def sigma_w(T):
    """ Surface tension of water for a given temperature.

    .. math::
        \\begin{equation}
        \sigma_w = 0.0761 - 1.55\\times 10^{-4}(T - 273.15)
        \end{equation}

    Parameters
    ----------
    T : float
        ambient air temperature, degrees K

    Returns
    -------
    float
        :math:`\sigma_w(T)` in J/m^2

    """
    return 0.0761 - 1.55e-4*(T-273.15)