1. Overturning, quasi-two-dimensional circulations parallel to mean wind in the layer they occupy in which individual particles move downwind in a helical motion. In the atmosphere, boundary layer rolls usually consist of alternating counterrotating helices, aligned nearly parallel to the mean boundary layer wind. When clouds are present, they form over the upwelling parts of the roll circulation. Called cloud streets, they are a good measure of roll wavelength. Typically, cloud streets are spaced at about two to three times the depth of the rolls, although larger spacings are not unusual. Several mechanisms have been proposed for forming rolls; formation and maintenance of atmospheric rolls is thought to involve both buoyancy and shear effects. In some cases, rolls are thought to result from the action of gravity waves on the boundary layer. Rolls occur in the convective boundary layer and have been observed with both stronger winds and midboundary layer wind maxima. They have also been observed for lighter winds with weaker buoyant forcing. 2. The overturning motion that results from breaking Kelvinâ€“Helmholtz waves.