Modding:Weather data

This page documents how the game generates weather in the game.

Data

The weather is controlled by the string field Game1::weatherForTomorrow. The possible values are:

value	constant	weather
`Sun`	`weather_sunny`	sunny
`Rain`	`weather_rain`	rain
`GreenRain`	`weather_green_rain`	green rain
`Wind`	`weather_debris`	windy
`Storm`	`weather_lightning`	lightning
`Festival`	`weather_festival`	festival
`Snow`	`weather_snow`	snow
`Wedding`	`weather_wedding`	wedding

Algorithm

Forced weather

Certain dates always have the same weather:

date	weather
spring 1	`weather_sunny`
spring 2 (year 1)	`weather_sunny`
spring 3 (year 1)	`weather_rain`
spring 4 (year 1)	`weather_sunny`
summer 1	`weather_sunny`
summer 13	`weather_lightning`
summer 26	`weather_lightning`
fall 1	`weather_sunny`
winter 1	`weather_sunny`

Green rain can only occur on Summer 5, 6, 7, 14, 16, 18 and 23.

Active festivals (defined in Data/Festivals) always have the Festival weather while passive festivals (defined in Data/PassiveFestivals) always have the Sun weather.

Generated weather

Weather in Stardew Valley is set within the Game1::newDayAfterFade() function (after the day change code, but before SMAPI's SaveEvents.BeforeSave event).

The game follows these steps to decide which weather and debris to set for the next day:

Check for forced weather.
If today is a festival, set the weather to weather_festival.
If today is a wedding, set the weather to weather_wedding.
Set Game1::wasRainingYesterday based on whether it was raining or storming.
Reset all weather flags, and sets them in the following pattern
1. If it is going to rain or storm, set the rain flag to true
2. If it is going to storm, set the storm flag to true
3. If the weather is (sunny, debris) or (festival || snowy || wedding), clear all the flags
4. and then if it's snowy, set snowy to true
5. It then sets the song based on this
6. Clear the debris weather array and clear the flag
7. if bloom isn't null, clear it's visibility
8. If the wedding is debris, populate the debris array.
If it's not raining, and the chance to rain tomorrow is less than .1, it will try to create a bloom day.
It then calculates the rain chance for tomorrow. It follows the following algorithm
1. Check to see if it's summer.
  1. If not, check to see if it's winter.
    1. If it's not, the rain chance is .183
    2. Else, it's .63
2. If it is, it uses the following: Check if it's day 1.
  1. If it's not, the chance is .12 + Game1::dayOfMonth * 3/1000
  2. If it is, the chance is 0. Which.. is kinda redundant, as the force days take care of this.
Check to see if a random number is less than the odds. If it is, it follows the following algorithm
1. Set Game1::weatherForTomorrow to weather_rain
2. If:
  1. It's summer, and if a random number is less than .85
  2. Or If it's not winter, and if a random number is less than .25 AND the day of the month is more than 2 and more than 27 days have been played
  3. Set Game1::weatherForTomorrow to weather_lightning
3. If it is winter
  1. Set Game1::weatherForTomorrow to weather_snow
Else, if it's over the rain odds.
1. If you've played less than 3 days.
2. Or: if
  1. It's not spring or a random number is greater than or equal to .2 (so 80%) and spring.
  2. And it's not fall or a random number is greater than or equal to .6 (so 40%) and fall.
  3. Or: If there is a wedding today
    1. Set Game1::weatherForTomorrow to weather_sunny if true
    2. Set Game1::weatherForTomorrow to weather_debris if false
3. Check if tomorrow is a festival, and set weather to weather_festival if true.
4. Again force the 3rd day to be weather_rain

At this point, the main function is done setting weather.

TV Channel

The TV will check the weather and show the following message:

weather	message
`Festival`	"It's going to be clear and sunny tomorrow... perfect weather for the Festival! The event will take place Location, starting between Time. Don't be late!" Failed to fetch the data: "Um... that's odd. My information sheet just says 'null'. This is embarrassing... "
`Snow`	50% chance each: "Bundle up, folks. It's going to snow tomorrow!" "Expect a few inches of snow tomorrow."
`Rain`	"It's going to rain all day tomorrow"
`GreenRain`	"Um... There appears to be some kind of... anomalous reading... I... don't know what this means..."
`Storm`	"Looks like a storm is approaching. Thunder and lightning is expected."
`Wind`	Per condition: Spring: "Partially cloudy with a light breeze. Expect lots of pollen!" Fall: "It's going to be cloudy, with gusts of wind throughout the day." Default: "It's going to snow all day. Make sure you bundle up, folks!"
default	50% chance each: "It's going to be clear and sunny all day." "It's going to be a beautiful, sunny day tommorow!"

