of such snow requires 1 extra 5-foot square of movement (this stacks with difficult terrain). Every hour of medium snow leaves 1 inch of snow on the ground. As long as at least 2 inches of snow remain on the ground, the requirement of an extra square of movement to enter a square of snow persists. If at least 1 foot of snow remains on the ground, entering a snow-filled square instead requires 2 extra squares of movement.

Thunderstorm: Thunderstorms feature powerful winds and heavy rain. To determine the type of wind associated with the thunderstorm, roll on Table 4–27: Thunderstorm Winds.

Table 4-27: Thunderstorm Winds

d%	Thunderstorm Wind Strength
1-50	Strong winds
51-90	Severe winds
91-100	Windstorm

In addition, there is a 40% chance that a thunderstorm features hail either up to an hour before or during the storm. An even greater danger presented by a thunderstorm is the lightning that occurs during the storm. These electrical discharges, generated by the roiling clouds, can pose a hazard to creatures that do not have proper shelters, especially creatures clad in metal armor. Every 10 minutes during a thunderstorm, a bolt of lightning strikes an unsheltered creature at random (though this can strike wildlife as easily as PCs). A creature struck by this lightning must succeed a DC 18 Reflex saving throw or take 10d8 points of electricity damage (a successful saving throw halves the damage). Creatures in metal armor take a –4 penalty on the Reflex saving throw.

There is a 10% chance that a thunderstorm with winds of windstorm strength also generates a tornado, while thunderstorms with windstorm-strength winds in temperatures higher than 85° F also have a 20% chance of being a precursor to a hurricane. There is a 20% chance that a thunderstorm of any strength in the desert also generates a haboob.

Deserts

Deserts are found in places where the weather must pass over mountains, causing a rain shadow, and in very cold environments. The baseline precipitation frequency in a desert is usually drought, but can be rare for a few weeks per year.

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