This thesis concentrates on the frequency dependence of PMSE properties based on the measurements with four radars. It reveals that the vast majority of all observations is consistent with theory which assumes PMSE originate from turbulence-induced scatter in combination with an enhanced Schmidt number due to the presence of charged ice particles. Based on this theory, a new algorithm has been developed to calculate Schmidt numbers and hence radii of the ice particles. Applying this algorithm to our observations, yields ice particle radii which are in excellent agreement with independent results.