Airmass (aka air mass) is a measure of the effect of Earth's atmosphere on observations by ground telescopes. It is the ratio of the atmosphere's effect on a given observation with that of an observation from sea level toward the zenith. The vertical sea-level observation would have an airmass of 1. A the angular distance from the zenith adds to it, the airmass at the horizon being on the order of 38. In theory, a higher altitude lessens it, though sometimes what is cited as the airmass is actually the sea-level airmass.
The term and concept is also used regarding solar energy, i.e., in determining the energy available to a solar panel.
Determining an accurate airmass is non-trivial, among the reasons being the curve of the atmosphere, and the varied effects of different layers of the atmosphere. Approximate formulas have been developed. For convenient accuracy, tables have been produced, and formulas have been developed to assist in interpolating table values.