Why does a cricket ball swing?

A ball that bends in the air

Watch a fast bowler closely and you will sometimes see the ball curve sideways as it flies down the pitch. Cricketers call this swing. It is not spin bowling, where the ball changes direction after it bounces. Swing happens in the air, and calculations from wind-tunnel measurements, backed up by photographs of real deliveries, show a well-bowled ball can move sideways by more than 80 centimetres.

The secret is the ball itself. A cricket ball is made of leather, and its two halves are joined by the seam: six rows of raised stitching around the middle, with roughly 60 to 80 stitches in each row. In men's Tests the ball must weigh between 155.9 and 163 grams and measure 22.4 to 22.9 centimetres around.

That raised seam, plus the difference between a shiny side and a rough side, is what makes a ball swing. To see why, look at a layer of air thinner than a coin.

The thin layer of air

As a ball flies, a very thin layer of air clings to its surface and slides around it. Scientists call this the boundary layer. It can flow in two ways. A laminar layer is smooth and orderly, like water sliding over glass. A turbulent layer is ruffled and swirly.

Here is the surprising part: the ruffled, turbulent layer holds on to the ball for longer. A laminar layer peels away early, near the widest part of the ball. A turbulent layer has more energy, so it hugs the surface further around towards the back before it lets go.

Now imagine the bowler releases the ball with the seam tilted about 20 degrees to one side. On the seam side, the air has to climb over the stitching, and that bump trips the smooth layer into turbulence. On the other side the leather is smooth, so the layer stays laminar. The air peels off late on the seam side and early on the shiny side. The flow is lopsided, the pressure on the seam side drops, and the ball is pushed sideways, towards the seam. That is conventional swing.

Seam pointing towards the slip fielders? The ball swings away from a right-handed batter: an outswinger. Seam towards fine leg, and it swings in. The bowler also gives the ball backspin along the seam as it rolls off the fingers, which keeps the seam steady in flight. A wobbling seam does not trip the air properly, which is why swinging a brand-new ball is harder than it looks.

Shine one side, let the other go rough

For conventional swing, the non-seam side must stay smooth so its air stays laminar. That is why fielders rub the ball on their trousers between deliveries: they polish one side only, and let the other scuff up on the pitch.

The Laws of Cricket are strict about this. A fielder may polish the ball on their clothing as long as no artificial substance is used; the only natural substance allowed is sweat. They may remove mud under an umpire's eye and dry a wet ball on an approved cloth. Anything else, such as scratching the leather or lifting the seam with a fingernail, is ball tampering. If the umpires decide the ball has been changed unfairly, the other side gets five penalty runs and the ball can be swapped.

Bowlers used to shine the ball with saliva too. That stopped during the COVID-19 pandemic for health reasons, and in 2022 the Laws made the ban permanent. Cricket Australia reported that the data showed little or no change in how much the ball swung. Sweat does the job.

Speed limits and reverse swing

In wind-tunnel tests at Imperial College London and NASA, scientist Rabindra Mehta and his colleagues measured how hard the air pushes a swinging ball. The sideways force appears at about 55 kilometres per hour and grows with speed, peaking at roughly 108 kilometres per hour, a brisk medium pace, with the seam at about 20 degrees. Those numbers come from a lab, with the seam held at one angle. Real bowlers change the seam angle and the state of the ball, and researchers say the lab numbers do not match every swing measured in real matches, so treat them as a guide, not a rule.

Bowl faster and something odd happens. Above about 110 kilometres per hour the smooth side's air also turns turbulent, the two sides match, and the swing fades. Past about 130 kilometres per hour with a new ball, the swing comes back the other way. Now both layers are turbulent, and the seam thickens the layer on its side so it peels away earlier. The ball swings away from the seam. This is reverse swing, made famous by Pakistani fast bowlers such as Wasim Akram and Waqar Younis in the 1980s and 1990s.

The rougher the ball, the lower the speed at which reverse swing kicks in. An old ball with about 100 overs of wear reversed at around 105 kilometres per hour in Mehta's tests. That is why a team keeps one side shiny and lets the other go rough: turn the ball over, and the same bowler can swing it both ways with the same action.

What about "late swing", when the ball seems to dart at the last moment? Mehta's measurements suggest it is built in. A swinging ball follows a curve called a parabola, and about three quarters of the sideways movement happens in the second half of the flight. The ball is swinging the whole way. It just looks sudden.

What scientists still argue about

Ask any cricketer and they will tell you the ball swings more on a humid, cloudy day. Here the science gets interesting, because the laboratory does not agree. Wind-tunnel tests going back decades have found no change in the side force when the air is more humid. Soaking a ball did not make its seam swell. And humid air is actually less dense than dry air, not "heavier", as commentators often say.

Researchers at Sheffield Hallam University in England tested balls in high humidity and found no evidence of extra swing. Either the effect is a myth, they concluded, or something not yet understood is going on. Mehta says the question is still open: he has seen the effect on the field, but there is not enough evidence to explain it. His best guess is that the varnish on a new ball turns slightly tacky in damp air, so the bowler grips it better and adds a little more spin without noticing.

Other puzzles remain. A 2022 University of Cambridge PhD thesis found "little consensus on the optimal strategies" for swing bowling, and said the effects of ball wear and weather have not been measured properly. Careful wind-tunnel experiments published in 2018 by researchers at the Indian Institute of Technology Kanpur showed the same three stages as a ball speeds up: no swing, then conventional swing, then reverse swing. But how gusts and turbulence on match day change things is still being worked out. Every time a bowler shines one side and runs in, they are running a physics experiment.

📚 Words to know

Seam

The raised rows of stitching around the middle of a cricket ball.

Boundary layer

The very thin layer of air that clings to the surface of a moving ball.

Laminar

Smooth, orderly flow, like water sliding over glass.

Turbulent

Ruffled, swirling flow full of tiny eddies.

Conventional swing

Swing towards the side the seam is pointing to.

Reverse swing

Swing away from the seam, which happens at high speed or with a rough ball.

🧪 Try this: get to know a cricket ball

1. Find a cricket ball (a tennis ball with a seam drawn on in marker works for the grip). Count the rows of stitching on the seam: a proper ball has six. Feel how the stitches stand up from the leather. That bump is what trips the air.
2. Hold the ball with your first two fingers either side of the seam, seam upright, and tilt it a little towards where first slip would stand. Practise letting it roll off your fingers so it spins backwards along the seam without wobbling. That steady seam is the hardest part of swing bowling.
3. Air clings to curved surfaces, and so does water. Hold a spoon lightly by the handle and bring the back of it close to a thin stream of water from a tap. The water wraps around the curve and the spoon gets pulled in. The air around a cricket ball clings to the leather in a similar way, and swing comes down to where that air lets go on each side.
4. Next time you watch cricket, look at which side of the ball the bowler is polishing and where the seam points as it leaves the hand. Then watch which way it swings. Does it match the physics?