Internal Weather System

Not to be confused with the Internal Clock, a similar anomaly that instead adapts the game to what time of day it is.

The Internal Weather System is an unused mechanic in Super Mario 64 that would detect the outside weather in the vicinity of the player and adapt the game to resemble these instances of weather. This mechanic was later removed, though it can be anomalously reimplemented by the Personalization A.I. The Internal Weather System appears to be very uncommon, potentially due to cartridge limitations or that only certain cartridges are compatible with the Internal Weather System.

Rainy Weather
Rainy weather is the most common documented change attributed to the Internal Weather System. It most commonly causes a unique rain effect to appear in outdoors areas never seen anywhere in the base game, and has been said to change the level Lethal Lava Land into the anomalous Raging Rain World level. An uncommon change associated with rainy weather is that the water level in levels where applicable can supposedly begin to rise, likely due to the rain.

Natural Disasters
If any type of natural disaster is detected by the Internal Weather System, it can reportedly take the player to a screen alerting them of the natural disaster and inform them of how to avoid the current disaster. However, the natural disaster detection of the Internal Weather System has been reported to be faulty, causing the screen to appear when no natural disaster is occurring, as well as strange alerts about nonexistent natural disasters such as Temporal Leakages appearing, potentially being one of the reasons that the Internal Weather System was scrapped.

Stormy Weather
Stormy weather has been reported to have a similar screen to the natural disasters if it is detected to be particularly dangerous, though it more commonly appears as a mix of rainy weather and an anomalous lightning hazard often associated with Raging Rain World and the stormy section of Sky Seas. Enemies have also been reported to attempt to run to shelter during stormy weather, potentially due to another reported anomaly of the lightning hazard damaging enemies. The clouds of Princess Peach's Castle's walls have also been reported to change into having angry eyes and have rain and thunder during stormy weather.

Clear Weather
Clear weather has had minimal reports, potentially due to being the default for Super Mario 64, though some reports of Raging Rain World disappearing, the storm section of Sky Seas disappearing, and other disappearances of weather-related anomalies or levels have been associated with clear weather, likely due to the weather-related anomalies not fitting with clear weather.

Snowy Weather
Similarly to the Internal Clock's changes during Christmas and the Winter season, snowy weather has been reported to cause anomalous snow levels such as Chroma Tundra to appear, anomalous Present Boxes to appear, the unused blizzard environmental effect to appear in some areas, and snow to begin appearing in all outdoors areas and levels in Super Mario 64.

Sunny Weather
Sunny weather has reportedly caused Cool, Cool Mountain and Snowman's Land to thaw, turning into new grassy levels or simply disappearing from the game and being replaced with entirely different levels. Notably, a screen similar to the type of screen reported with natural disasters has also been reported, instead telling the player that the weather conditions are good and to consider going to play outside.

General Changes
Several general changes have been reported between all weather changes. Dynamic Level Rearrangement and Enemy Discoloring can occur to fit the weather changes, new levels and anomalies can appear, and some rare reports state that Mario's outfit can change depending on the weather, such as Mario wearing a swimsuit during sunny weather or a coat during snowy weather.