def msi(b8a, b11):
    """
    Moisture Stress Index (Rock, Williams, and Vogelmann, 1985).

    .. math:: MSI = b11 / b8a

    :param b8a: NIR narrow.
    :type b8a: numpy.ndarray or float
    :param b11: SWIR 1.
    :type b11: numpy.ndarray or float

    :returns MSI: Index value

    .. Tip::

        Rock, B.N., Williams, D.L., Vogelmann, J.E. 1985. Field and airborne \
        spectral characterization of suspected acid deposition damage in red \
        spruce (picea rubens) from vermont. in: Proceedings of the 11th \
        International Symposium—Machine Processing of Remotely Sensed Data, \
        West Lafayette, IN, USA, 25-27 June 1985; pp. 71-81.
    """

    MSI = b11 / b8a
    return MSI