Transparency
Also called: clarity, haze.
How clear the air is: on a transparent night faint stars and galaxies show; haze, humidity and dust hide them.
Transparency is about how much light gets through: thin high cloud, humidity, haze, dust and smoke all dim faint objects and scatter town lights into a brighter sky. It matters most for faint galaxies and nebulae; planets barely notice.
A sky can be transparent but turbulent, or murky but steady — so a bad deep-sky night can be a good planetary night, and vice versa.
In AstroPilot
Hour by hour on Tonight rates transparency from 1 to 5 from humidity, high cloud and aerosols.