Razor rain is a pseudo-meteorological event that occurs when pieces of arks fall to Earth in unpredictable volleys of razor-sharp metals, nano-tech and other materials. These outbreaks differ from ark falls in that they are comprised of nothing but deadly shards of debris rather than salvagable arks.
Within the stratosphere, the phenomenon mimics a normal Earth thunderstorm and, once underway, can appear anywhere and at any time. Most buildings provide adequate shelter from razor rain; it is only out in the open that the phenomenon is deadly.
Razor rain was introduced in this episode, threatening the citizens of Defiance into seeking shelter, and taking many lives.