The growth of particle clusters is modeled by allowing the particles to be moved by the atmospheric winds and a random velocity component. The dispersion of the cluster is followed for 432 hours. The results from many model runs using wind fields during a 2-year period indicate that clusters grow with a time-varying exponent. The growth between 40 and 150 hours is exponential in time. The rapid growth can be explained as resulting from horizontal wind shear. ¿ American Geophysical Union 1987