def _sat_press(T):
    """ 
    Return the saturation pressure in mb of the water vapour, given 
    temperature in deg C.  Equation from:
    http://wahiduddin.net/calc/density_altitude.htm
    """
    eso = 6.1078
    c0 = 0.99999683
    c1 = -0.0090826951
    c2 = 7.8736169e-05
    c3 = -6.1117958e-07
    c4 = 4.3884187e-09
    c5 = -2.9883885e-11
    c6 = 2.1874425e-13
    c7 = -1.7892321e-15
    c8 = 1.1112018e-17
    c9 = -3.0994571e-20
    p = c0 + T * (c1 + T * (c2 + T * (c3 + T * (c4 + T * (c5 + T * (c6 + T *
        (c7 + T * (c8 + T * c9))))))))
    sat_press = eso / p ** 8
    return sat_press
