def poa_horizontal_ratio(surface_tilt, surface_azimuth,<EOL>solar_zenith, solar_azimuth):
cos_poa_zen = aoi_projection(surface_tilt, surface_azimuth,<EOL>solar_zenith, solar_azimuth)<EOL>cos_solar_zenith = tools.cosd(solar_zenith)<EOL>ratio = cos_poa_zen / cos_solar_zenith<EOL>try:<EOL><INDENT>ratio.name = '<STR_LIT>'<EOL><DEDENT>except AttributeError:<EOL><INDENT>pass<EOL><DEDENT>return ratio<EOL>
Calculates the ratio of the beam components of the plane of array
irradiance and the horizontal irradiance.

Input all angles in degrees.

Parameters
----------
surface_tilt : numeric
    Panel tilt from horizontal.
surface_azimuth : numeric
    Panel azimuth from north.
solar_zenith : numeric
    Solar zenith angle.
solar_azimuth : numeric
    Solar azimuth angle.

Returns
-------
ratio : numeric
    Ratio of the plane of array irradiance to the horizontal plane
    irradiance