The deadly rain in Rain World is primarily a byproduct of the cooling systems used by the enormous artificial intelligences known as Iterators.
The Role of Iterators
Iterators, like Five Pebbles, are massive structures and sophisticated artificial intelligences. They were originally created by an ancient civilization called "them" with the specific goal of calculating a way for all living organisms to achieve ascension, essentially ending the cycle of life and death.
The Cooling Process
To perform their vast calculations and maintain their systems, Iterators require substantial cooling. According to the lore:
- Iterators use vast quantities of water as a coolant. This water absorbs the immense heat generated by their internal processes.
From Vapor to Deadly Rain
Once the water has served its purpose as a coolant, the Iterators must expel it. The method of expulsion directly leads to the formation of the hazardous weather:
- They then release [the water] as vapor. This large-scale release of water vapor into the atmosphere is the crucial step.
- This vapor then falls back down in the form of deadly rain. As the vapor cools in the upper atmosphere, it condenses and precipitates back down onto the world below.
The sheer volume and consistent nature of this process, driven by the needs of countless Iterators, result in the relentless and dangerous downpours that define the world's climate and pose a significant threat to creatures like the Slugcat.
In summary, the rain isn't a natural weather phenomenon in the traditional sense but an environmental consequence of the colossal Iterators managing their internal temperatures.
