Zonal bands of stratospheric aerosol optical depth are observed by various sensors under both volcanic and non-volcanic conditions. Maximum optical depths occur within the tropics and at high latitudes, while minima are found between latitudes of approximately 15¿ and 45¿. This structure is shown to result from a reservoir of aerosol over the tropics, poleward transport, and the departure of the tropopause from isentropic surfaces. ¿ American Geophysical Union 1994