The clearness index kt (the ratio of surface-level GHI to ET irradiance) can be calculated via pvlib.irradiance.clearness_index and is used in the DIRINT and DISC models. Though other metrics in the same vein are not necessary for the irradiance models implemented in pvlib, including them in pvlib could be a convenience to users. Is there interest in implementing other irradiance metrics for users to access? Some examples:

• clear-sky index: the ratio of surface GHI to modeled clear-sky GHI
• Variability Scores as described in this Sandia paper
• Variability Index as described in this Sandia paper, coauthored by Cliff Hansen