pvlib.irradiance.clearness_index¶
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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.
Parameters: - 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.
Returns: - kt : numeric
Clearness index
References
[1] 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.