The science of swing

Sports aerodynamics consultant and NASA scientist Rabindra Mehta on how weather conditions affect the ball’s aerodynamics, in the Dawn.

A popular theory that has circulated for years suggests that in damp conditions, the primary seam swells by absorbing moisture, thus making it a more efficient boundary layer trip. We investigated this possibility in detail by first measuring profiles of the primary seam using a precision stylus device on a new ball before and after a few minutes soaking in water. A similar test was also performed on a used ball, where the varnish on the seam had worn-off. In both cases, no swelling of the seam was observed. These two balls were then tested in a wind tunnel to measure the side force. These tests showed conclusively that there was no increase in side force for the wet balls. Other investigators have also failed to find any positive effects of humid conditions on cricket ball swing in laboratory tests. Is it possible that changes in the atmospheric properties can cause the ball to swing more?