def calc_rso(ra, frac=0.75):
    """
    Clear-sky solar radiation.

    Parameters
    ----------
    ra : numpy ndarray
        Extraterrestrial solar radiation.
    frac : float <= 1
        Fraction of extraterrestrial solar radiation that
        reaches earth on clear-sky days.

    """
    s = 'Please enter a fractional value less than or equal to 1.0 and ' \
        'greater than or equal to 0.0.'
    assert 0 <= frac <= 1, s
    return ra * frac