def estimate_Cn(P=1013, T=273.15, Ct=1e-4):
    """Use Weng et al to estimate Cn from meteorological data.

    Parameters
    ----------
    P : `float`
        atmospheric pressure in hPa
    T : `float`
        temperature in Kelvin
    Ct : `float`
        atmospheric struction constant of temperature, typically 10^-5 - 10^-2 near the surface

    Returns
    -------
    `float`
        Cn

    """
    return (79 * P / (T ** 2)) * Ct ** 2 * 1e-12