From Glossary of Meteorology
A velocity scale used in the transport of trace constituents across the air–sea interface, given by
where F is the flux of constituent C across the interface and v is the transfer velocity.
The indices a and w refer to air and water, respectively. The constituent Cw must be considered the concentration in the air in equilibrium with the water.
Kraus, E. B., and J. A. Businger 1994. Atmosphere–Ocean Interaction. Oxford University Press, . p. 163.