def bird_hulstrom80_aod_bb(aod380, aod500):
<EOL>return <NUM_LIT> * aod380 + <NUM_LIT> * aod500<EOL>
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`