def clearness_index_zenith_independent(clearness_index, airmass,<EOL>max_clearness_index=<NUM_LIT>):
if airmass is not None:<EOL><INDENT>kt_prime = clearness_index / (<EOL><NUM_LIT> * math.exp(-<NUM_LIT> / (<NUM_LIT> + <NUM_LIT> / airmass)) + <NUM_LIT:0.1>)<EOL>kt_prime = max(kt_prime, <NUM_LIT:0>)<EOL>kt_prime = min(kt_prime, max_clearness_index)<EOL>return kt_prime<EOL><DEDENT>else:<EOL><INDENT>return <NUM_LIT:0><EOL><DEDENT>
Calculate the zenith angle independent clearness index.

Note:
    [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

Args:
    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