Note: the TV will not necessarily be accurate for any other day than non festival days, the first of the month and the 3rd of spring, which are force-set by the Game1::newDayAfterFade method. Farmhands (and not host players) may not even get that accuracy for the first of the month and 3rd of spring.

Note 2: Passive festivals such as the Night Market are not considered to have "festival" weather. Instead, all passive festivals in the valley have "sunny" weather.

Weather Icon

The weather icon is set in Game1::updateWeatherIcon, which sets an index in LooseSprite/Cursors.xnb:

If snowing: 7.
If sunny: 2.
If wedding: 0.
If festival: 1.
If raining: 4.
If stormy: 5.
If spring: debris weather is 3.
If summer: unset (defaults to sunny).
If fall: 6.
If winter: 7 (same as snowing).

Rain Totem

The rain totem (item #681) and is controlled by Object::rainTotem, which is invoked by Object::performUseAction. If tomorrow is a festival day, the item is used up but nothing happens. Otherwise, it uses up the item and sets the weather for tomorrow to weather_rain and displays the message.

Weather probability by type

This section explains the probability of each weather type. This only applies on dates with no forced weather.

Sunny

Sunny weather covers weather variables 0, 4, and 6.

Spring: there's an 18.3% base chance of rain (81.7% chance remaining for other weathers). If it doesn't rain, there's an 80% chance to remain sunny (except on spring 3, which will always be rainy). That means that in spring there's a 66.4% chance of sunny weather.
Summer: The chance of sunny weather diminishes steadily per day from 87.4% on summer 1 to 79.9% on summer 28. The precise chance is 1 - [12% + (0.3% * day of the month)] per day, with a 0% chance on day 1.
Fall: identical to spring, but no forced weather.
Winter: there's a 63% chance of precipitation, so only a 37% chance of sunny weather.

Rainy

Spring: there's an 18.3% base chance of rain. If it rains, there's a 25% chance of storms except in year 1. So the rain odds in Spring is a flat 18.3% in year 1, and 13.725% in year 2 or later. It will always rain on spring 3 (year 1).
Summer: the chance of rainy weather increases steadily per day from 12.6% on summer 2 to 20.1% on Summer 27. There's an 85% chance that rain becomes storms. So you have a scale of [12% + (0.3% * day of the month)] * .15 to determine your chances of rainfall.
Fall: same as spring, except fall in year 1 can be stormy so it's a flat 13.725% chance.
Winter: never rains.

Debris

Debris weather covers weather variable 2.

Spring: there's a 20% chance of this after rain, so approximately 16.6% chance of debris weather.
Summer: no debris weather.
Fall: there's a 60% chance for this after rain, so approximately 49.8% chance of debris weather.
Winter: no debris weather.

Stormy

Stormy weather covers weather variable 3.

Spring: 4.57% chance of storms.
Summer: variable chance of storms; starts at 10.71% and increases to 17.085%.
Fall: 4.57% chance of storms.
Winter: no stormy weather.

Snowy

In winter, there's a 63% chance of snow. No other season has snow.

Save files

The save file is a snapshot of the day at 0600 the next morning. The WeatherForTommorow field was used to calculate the weather, but changing it has no effect on the weather since the weather flags have already been set by this point. To change the weather, you need to set one of these combinations of flags:

weather	`isRaining`	`isDebrisWeather`	`isLightning`	`isSnowing`
sunny	☐	☐	☐	☐
rainy	☑	☐	☐	☐
stormy	☑	☐	☑	☐
debris	☐	☑	☐	☐
snowy	☐	☐	☐	☑
festival	☐	☐	☐	☐
wedding	☐	☐	☐	☐

Note: changing isDebrisWeather during an active game will not create the array. You'll need to call Game1::populateDebrisWeatherArray to get the debris populated. Correspondingly, if you're removing the debris weather flag, remember to call Game1::debrisWeather::Clear.

Notes/FAQ

The TV can be fixed by overriding it - or just by using Entoroax's Framework.
You can set snow and debris at any time, the game just won't.
You cannot set debris and rain at the same time.