def svp_water(t):
    """Saturation vapour pressure over water at given temperature.

    Parameters
    ----------
    t : float
        Air temperature in degree Celsius.

    Returns
    -------
    p_sv : float
        Saturation vapour pressure over water, at the given
        temperature, in Pascal.

    Notes
    -----
    From section A-I of
    http://emtoolbox.nist.gov/Wavelength/Documentation.asp.

    """
    import math
    K1 = 1167.05214528
    K2 = -724213.167032
    K3 = -17.0738469401
    K4 = 12020.8247025
    K5 = -3232555.03223
    K6 = 14.9151086135
    K7 = -4823.26573616
    K8 = 405113.405421
    K9 = -0.238555575678
    K10 = 650.175348448
    T = t + 273.15
    omega = T + K9 / (T - K10)
    A = omega ** 2 + K1 * omega + K2
    B = K3 * omega ** 2 + K4 * omega + K5
    C = K6 * omega ** 2 + K7 * omega + K8
    X = -B + math.sqrt(B ** 2 - 4 * A * C)
    p_sv = 1000000.0 * (2.0 * C / X) ** 4
    return p_sv
