The effective temperature of the clear night sky being well below freezing is a matter of common knowledge based on everyday atmospheric physics and radiative cooling observations.
The claim refers to the well-known phenomenon of atmospheric radiative cooling under clear night skies, where the effective sky temperature drops significantly below freezing due to outgoing longwave radiation into space. Because this is a direct, everyday physical observation and fundamental meteorological fact, no specific citation is needed, resulting in a COMMON_KNOWLEDGE verdict.