def svp_ice(t):
    """Saturation vapour pressure over ice at given temperature.


    Parameters
    ----------
    t : float
        Temperature in degree Celsius.

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

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

    """
    import math
    A1 = -13.928169
    A2 = 34.7078238
    t += 273.15
    theta = t / 273.16
    Y = A1 * (1 - theta ** -1.5) + A2 * (1 - theta ** -1.25)
    p_sv = 611.657 * math.exp(Y)
    return p_sv
