def _vpres(T):
    """ Polynomial approximation of saturated water vapour pressure as
    a function of temperature.

    Parameters
    ----------
    T : float
        Ambient temperature, in Kelvin

    Returns
    -------
    float
        Saturated water vapor pressure expressed in mb

    See Also
    --------
    es

    """
    # Coefficients for vapor pressure approximation
    A = [
        6.107799610e0,
        4.436518521e-1,
        1.428945805e-2,
        2.650648471e-4,
        3.031240396e-6,
        2.034080948e-8,
        6.136820929e-11,
    ]
    T -= 273  # Convert from Kelvin to C
    vp = A[-1] * T
    for ai in reversed(A[1:-1]):
        vp = (vp + ai) * T
    vp += A[0]
    return vp