@staticmethod
def daylight_hours(ws):
    """Return the daylight hours (:math:`N`) *[hour]* as defined in
        `FAO 56 <http://www.fao.org/tempref/SD/Reserved/Agromet/PET/FAO_Irrigation_Drainage_Paper_56.pdf>`_
        (eq. 34, p. 49):

        .. math::

            N = \\frac{24}{\\pi} \\omega_s

        :param ws: sunset hour angle (:math:`\\omega_s`) *[rad]*. See :meth:`sunset_hour_angle`
        :type ws: float or np.numpy
        :return: (*float or np.numpy*) daylight hours (:math:`N`) *[hour]*
        """
    return 7.639437268410976 * ws
