A heuristic test for the presence of vorticity is to imagine a small paddle wheel in the flow; the paddle wheel acts as a ‘circulation-meter’, and rotates if the vorticity is non-zero. Vorticity might seem to be similar to angular momentum, in that it is a measure of spin. However, unlike angular momentum, the value of vorticity at a point does not depend on the particular choice of an axis of rotation; indeed, the definition of vorticity makes no reference at all to an axis of rotation or to a coordinate system. Rather, vorticity is a measure of the local spin of a fluid element.

• If the paddle wheel is rotating clockwise then there is negative (or anticyclonic) relative vorticity.
• If the paddle wheel is rotating counter-clockwise then there is positive (or cyclonic) relative vorticity.