The Rossby number, named for Carl-Gustav Arvid Rossby, is a dimensionless number used in describing geophysical phenomena in the oceans and atmosphere. It characterises the ratio of inertial forces in a fluid to the fictitious forces arising from planetary rotation. It is also known as the Kibel number.

It is defined as:

<math>R_o=\frac{U}{2L\Omega\sin\varphi}<math>

- where U and L are, respectively, characteristic velocity and length scales of the phenomenon, Ω the angular velocity of planetary rotation, and φ the latitude. The term 2 Ω sin φ is the Coriolis frequency.

When the Rossby number is large (such as in the tropics and at lower latitudes), the effects of planetary rotation are unimportant and can be neglected. When the Rossby number is small, then the effects of planetary rotation are large and the geostrophic approximation is valid.