Optical properties of aerosols and clouds: the software package OPAC. OPAC is a software package which contains the optical properties in the solar and terrestrial spectral range of atmospheric particles, i.e. water droplets, aerosol and ice crystals. In all 3 groups data of important or typical examples are available in OPAC. The aerosol related data are also used in the Global Aerosol Data Set (GADS). In order to make OPAC available on most of the common computer systems, the data are provided in ASCII files and the enclosed software as a FORTRAN program. The data consist of optical properties of clouds and aerosol components, all calculated for 1 particle/cm^3. The FORTRAN program allows to read these files, to mix aerosol components to aerosol types consisting of several components, and to calculate additional optical properties which are not stored in the ASCII files. Aerosol in the atmosphere is assumed to be a mixture of different components. This mixture can be achieved with data given by the user or by the use of typical mixtures, called aerosol types, which are presented in OPAC. Multiplying the stored data with realistic number densities, either with data given by the user or available in OPAC, gives absolute optical properties. Moreover, the height distribution of the particles can be varied, likewise with data given by the user or with prefixed data. For those aerosol components, which are assumed to change their size with relative humidity, values for humidity classes are given. The data are given for up to 61 wavelengths between 0.3 µm and 40 µm and for up to 8 relative humidities. Conception and design of OPAC are due to G.A. d´Almeida. Nevertheless, the aerosol data used for calculating the optical properties have been completely revised compared to the original version in d´Almeida et al. (1991). This revision has been performed mainly by P. Koepke in collaboration with Ingrid Schult at MPI Hamburg. The cloud properties in OPAC are compiled by M. Hess.