Abstract: This study investigates how CdTe thin film solar cells perform under varying diffuse solar spectrum due to the changes of environmental parameters. These parameters are, generally, the air mass, the atmospheric water vapour content, the turbidity and the albedo. The solar irradiance striking the solar cell is simulated using the spectral irradiance model SMARTS2 (Simple Model of the Atmospheric Radiative Transfer of Sunshine) for clear skies. The results show the short circuit current increases with increasing turbidity and albedo and it decreases with increasing air mass and atmospheric water vapour content but with different proportions. The performance of the cells is notably reduced, both in terms of efficiency and open circuit voltage, with increasing air mass but the cell performs better with increasing water vapour content, albedo and turbidity.