def Lv(parameters, T):
    """
    Returns an expression for the latent heat of vaporisation of water.

    :arg parameters: a CompressibleParameters object.
    :arg T: the temperature in K.
    """

    L_v0 = parameters.L_v0
    T_0 = parameters.T_0
    c_pl = parameters.c_pl
    c_pv = parameters.c_pv

    return L_v0 - (c_pl - c_pv) * (T - T_0)