*2.1.4. Temperature*

solar radiation, reaches the Earth's surface unmodified by any of the above atmospheric

processes, as depicts Figure 2.

92 Sustainable Energy - Recent Studies

**Figure 2.** Atmosphere effects on incoming solar radiation.

**Figure 3.** Solar radiation path across the Earth's atmosphere.

ed as reference at STC.

*2.1.3. Air Mass*

Since the direct radiation on a clear day, at noon, is typically 1000 W/m2

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

, this value is adopt‐

.

The third parameter used to characterize a PV module is its surface temperature. The STC temperature value adopted for PV modules characterization is *T=*25 ° C.
