def forestvar(z_in):
    """  Return intrinsic variance of LyaF variance for weighting. This
    estimate is roughly from McDonald et al 2006

    Parameters
    ----------
    z_in : float or ndarray

    Returns
    -------
    fvar : float or ndarray
      Variance
    """
    fvar = 0.065 * ((1.+z_in)/(1.+2.25))**3.8
    # Return
    return fvar