The study compares radiance measurements taken by the Airborne Research Interferometer Evaluation System (ARIES) above a cirrus cloud over the sea with multiple scattering line-by-line simulations. The complete data set allows only a partial reconstruction of the main microphysical and geometrical features of the cirrus. However, spectral standard deviation of the radiance signal, which includes the spatial variability of the signal, is also available and is of great importance when comparing the simulations with the measurements. Two measurement sections are analyzed where the cloud shows different transmissivity and the comparison of simulations with measurements shows deviations that are below 1 standard deviation from 600 to 2250 cm-1, except in small spectral intervals. The quality of the simulation results has justified an in-depth theoretical examination of the diabatic heating of the atmosphere. The main result is that different geometrical configurations, maintaining the cloud top elevation and varying cloud depth but keeping the ice water path constant, produce very similar results in the radiance and fluxes above the cloud and at the surface and, at the same time, very different diabatic heating of the atmosphere inside and below the cloud, which implies very different effects on the evolution of the cloud and its microphysics. From these results the use of additional remote-sensing tools to define the cloud geometry (for example, a lidar sensor) appears of fundamental importance when the dynamical and microphysical evolution of a cloud layer needs to be investigated.