Black rain is a type of rain that has been polluted and contains ash, dust, or other particles. In modern terminology, it also refers to rain that gathers soot and falls as part of nuclear fallout, though of course this definition wouldn’t have existed during Shelley’s lifetime.
"a vast sepulchre..."See in text(Ode to the West Wind)
A sepulchre is a stone tomb. The term also has religious connotations, as a sepulchre can refer to a place where religious artifacts are kept. When taking this second meaning into account, the use of the word could signify a tomb not just for the dying season, but for the dying belief system of a king and society being ousted by revolutionaries. The use of the phrase “congregated might” also suggests religious connotations, as it brings to mind a church congregation.
This word is an alternate spelling of airy; in a poetical sense, aëry also meant something that was ethereal or of the air, not just something that was in the air.
