Solar radiation emitted by the sun is absorbed by the atmosphere to some extent. Absorbance levels depend on wavelength (ozone absorbs at wavelengths below 350nm, water vapor and carbon dioxide absorb at wavelengths above 2500nm), cloud cover, pollution and atmospheric
particles. These various factors determine the spectrum which reaches the ground. The spectrum also depends on the thickness of the atmosphere it has to cross. Definitions have been drawn up for standard atmospheres under the name AM (Air Mass). AM 1 refers to the thickness of a standardized atmosphere crossed when the sun is at its zenith.