Serein (/sɪˈriːn/; French: [səʁɛ̃]) refers to rain falling from a cloudless sky.
[1] This sort of rain is said to take the form of a fine, light drizzle, typically after dusk.
[1] The name derives from French serein, meaning "serene", or "clear" (as in unclouded).
One possible explanation is cloud droplets evaporating at the same time as rain drops are formed.
[2] Another explanation is that the rain is not actually originating above where it falls,[3] but rather is being blown from a distant cloud by strong winds at altitude (which may not be felt at ground level due to a high vertical wind shear).