article_snippet.sent: One theory, suggested by Steven Hanna in 1969 in Steven Hanna Formation of Longitudinal Sand Dunes by Large Helical Eddies in the Atmosphere paper, is that linear dunes are created by helical roll vortices.
article_snippet.sent: The necessary conditions for the formation of these roll vortices are fulfilled over large deserts and these roll vortices spacings agree with the observed spacings of Linear dunes.'
article_snippet.sent: One theory, suggested by Steven Hanna in 1969, is that the linear dunes are created by helical roll vortices (illustrated left).
article_snippet.sent: his said Steven Hanna Steven Hanna compared data on the spacing of linear dunes with the expected dimensions of helical roll vortices in subtropical desert areas, concluding their 'probable wavelength was 1.9 miles to 3.7 miles (2km to 6km).'
article_snippet.sent: The observed scale of helical roll vortices is in many cases much larger than average dune spacing, suggesting that linear dunes are not the product of such atmospheric motions,' explained Nicholas Lancaster. '
article_snippet.sent: Evidence for the existence of helical roll vortices in linear dune landscapes is slight.