Weather

The skies are unpredictable and wild, and often treacherous to explorers. Weather has the potential to change on each watch, but won’t necessarily do so. The weather on a given watch has three basic components: precipitation, wind speed, and temperature. Each of these is randomly determined each watch, based on different factors.

Precipitation
Precipitation has three modes: no precipitation, a clear day; light precipitation, such as a shower or flurry; and heavy precipitation, such as a downpour or whiteout. Each of these modes has, at the turning of the current watch, a chance to change to one of the other modes. Each mode has its own chances for determining how the weather changes.

The chance to determine which mode will occur next are as follows. This table is just a base line and can change depending on the climate of the region.

Precipitation has one unusual aspect, however, which is that the temperature of the watch where it is occurring may have an effect. Specifically, if it is below freezing, precipitation will come down as snow, rather than rain. Given this, there are individual effects for snow and rain. However, you still determine them in the same fashion, and it is possible to have snow during the night and rain during the day, and vice versa.

Wind Speed
Wind Speed, like precipitation, has three basic modes: no wind, low winds, and high winds. Each of those modes changes at the end of the current watch, and have varying chances based on the current mode.

The chance to determine which mode will occur next are as follows. As before, if your climates are more windy than temperate ones, or if there are seasonal changes, feel free to alter these rolls.

When there are high winds occurring simultaneously with heavy precipitation, a serious storm results. Storms have additional effects that occur in addition to all other effects.

The effects of wind speeds are as follows:

Temperature
Temperature changes on watch as the day progresses, like the other components, but unlike the others, it is based far more on the time of day and season.

In a stereotypical temperate climate, weather in the summer typically gets to about 80-100 degrees Fahrenheit, and in the winter drops to 0-20 degrees Fahrenheit. This has a fair degree of leeway, obviously, so you should adjust that to match your climate.

There are, broadly speaking, four basic levels of temperature that matter: “regular” weather, which spans roughly 32 degrees Fahrenheit to 100 degrees Fahrenheit; freezing cold, which occurs between 32 degrees Fahrenheit and 0 degrees Fahrenheit; extreme cold, which is anything beneath 0 degrees Fahrenheit; and extreme heat, which is any temperature above 100 degrees.

Regular and freezing weathers’ key distinction—below 32 degrees Fahrenheit, water freezes and precipitation occurs as snow, and above that mark precipitation comes down as rain instead. Other than that, there are no mechanical differences.

Extreme Weather
In addition to ordinary weather patterns—which can pose a challenge all on their own—there are rarer, more dangerous kinds of weather or climate patterns that might pose fresh hazards to players exploring the wilderness.

These hazards often vary with the seasons: extreme cold and icy waterways are highly dangerous in winter, but sandstorms and extreme heat might rarely occur outside of the summer.

Extreme Cold
It is assumed that the ordinary clothing and gear adventurers carry with them can withstand normal temperature swings of cold winters. However, when the temperature drops with severity, simple exposure can be deadly. When a creature is exposed to temperatures below 0 degrees Fahrenheit, they must make a DC 10 Constitution saving throw at the end of each hour or suffer one level of exhaustion.

If a creature has resistance or immunity to cold damage, is naturally adapted to cold-weather climates, or is wearing cold-weather gear, they automatically succeed on this saving throw.

Extreme Heat
As with cold weather, it is assumed that an adventurer’s ordinary gear is sufficiently light (or can be taken off with enough ease) to deal with ordinary heat swings of summer. That said, severe heat can still be deadly. When a creature is exposed to temperatures above 100 degrees Fahrenheit, they must make a Constitution saving throw at the end of each hour or suffer one level of exhaustion. If a creature has not had sufficient drinking water, they have disadvantage on these saving throws. The DC begins at 5, and increases by 1 for each additional hour; if a creature is wearing heavy or medium armor, the DC increases by 2 each hour instead.

If a creature has resistance or immunity to fire damage or is naturally adapted to hot-weather climates, they automatically succeed on this saving throw.

Extreme Storms
Occasionally, extreme winds will combine with heavy rain or snow to form an extreme storm: a cyclone, hurricane, tornado, or worse. Inside the storm, all movement speed is halved, the area is heavily obscured, all creatures automatically fail all Wisdom (Perception) checks relying on sight, sound, or smell.

Any creature or object weighing less than 300 lb. must make a DC 20 Strength saving throw at the end of each round or be thrown 6d10 feet in a random direction, suffering 6d6 bludgeoning damage on impact with any other object or the ground. Creatures that weigh less than 150 pounds have disadvantage on these saving throws.

Fog
There are two basic kinds of fog: light fog and heavy fog. An area covered in light fog - such as a mist-clad mountainside—is considered lightly obscured. An area covered in heavy fog - such as a shrouded ghost town—is considered heavily obscured.

Fog is somewhat irregular compared to ordinary weather patterns, but is most prevalent early in the morning, around dawn, and is more common in autumn than other seasons.

Sandstorms
In especially arid environments, high winds can kick up huge storms of sand, dust, and gritty dirt. Sandstorms follow the same rules as regular storms, but with the addition that each exposed character must make a DC 10 Constitution saving throw at the end of each minute or suffer 1d10 slashing damage.

Frigid Water
When submerged in water that is 40 degrees Fahrenheit or less, a creature can spend a number of minutes equal to its Constitution modifier without issue.

After that, at the end of each minute, the creature must make a DC 10 Constitution saving throw or suffer one level of exhaustion.

If a creature has resistance or immunity to cold damage or is naturally adapted to living in frigid waters, they automatically succeed on this saving throw.

Slippery Ice
When a creature moves across slippery ice, they must succeed on a DC 10 Dexterity saving throw or be knocked prone.

If a creature moves at half speed while on the ice, they have advantage on the saving throw. If a creature dashes while on the ice, they have disadvantage on the saving throw.

Thin Ice
A given 10-foot patch of thin ice can hold approximately 3d10 ✕ 10 pounds. As soon as more weight is placed on the patch, the ice shatters. Thin ice almost always has frigid water beneath.