This work addresses three aspects of ice formation in clouds: the effects of silver iodide ice nucleating aerosols; measurements of natural ice nuclei; and simulations of ice formation in cirrus clouds. Two complementary approaches will be used: laboratory experiments in CSU's cloud chambers, and simulations with numerical cloud models. The experiments will simulate ice forming processes in a variety of well-defined cloud conditions, equivalent to atmospheric clouds. Measuring natural ice nuclei is a new application of the facilities; this work will establish the utility of a large expansion chamber to make measurements of natural ice nuclei. Ice nucleation processes in cirrus clouds will be simulated for both homogeneous and heterogeneous freezing processes. The laboratory experiments will be used to derive functional quantitative descriptions of ice nucleation. The laboratory results will be compare with those of the models, and revisions to the models will be made to improve their predictive capabilities. Several case studies of winter orographic cloud seeding will be simulated with a dynamical microphysical model using the new nucleation formulations. //
