A dryline is a narrow zone, other than a classical polar front, across which a sharp horizontal surface moisture gradient occurs. It often exists and moves under synoptically quiescent conditions. One explanation of dryline motion is that it is caused by turbulent vertical mixing. A slab symmetric numerical model of the boundary layer is developed to test this hypothesis. This model, when applied to a typical dryline environment, produces dryline motions similar to those observed in the atmosphere. The model is applied to several real dryline cases, and a close correspondence between simulated and observed motions are found. It is concluded that vertical mixing is the primary cause of dryline motion.