def nhi(b3, b11):
    """
    Normalized Humidity Index (Lacaux et al., 2007).

    .. math:: NHI = (b11 - b3)/(b11 + b3)

    :param b3: Green.
    :type b3: numpy.ndarray or float
    :param b11: SWIR 1.
    :type b11: numpy.ndarray or float

    :returns NHI: Index value

    .. Tip::

       Zarco-Tejada, P. J., Miller, J. R., Noland, T. L., Mohammed, \
       G. H., Sampson, P. H. 2001. Scaling-up and model inversion methods \
       with narrowband optical indices for chlorophyll content estimation in \
       closed forest canopies with hyperspectral data. IEEE Transactions on \
       Geoscience and Remote Sensing 39, 1491-1507. doi:10.1109/36.934080.
    """

    NHI = (b11 - b3)/(b11 + b3)
    return NHI