D20:Weather Rules

Sometimes weather can play an important role in an adventure.
Table: Random Weather is an appropriate weather table for general use, and can be used as a basis for a local weather tables. Terms on that table are defined as follows.

Wind speeds are light (0 to 10 mph).

Between 0° and 40° Fahrenheit during the day, 10 to 20 degrees colder at night.

Lowers temperature by ?10° F.

Treat as rain (see Precipitation, below), but conceals as fog. Can create floods (see above). A downpour lasts for 2d4 hours.

Raises temperature by +10° F.

Between 85° and 110° Fahrenheit during the day, 10 to 20 degrees colder at night.

Between 40° and 60° Fahrenheit during the day, 10 to 20 degrees colder at night.

Wind speeds are over 50 mph (see Table: Wind Effects). In addition, blizzards are accompanied by heavy snow (1d3 feet), and hurricanes are accompanied by downpours (see above). Windstorms last for 1d6 hours. Blizzards last for 1d3 days. Hurricanes can last for up to a week, but their major impact on characters will come in a 24-to-48-hour period when the center of the storm moves through their area. Tornadoes are very short-lived (1d6×10 minutes), typically forming as part of a thunderstorm system.

Roll d% to determine whether the precipitation is fog (01?30), rain/snow (31?90), or sleet/hail (91?00). Snow and sleet occur only when the temperature is 30° Fahrenheit or below. Most precipitation lasts for 2d4 hours. By contrast, hail lasts for only 1d20 minutes but usually accompanies 1d4 hours of rain.

Wind speeds are severe (30 to 50 mph) and visibility is cut by three-quarters. Storms last for 2d4?1 hours. See Storms, below, for more details.

Between 60° and 85° Fahrenheit during the day, 10 to 20 degrees colder at night.

Wind speeds are moderate to strong (10 to 30 mph); see Table: Wind Effects on the following page.

[top]Table: Random Weather

d%	Weather	Cold Climate	Temperate Climate1	Desert
01?70	Normal weather	Cold, calm	Normal for season2	Hot, calm
71?80	Abnormal weather	Heat wave (01?30) or cold snap (31?100)	Heat wave (01?50) or cold snap (51?100)	Hot, windy
81?90	Inclement weather	Precipitation (snow)	Precipitation (normal for season)	Hot, windy
91?99	Storm	Snowstorm	Thunderstorm, snowstorm3	Duststorm
100	Powerful storm	Blizzard	Windstorm, blizzard4, hurricane, tornado	Downpour
Temperate includes forest, hills, marsh, mountains, plains, and warm aquatic. Winter is cold, summer is warm, spring and autumn are temperate. Marsh regions are slightly warmer in winter.

Bad weather frequently slows or halts travel and makes it virtually impossible to navigate from one spot to another. Torrential downpours and blizzards obscure vision as effectively as a dense fog.
Most precipitation is rain, but in cold conditions it can manifest as snow, sleet, or hail. Precipitation of any kind followed by a cold snap in which the temperature dips from above freezing to 30° F or below may produce ice.

Rain reduces visibility ranges by half, resulting in a ?4 penalty on Spot and Search checks. It has the same effect on flames, ranged weapon attacks, and Listen checks as severe wind.

Falling snow has the same effects on visibility, ranged weapon attacks, and skill checks as rain, and it costs 2 squares of movement to enter a snow-covered square. A day of snowfall leaves 1d6 inches of snow on the ground.

Heavy snow has the same effects as normal snowfall, but also restricts visibility as fog does (see Fog, below). A day of heavy snow leaves 1d4 feet of snow on the ground, and it costs 4 squares of movement to enter a square covered with heavy snow. Heavy snow accompanied by strong or severe winds may result in snowdrifts 1d4×5 feet deep, especially in and around objects big enough to deflect the wind?a cabin or a large tent, for instance. There is a 10% chance that a heavy snowfall is accompanied by lightning (see Thunderstorm, below). Snow has the same effect on flames as moderate wind.

Essentially frozen rain, sleet has the same effect as rain while falling (except that its chance to extinguish protected flames is 75%) and the same effect as snow once on the ground.

Hail does not reduce visibility, but the sound of falling hail makes Listen checks more difficult (?4 penalty). Sometimes (5% chance) hail can become large enough to deal 1 point of lethal damage (per storm) to anything in the open. Once on the ground, hail has the same effect on movement as snow.

The combined effects of precipitation (or dust) and wind that accompany all storms reduce visibility ranges by three quarters, imposing a ?8 penalty on Spot, Search, and Listen checks. Storms make ranged weapon attacks impossible, except for those using siege weapons, which have a ?4 penalty on attack rolls. They automatically extinguish candles, torches, and similar unprotected flames. They cause protected flames, such as those of lanterns, to dance wildly and have a 50% chance to extinguish these lights. See Table: Wind Effects for possible consequences to creatures caught outside without shelter during such a storm. Storms are divided into the following three types.

These desert storms differ from other storms in that they have no precipitation. Instead, a duststorm blows fine grains of sand that obscure vision, smother unprotected flames, and can even choke protected flames (50% chance). Most duststorms are accompanied by severe winds and leave behind a deposit of 1d6 inches of sand. However, there is a 10% chance for a greater duststorm to be accompanied by windstorm-magnitude winds (see Table: Wind Effects). These greater duststorms deal 1d3 points of nonlethal damage each round to anyone caught out in the open without shelter and also pose a choking hazard (see Drowning?except that a character with a scarf or similar protection across her mouth and nose does not begin to choke until after a number of rounds equal to 10 × her Constitution score). Greater duststorms leave 2d3?1 feet of fine sand in their wake.

