WIIS IV comprises the investigation of solid and mixed-phase precipitation and cloud particles, and their influence on microwave propagation (scattering, absorption and attenuation), applying an instrument for precipitation micro structure measurement, the 2D-Video-Distrometer (refer to http://www.distrometer.at). Single scattering calculations of close to reality modelled precipitation particles are carried out using Mie-codes and Finite Element Methods. A further activities is the studying of the influence of acidic precipitation on the scattering parameters of single hydrometeors. A long term objective is a classification of winterly precipitation particles based on rigorously derived scattering amplitudes even for complicated shapes and mixed-phase constituents.