length of path through the earth's atmosphere traversed by the direct solar beam, expressed as a multiple of the path traversed to a point at sea level with the sun directly overhead

NOTE The air mass index is 1,0 at sea level with cloudless sky when the sun is directly overhead and the local air pressure equals P0. Standard operating conditions, 3.4.16d), and standard test conditions, 3.4.16e), use an AM of 1,5