There are two dimensionless groups given the name Rossby number.

The first represents the ratio of inertia force to **Coriolis Force** exerted on a flowing liquid, in particular that associated with the earth's rotation.

Rossby number 1,

where u is fluid velocity, ω_{e} is the earth’s angular velocity (s^{–1}), L length and the angle between the earth's axis of rotation and the direction of fluid flow.

The second Rossby number represents the ratio of fluid velocity to velocity associated with rotation.

Rossby number 2,

where ω is angular velocity (s^{–1}).