def bird_hulstrom80_aod_bb(aod380, aod500):
    """
    Approximate broadband aerosol optical depth.

    Bird and Hulstrom developed a correlation for broadband aerosol optical
    depth (AOD) using two wavelengths, 380 nm and 500 nm.

    Parameters
    ----------
    aod380 : numeric
        AOD measured at 380 nm
    aod500 : numeric
        AOD measured at 500 nm

    Returns
    -------
    aod_bb : numeric
        broadband AOD

    See also
    --------
    kasten96_lt

    References
    ----------
    [1] Bird and Hulstrom, "Direct Insolation Models" (1980)
    `SERI/TR-335-344 <http://www.nrel.gov/docs/legosti/old/344.pdf>`_

    [2] R. E. Bird and R. L. Hulstrom, "Review, Evaluation, and Improvement of
    Direct Irradiance Models", Journal of Solar Energy Engineering 103(3),
    pp. 182-192 (1981)
    :doi:`10.1115/1.3266239`
    """
    # approximate broadband AOD using (Bird-Hulstrom 1980)
    return 0.27583 * aod380 + 0.35 * aod500