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AM stands for Air Mass and describes the reduction of radiant power by the atmosphere. In central Europe, from northern Italy to central Sweden, the AM value is usually 1.5, as the sun's rays hit the atmosphere at an angle and pass through 1.5 times the height of the atmosphere. This is particularly significant in summer. During the winter months, the sun is much lower in our latitudes, resulting in a more realistic AM value between 4 and 6.