Air may move, with a circular motion, northwards or southwards in a vertical cell; see Trump et al. (1982) Boundary-Layer Met. 24 for a model of a closed mesoscale atmospheric cell. Atmospheric cells are major components in the transfer of heat and momentum in the atmosphere from the equator to the poles. Horizontal cells (winds) also fulfil this role; see Abitol and Ben-Yosef (1994) J. Opt. Soc. Am. A 11, 7.