# Rossby number

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## Rossby number

(**ross**-bee) Symbol:

*R*

_{0}. The ratio of the rotation period

*P*to the turnover time τ

_{c}of a convective cell in a star's interior or a planetary atmosphere; τ

_{c}is the time required for hot gas to rise from the bottom to the top of a convective cell and then fall back as it cools. The Rossby number is a measure of the efficiency of a star's dynamo (which ‘drives' its activity), smaller values of

*R*

_{0}indicating a greater influence of rotation upon convection, and therefore higher activity, irrespective of the surface temperature of the star.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006

## Rossby number

[′rȯs·bē ‚nəm·bər] (fluid mechanics)

The nondimensional ratio of the inertial force to the Coriolis force for a given flow of a rotating fluid, given as

*R*_{0}=*U/fL,*where*U*is a characteristic velocity,*f*the Coriolis parameter (or, if the system is cylindrical rather than spherical, twice the system's rotation rate), and*L*a characteristic length.McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

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