def germano_tau(f, g, operator, scale=45, mask=False):
    """Small scale variance according to M. Germano 1990 paper where
       <tau>_ss = <f * g> - <f> * <g> and bracket terms are convolved inputs

       Scale is analogous to sigma if using a gaussian kernel.
    """
    return operator(f * g, scale=scale, mask=mask) - (operator(f, scale=scale, mask=mask) *
                    operator(g, scale=scale, mask=mask))