We discuss a climatological model named Extinction Coefficient for Stratospheric Aerosols (ECSTRA), describing the vertical and spectral dependence of the stratospheric aerosol extinction in the UV-visible range. The basic variables used for the analytical formulation of the model are the wavelength, a reduced altitude referenced from the tropopause level, the latitude, and a parameter that describes the volcanic status of the atmosphere. The evolution of the spectral and vertical features of the extinction profile is discussed with respect to the different basic variables. The behavior of the spectral coefficients is qualitatively interpreted as a function of volcanism. The volcanism dependence of the parameters characterizing the Junge layer is also investigated: the peak position shows variations of several kilometers, with a minimum value reached between 0 and 2 km above the tropopause, while the peak width presents a weak volcanism dependence. The most important discrepancies between the model and the SAGE II input data set are found to be related to dynamical effects, and particularly to the influence of the quasi-biennial oscillation.