AbstractThe approach of an upper‐level trough is accompanied by a local ascent of the isentropic surfaces in the troposphere. The associated cooling and moistening destabilizes the atmosphere to convection. In this paper we attempt to quantify the maximum amount of destabilization as a function of the amplitude and horizontal scale of the trough. The trough is represented by an elongated potential‐temperature perturbation on the tropopause. Its structure, including the isentrope displacement it produces, is calculated from the balanced shear‐line solutions of Juckes. These solutions are used to estimate the amount by which mid‐ and lower‐tropospheric air masses are lifted when an upper‐tropospheric trough passes overhead on the assumption that isentropic potential‐vorticity anomalies are localized in the upper troposphere. The amount of destabilization is characterized by the maximum change in convective available potential energy (CAPE) and convective inhibition (CIN) brought about by the passage of the trough. The calculations seek to isolate one aspect of the interaction between an upper‐level trough and a tropical cyclone. We show that the changes in CAPE are significant in the Tropics (up to 83% for the soundings examined) and even more so in the middle latitudes (up to 130%) and that the CIN may be com