<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def poa_horizontal_ratio(surface_tilt, surface_azimuth, solar_zenith, solar_azimuth):
 """ 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 """


    cos_poa_zen = aoi_projection(surface_tilt, surface_azimuth,
                                 solar_zenith, solar_azimuth)

    cos_solar_zenith = tools.cosd(solar_zenith)

    # ratio of tilted and horizontal beam irradiance
    ratio = cos_poa_zen / cos_solar_zenith

    try:
        ratio.name = 'poa_ratio'
    except AttributeError:
        pass

    return ratio