def isotropic(surface_tilt, dhi):
sky_diffuse = dhi * (<NUM_LIT:1> + tools.cosd(surface_tilt)) * <NUM_LIT:0.5><EOL>return sky_diffuse<EOL>
r'''
    Determine diffuse irradiance from the sky on a tilted surface using
    the isotropic sky model.

    .. math::

       I_{d} = DHI \frac{1 + \cos\beta}{2}

    Hottel and Woertz's model treats the sky as a uniform source of
    diffuse irradiance. Thus the diffuse irradiance from the sky (ground
    reflected irradiance is not included in this algorithm) on a tilted
    surface can be found from the diffuse horizontal irradiance and the
    tilt angle of the surface.

    Parameters
    ----------
    surface_tilt : numeric
        Surface tilt angle in decimal degrees. Tilt must be >=0 and
        <=180. The tilt angle is defined as degrees from horizontal
        (e.g. surface facing up = 0, surface facing horizon = 90)

    dhi : numeric
        Diffuse horizontal irradiance in W/m^2. DHI must be >=0.

    Returns
    -------
    diffuse : numeric
        The sky diffuse component of the solar radiation.

    References
    ----------
    [1] Loutzenhiser P.G. et. al. "Empirical validation of models to
    compute solar irradiance on inclined surfaces for building energy
    simulation" 2007, Solar Energy vol. 81. pp. 254-267

    [2] Hottel, H.C., Woertz, B.B., 1942. Evaluation of flat-plate solar
    heat collector. Trans. ASME 64, 91.