def clearness_index_zenith_independent(clearness_index, airmass,<EOL>max_clearness_index=<NUM_LIT>):
<EOL>kt_prime = clearness_index / _kt_kt_prime_factor(airmass)<EOL>kt_prime = np.maximum(kt_prime, <NUM_LIT:0>)<EOL>kt_prime = np.minimum(kt_prime, max_clearness_index)<EOL>return kt_prime<EOL>
Calculate the zenith angle independent clearness index.

Parameters
----------
clearness_index : numeric
    Ratio of global to extraterrestrial irradiance on a horizontal
    plane

airmass : numeric
    Airmass

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_prime : numeric
    Zenith independent clearness index

References
----------
.. [1] Perez, R., P. Ineichen, E. Maxwell, R. Seals and A. Zelenka,
       (1992). "Dynamic Global-to-Direct Irradiance Conversion Models".
       ASHRAE Transactions-Research Series, pp. 354-369