def e(parameters, p, r_v):
    """
    Returns an expression for the partial pressure of water vapour
    from the total pressure and the water vapour mixing ratio.

    :arg parameters: a CompressibleParameters object.
    :arg p: the pressure in Pa.
    :arg r_v: the mixing ratio of water vapour.
    """

    epsilon = parameters.R_d / parameters.R_v

    return p * r_v / (epsilon + r_v)