Figure 3

Ball moving below the critical speed, but flow rendered unsymmetrical by the effect of the seam. Ball viewed from directly above.

The roughness of the seam at the front of the ball (on the left as shown here) makes the boundary layer cling much further around this side. On the smooth side the layer breaks away early on. This produces an asymmetrical pressure distribution over the surface, and the resulting sideways force component makes the ball swerve.

The seam stays in position because of the backward spin given to it as bowled; the direction of this is indicated by small arrows along the central line of the seam.

(The drawing is not to scale : the thickness of the boundary layer is exaggerated in order to show it.)