Air mass is the optical path length through the Earth’s atmosphere. As it passes through the Earth’s atmosphere, light is reduced (or attenuated) by scattering and absorption. The more atmosphere the light must pass through, the greater the attenuation. An air mass of 1.0 is looking straight up from sea level at the sun when it is directly overhead. As an this condition never occurs north of the Tropics (23.5o latitude), the STC air mass number used for testing (e.g., 1.5) has been agreed upon as a more representative condition for countries north of the Tropics such as the U.S. and Europe.