Rossby number

[1][2] It is commonly used in geophysical phenomena in the oceans and atmosphere, where it characterizes the importance of Coriolis accelerations arising from planetary rotation.

A small Rossby number signifies a system strongly affected by Coriolis forces, and a large Rossby number signifies a system in which inertial and centrifugal forces dominate.

For example, in tornadoes, the Rossby number is large (≈ 103), in low-pressure systems it is low (≈ 0.1–1), and in oceanic systems it is of the order of unity, but depending on the phenomena can range over several orders of magnitude (≈ 10−2–102).

[5][6] Cyclostrophic balance also commonly occurs in the inner core of a tropical cyclone.

In the oceans all three forces are comparable (called cyclogeostrophic balance).

[6] For a figure showing spatial and temporal scales of motions in the atmosphere and oceans, see Kantha and Clayson.

[8] When the Rossby number is large (either because f is small, such as in the tropics and at lower latitudes; or because L is small, that is, for small-scale motions such as flow in a bathtub; or for large speeds), the effects of planetary rotation are unimportant and can be neglected.