pvlib.irradiance.clearness_index(ghi, solar_zenith, extra_radiation, min_cos_zenith=0.065, max_clearness_index=2.0)[source]

Calculate the clearness index.

The clearness index is the ratio of global to extraterrestrial irradiance on a horizontal plane.

  • ghi (numeric) – Global horizontal irradiance in W/m^2.

  • solar_zenith (numeric) – True (not refraction-corrected) solar zenith angle in decimal degrees.

  • extra_radiation (numeric) – Irradiance incident at the top of the atmosphere

  • min_cos_zenith (numeric, default 0.065) – Minimum value of cos(zenith) to allow when calculating global clearness index kt. Equivalent to zenith = 86.273 degrees.

  • max_clearness_index (numeric, default 2.0) – Maximum value of the clearness index. The default, 2.0, allows for over-irradiance events typically seen in sub-hourly data. NREL’s SRRL Fortran code used 0.82 for hourly data.


kt (numeric) – Clearness index



Maxwell, E. L., “A Quasi-Physical Model for Converting Hourly Global Horizontal to Direct Normal Insolation”, Technical Report No. SERI/TR-215-3087, Golden, CO: Solar Energy Research Institute, 1987.