Diabatic processes are essential in shaping the thermodynamic and chemical structure of the extra-tropical transition layer (ExTL). Cirrus may play a vital role due to associated latent heating and their influence on radiative and turbulent properties. Here we present for the first time in situ observations of the ExTL thermodynamic structure in- and outside cirrus by utilizing a dual-platform approach. The observational data were collected during the AIRTOSS-ICE campaign. Earlier analysis by Müller et al. (2015) suggests that the observed cirrus had formed in stratospherically influenced air masses based on measured N<sub>2</sub>O mixing ratios. The dual-platform approach reveals substantial disturbances in the vertical profile of potential temperature with a weakened stratification inside the cirrus and sharpening above.</p> <p>Lagrangian analysis based on high-resolution ICON simulations suggests that cirrus related radiative cooling and latent heating are instrumental in the formation of the observed disturbed potential temperature profile. Radiative cooling and to a lesser degree turbulent heat and momentum transport result in substantial PV production in the upper part of the cirrus and a steepening of the vertical potential vorticity gradient. The simulation reproduces key aspects of the in situ observations and the larger-scale evolution as evident from satellite and radiosonde data. Our analysis further indicates that the cirrus particles formed in an already moist ExTL air mass over Southern Germany about 12 hours before it being sampled over the North Sea.</p> <p>Our findings underline the importance of diabatic cloud processes for the thermodynamic structure of the ExTL and potential cross tropopause exchange.