Abstract: The dispersion in the convective boundary layers (CBL) is non Gaussians. A double Gaussians PDF model is presented on the basis of three assumptions: 1. the probability distribution function of vertical velocity. pw at any given height is the sum of two Gaussian distributions; 2. the tracers are responding to pw. the probability distribution function. at the source height with straight line trajectories; 3. the reflection of tracers at surface and Zi, the mixing depth, is simple reflection, or some of the tracers are absorbed at Zi. Some statistical characteristics of vertical velocity W at three heights are analysed. The contrast of crosswind integral concentration between this model and Lamb's numerical modeling. Deardorff's tank experiment shows considerable agreement. The double gaussians model is also supported by the fields observed data of project CONDORS in the U. S. A.