From AMS Glossary
A method of evaluating the static stability over a limited area at any reference level in the atmosphere.
Unlike the parcel method, the slice method takes into account continuity of mass by considering both upward and downward motion but not mixing. The temperature excess of the rising air will be 1) negative (stable conditions), 2) zero (neutral or indifferent conditions), or 3) positive (unstable conditions) according to whether the expression
is 1) less than zero, 2) equal to zero, or 3) greater than zero, respectively, where A is the total area of the region under consideration, Ad the area covered by the descending current, γ the environmental lapse rate of temperature, and γ′ and γd the adiabatic process lapse rates followed, respectively, by the ascending and descending currents. Since the ratio Ad/A can be evaluated only after convection has set in, the slice method is used mainly as a diagnostic, rather than a prognostic, tool.
Emanuel, K. 1994. Atmospheric Convection. Oxford University Press, . 175–178.