*2.1.3. Air Mass*

The Air Mass (*AM*) index quantify the solar radiation path (*L*) across the atmosphere, nor‐ malized by the shorter path (*L* 0), measured from the zenith angle, as is depicted in Figure 3 and mathematically described by (1) [19]. The index *AM*0 is used to describe the radiation path out of the atmosphere, where the irradiance is constant and equal to 1366 W/m2 .

An Optimized Maximum Power Point Tracking Method Based on PV Surface Temperature Measurement http://dx.doi.org/10.5772/51020 93

$$AM = \frac{L}{L\_{\
u}} = \frac{1}{\cos(\Theta)}\tag{1}$$

On industry, PV modules are standardized considering an air mass index of AM=1.5. This value comes from a angle of aproximatly 48° , proper representing the PV instalitions around the most populed centres across Europe, China, Japan and United States of America, located in mid-latitudes. This value is also adopted as reference at STC.
