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
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