The Air mass coefficient (AM) describes the thickness of the atmosphere. The light radiation has to penetrate different layers of the atmosphere before hitting the earth's surface, depending on the sun's posiotion. The measured value depicts the relation between the path of light and the shortes way through the atmosphere.
An AM of 1 defines a vertical entry of radiation in the atmosphere and a vertical impingement on the earth's surface. The lower the sun is, the longer is the path of light through the atmosphere. During that process certain wavelengths of the light spectrum are mitigated through interaction with air molecules. This has a negative impact on the power of solar plants.
The standard test conditions of photovoltaic plants state an AM of 1,5. The measured value above the atmosphere is 0.
Air mass factor