def _dT_d_dt(U, T_g, T_d, RTI):
    """[Eq. 11.5]
    :param U: [m/s]
    :param T_g: [K]
    :param T_d: [K]
    :param RTI: [-]
    :return dT_d_dt: [K/s]
    """

    dT_d_dt = U ** (1 / 2) * (T_g - T_d) / RTI

    return dT_d_dt