Extended (7-y) polarization lidar data, which are sensitive to ice crystal shape and orientation, are used to search for diurnal changes in cirrus clouds caused by solar heating. Average linear depolarization ratios are ~0.02 lower during the day than night, minimizing at a 45¿ sun angle at ~0.05 lower than the 0.350 nighttime average. Analysis of the diurnal pattern using scattering simulations suggests that plate crystals are more common during daylight, and that a feedback occurs between solar heating and the maintenance of horizontal plate orientations. This anisotropy also affects the cirrus solar albedo, representing an additional complication confronting climate modelers.