Cirrus clouds play an important role in the earth's climate system. The analysis of clouds and radiative transfer inside clouds is important to obtain a better understanding of the origin of the polarization difference signal and the relation to the cloud properties.

The vector radiative transfer equation is solved numerically by the fully polarized scattering model ARTS-1-1 which uses spherical geometry.

Several simulations for microwave frequencies were carried out that show the relation of the polarization difference Q to different cloud properties, e.g., particle size and shape.

This talk will give an overview on the dependencies of Q on cloud properties based on the results of the calculations.

Additionally a simple conceptual model will be presented to explain that even spherical scatterers lead to a polarization difference signal for microwave limb radiances. The conceptual model relates the polarization difference measured by a limb-looking sensor situated inside a cloud with the anisotropy of the radiation.