Radar observations made with a polarization diversity radar show the existence of a high degree of common orientation of the particles in the upper levels of thunderstorms. Abrupt changes in orientation associated with lightning discharges have frequently been observed, the particles becoming disoriented, or in some cases reorientated, in less than 1 s. Recovery times of the order of 10--15 s were observed. A consequence of the oriented states is the existence of differential propagation charactersitics for radio waves propagating through the medium. A diffential phase shift of 2.5¿/km at 1.8-cm wavelength is reported for a thunderstorm.