In addition to the wind and precipitation common to other storms, snowstorms leave 1d6 inches of snow on the ground afterward.

In addition to wind and precipitation (usually rain, but sometimes also hail), thunderstorms are accompanied by lightning that can pose a hazard to characters without proper shelter (especially those in metal armor). As a rule of thumb, assume one bolt per minute for a 1-hour period at the center of the storm. Each bolt causes electricity damage equal to 1d10 eight-sided dice. One in ten thunderstorms is accompanied by a tornado (see below).

Very high winds and torrential precipitation reduce visibility to zero, making Spot, Search, and Listen checks and all ranged weapon attacks impossible. Unprotected flames are automatically extinguished, and protected flames have a 75% chance of being doused. Creatures caught in the area must make a DC 20 Fortitude save or face the effects based on the size of the creature (see Table: Wind Effects). Powerful storms are divided into the
following four types.

While accompanied by little or no precipitation, windstorms can cause considerable damage simply through the force of their wind.

The combination of high winds, heavy snow (typically 1d3 feet), and bitter cold make blizzards deadly for all who are unprepared for them.

In addition to very high winds and heavy rain, hurricanes are accompanied by floods. Most adventuring activity is impossible under such conditions.

One in ten thunderstorms is accompanied by a tornado.

Whether in the form of a low-lying cloud or a mist rising from the ground, fog obscures all sight, including darkvision, beyond 5 feet. Creatures 5 feet away have concealment (attacks by or against them have a 20% miss chance).

The wind can create a stinging spray of sand or dust, fan a large fire, heel over a small boat, and blow gases or vapors away. If powerful enough, it can even knock characters down (see Table: Wind Effects), interfere with ranged attacks, or impose penalties on some skill checks.

A gentle breeze, having little or no game effect.

A steady wind with a 50% chance of extinguishing small, unprotected flames, such as candles.

Gusts that automatically extinguish unprotected flames (candles, torches, and the like). Such gusts impose a ?2 penalty on ranged attack rolls and on Listen checks.

In addition to automatically extinguishing any unprotected flames, winds of this magnitude cause protected flames (such as those of lanterns) to dance wildly and have a 50% chance of extinguishing these lights. Ranged weapon attacks and Listen checks are at a ?4 penalty. This is the velocity of wind produced by a gust of wind spell.

Powerful enough to bring down branches if not whole trees, windstorms automatically extinguish unprotected flames and have a 75% chance of blowing out protected flames, such as those of lanterns. Ranged weapon attacks are impossible, and even siege weapons have a ?4 penalty on attack rolls. Listen checks are at a ?8 penalty due to the howling of the wind.

All flames are extinguished. Ranged attacks are impossible (except with siege weapons, which have a ?8 penalty on attack rolls). Listen checks are impossible: All characters can hear is the roaring of the wind. Hurricane-force winds often fell trees.

All flames are extinguished. All ranged attacks are impossible (even with siege weapons), as are Listen checks. Instead of being blown away (see Table: Wind Effects), characters in close proximity to a tornado who fail their Fortitude saves are sucked toward the tornado. Those who come in contact with the actual funnel cloud are picked up and whirled around for 1d10 rounds, taking 6d6 points of damage per round, before being violently expelled (falling damage may apply). While a tornado?s rotational speed can be as great as 300 mph, the funnel itself moves forward at an average of 30 mph (roughly 250 feet per round). A tornado uproots trees, destroys buildings, and causes other similar forms of major destruction.

Table: Wind Effects

Wind Force	Wind Speed	Ranged Attacks Normal/Siege Weapons1	Creature Size2	Wind Effect on Creatures	Fort Save DC
Light	0?10 mph	?/?	Any	None	?
Moderate	11?20 mph	?/?	Any	None	?
Strong	21?30 mph	?2/?	Tiny or smaller	Knocked down	10
			Small or larger	None
Severe	31?50 mph	?4/?	Tiny	Blown away	15
			Small	Knocked down
			Medium	Checked
			Large or larger	None
Windstorm	51?74 mph	Impossible/?4	Small or smaller	Blown away	18
			Medium	Knocked down
			Large or Huge	Checked
			Gargantuan or Colossal	None
Hurricane	75?174 mph	Impossible/?8	Medium or smaller	Blown away	20
			Large	Knocked down
			Huge	Checked
			Gargantuan or Colossal	None
Tornado	175?300 mph	Impossible/impossible	Large or smaller	Blown away	30
			Huge	Knocked down
			Gargantuan or Colossal	Checked
The siege weapon category includes ballista and catapult attacks as well as boulders tossed by giants. Flying or airborne creatures are treated as one size category smaller than their actual size, so an airborne Gargantuan dragon is treated as Huge for purposes of wind effects. Checked: Creatures are unable to move forward against the force of the wind. Flying creatures are blown back 1d6×5 feet. Knocked Down: Creatures are knocked prone by the force of the wind. Flying creatures are instead blown back 1d6×10 feet. Blown Away: Creatures on the ground are knocked prone and rolled 1d4×10 feet, taking 1d4 points of nonlethal damage per 10 feet. Flying creatures are blown back 2d6×10 feet and take 2d6 points of nonlethal damage due to battering and buffeting.

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System Reference Document