The effect of (primarily) aerosols, through their total optical depth, in reducing the transmission of direct solar radiation to the surface below that through a purely molecular atmosphere. Measures of turbidity refer to the total aerosol optical depth, either directly at a specified wavelength (e.g., the Volz turbidity factor or the Ängström turbidity coefficient, which is referenced to a wavelength of 1 μm), or indirectly by the ratio of aerosol to Rayleigh optical depth (e.g., the Linke turbidity factor).