Aircraft observations during the summer over the eastern Pacific Ocean, ≥400 km offshore, show that the free troposphere has a distinctly nonuniform, layered structure. Analysis of plots of ozone versus total water mixing ratio indicates that the layers typically consist of a mixture of air from several sources: (1) moist, ozone-depleted boundary layer air, (2) very dry air with high ozone content, which probably originated in the middle or upper troposphere, and (3) air with relatively low ozone and moderate, varying moisture contents, which may represent residues of convective clouds that had formed over the ocean upstream of the research area. On two research days we also observed what we infer to be continental air. On one day in the usual research area we encountered air with elevated concentrations of aerosols whose trajectories, traced backward in time, indicated that it had passed over the Alaskan continent. On another day, when soundings were made about 100 km off the California coast, we observed a layer just above the marine inversion containing unusually high amounts of ozone, and moisture. This layer probably represents a direct intrusion of polluted air from the coast, which could have been the result of baroclinic flow associated with boundary layer warming over land. Over the eastern Pacific Ocean in summer the subtropical high results in stably stratified, subsiding air, where discrete horizontal layers with differing flow velocities transport air over long distances with little vertical mixing. Here, localized large-scale flows (such as tropospheric folding, deep convection, or baroclinic flow from coastal areas) ''inject'' air of different properties at various levels in the lower troposphere. This air is then subject to buoyancy sorting and differential advection. Because the lower troposphere is very stably stratified, the mixing eddies are small compared to the air mass size, and small-scale turbulent diffusion is slow. Here, wind shear plays an important role through stretching and thinning the different air masses until small-scale diffusion completes the mixing process. ¿ American Geophysical Union 1992