Extinction
Also called: airmass, atmospheric extinction.
The dimming of starlight by the atmosphere, which grows quickly towards the horizon.
Light from overhead passes through one “airmass” of atmosphere; at 30° altitude it passes through two, and near the horizon through ten or more. Each airmass dims stars by roughly 0.2–0.3 magnitudes on a clear night, more in hazy air — so a faint galaxy low in the sky can lose a whole magnitude or more.
In AstroPilot
AstroPilot's ratings include extinction for each object's altitude and the forecast haze.