atmospheric extinction

Atmospheric extinction is the reduction in the intensity of light from a celestial body due to absorption and scattering by Earth's atmosphere. It increases from the zenith to the horizon and affects short wavelengths more than long wavelengths, so that objects near the horizon appear redder than they do at the zenith. The brightness of a star in the zenith will be reduced by only about 0.3 magnitudes, whereas the extinction at 20° altitude is about 0.9 magnitudes and at 10° altitude about 1.6 magnitudes.