Howzat work? The science of swing debunked

By Lauren Davis
Friday, 01 June, 2012

“The atmosphere feels very heavy, so you will have to wonder if the new ball will swing.” These are the words of former English cricket captain Michael Atherton, stating the commonly held belief that high humidity equals high swing. But as researchers from the UK and NZ recently discovered, this is not the case - in fact it’s rather the opposite.

The idea goes that the moisture in the air from damp or humid conditions will make it easier for a cricket ball to swing, ie, unexpectedly deviate mid-delivery. But despite scientific literature covering the topic since the 1950s, there has been no experimentally validated theory to support it. Now this long-held myth has been busted following testing by Dr David James and Dr John Hart from Sheffield Hallam University and PhD student Danielle C MacDonald from Auckland University of Technology.

In the paper ‘The effect of atmospheric conditions on the swing of a cricket ball’, published in the journal Procedia Engineering, the researchers investigate the effect of atmospheric conditions on swing delivery. Using a 3D trajectory model which incorporated features of a right-handed swing bowler, they discovered that as they increased the humidity from 0 to 100%, the deviation actually decreased - but only by 5.4 mm.

“This lateral deviation is very small indeed and is unlikely to be perceivable by players or audiences,” explained the researchers. “Furthermore, the small change in displacement is actually in the opposite direction to the commonly held belief that increasing humidity increases swing. Of course, this is because whilst humid or damp air is often referred to as being ‘heavy’, humid air is actually less dense than dry air.”

Other atmospheric conditions that were tested included temperature and altitude. As temperature increased from 15 to 35°C, swing displacement decreased by 7 mm. Altitude was the only condition to have a large effect on swing displacement, with the difference between Lords, London (14 m altitude), and The Wanderers, Johannesburg (1750 m altitude), having an average difference of 210 mm, which varied at different bowling speeds.

The researchers then proceeded to test the theory that higher humidity may subtly change the geometry of the ball through a swelling of the hand-stitched seam, with a larger seam possibly creating more asymmetrical airflow around the ball and thus more swing.

Measuring five balls in various stages of wear and tear in a climate-controlled chamber, the researchers found that all five balls did indeed lose mass (average 0.16 g) as the humidity was reduced, and increased in mass as it was raised, with the more worn balls increasing in mass the most - by almost 1 g if humidity was increased from 50 to 100%.

“However,” noted the researchers, “this change in mass is still relatively small and will not affect its flight trajectory by any considerable degree. Indeed, all other variables remaining constant, if a cricket ball’s mass were to increase by 1 g its swing deviation would actually decrease by 66 mm assuming a 30 ms-1 delivery in 50% humidity at 25°C. Once again, there is a small effect, but it actually works in the opposite direction to the commonly held belief that humid conditions are good for swing.”

So why do people associate humidity with high swing? The researchers propose that scientific literature on the topic is actually misinterpreting what the majority of cricketers are saying, and that these players, including England’s Andrew Flintoff, actually believe that cloud cover is responsible for swing, not humidity.

“Players are convinced that the ball swings more on cloudy, overcast days and whilst the humidity levels on these days may tend to be higher than normal, humidity is not the key factor.”

The researchers have therefore put forth the hypothesis that cloud cover leads to stillness in the air and low levels of air turbulence above the cricket pitch.

“This is because cloud cover reduces the effect of sunlight heating the ground and this tends to reduce the convection currents within the air. Theoretically, low levels of air turbulence are good for swing bowling as the bowler will be more able to create an asymmetry within the boundary layer of airflow around the ball.”

While this theory seems to hold more water than its predecessor, the researchers emphasise that it is still little more than a loosely formed idea. Rigorous scientific testing will be the only way to see if the concept will stay in, or be swiftly dismissed.

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