Longitudinal dune forms can be almost exactly simulated by running a roller across a layer of grease. This technique particularly duplicates the uniform spacing, the proper number and angle of dune junctures, and the fact that dune junctures always open upwind (or up-roller). Movement in the viscous grease layer accurately models helicoidal movement in the lower, boundary layer of the atmosphere. The longitudinal dune form is produced by alternately rotating pairs of horizontal roller vortices beneath the main movement of the geostrophic winds. Oghurd (star) dunes are simulated by lifting a sheet of cardboard vertically off a grease spot, thus indicating that convection cells developed by predominantly vertically moving air currents form the dunes.