An Etymological Dictionary of Astronomy and Astrophysics

A → dimensionless quantity that measures the strength of → viscous forces relative to the → Coriolis force in a rotating fluid.
It is given by E_k = ν/(ΩH²), where ν is the → kinematic viscosity of the fluid, Ω is the → angular velocity, and H is the depth scale of the motion. The Ekman number is usually used in describing geophysical phenomena in the oceans and atmosphere. Typical geophysical flows, as well as laboratory experiments, yield very small Ekman numbers. For example, in the ocean at mid-latitudes, motions with a viscosity of 10^-2 m²/s are characterized by an Ekman number of about 10^-4.

See also: → Ekman layer; → number.