Red Rain is a story-related weather phenomenon introduced in God Eater 2, which literally as it sounds, crimson clouds raining down crimson rain. It will not appear while or during missions, as it is merely for story plot.
Ingame Description
Red Rain
A weather phenomenon which began within the last six months (of the year 2074). First recorded in the Far East, it is named for the red color of the raindrops.
When a human comes in physical contact with this rain, there is a high likelihood that they will contract the black plague.
A brief time after the rain has fallen, the red color vanishes and it changes into regular rainwater. The cause is still unknown.
According to the research of Dr. Sasaki, red rain is a natural process by which the Earth, having lost its Singularity, attempts to create a new one. When this rain enters the human body, the body's cells are altered to become like those of the Singularity and a Bias Factor is produced. When an Aragami touches red rain, it is predicted that the effect of the Bias Factor causes them to grow much stronger. The psion-type Aragami are theorized to be the product of this process.
Red Nimbus
A cloud with a cumulonimbus shape from which red rain falls. It forms overhead at an unnatural speed, and rainfall begins in under thirty minutes, leaving very little time for safe evacuation after a sighting. The cloud covers a land area of several tens kilometers, similar to a natural cumulonimbus cloud.
Using the red rain data collected by the Far East Branch, Dr. Rachel developed a highly accurate system for predicting the formation of red nimbus clouds. The system can predict formation time, as well as approximate size, allowing for much more efficient unit deployment and satellite base evacuation procedures.
With the appearance of the Spiral Tree, red nimbus formation has ceased. Dr. Sasaki has theorized that this is because the Oracle Cells have succeeded in repeating the Devouring Apocalypse sequence. In other words, because red rain only exists to reinitiate the Devouring Apocalypse and because that Apocalypse is currently underway on the Earth's surface, it can be expected that there will be no more red nimbus formation.