Most cloud formations are the result of water vapor accreting around nuclei. A cirrus cloud model presented by Paul J. Demott, at the Department of Atmospheric Science at Colorado State University deals with laboratory simulations of cirrus cloud processes. The primary variables of the model are temperature, relative humidity, and aerosol size. Special attention should be given to this last variable mentioned. This source states that the material being delivered by aircraft is composed of barium salts, and that it is being used in connection with advanced radar studies.
Barium oxide is a whitish powder.