2023 Ashes Preview - A guide to watching swing bowling
Things to look and listen for in the next few weeks while watching the best series in cricket!
At 10:30am on Friday 16th June, Ben Stokes and Pat Cummins will stride out to the middle at Edgbaston and toss a coin to choose who takes the field for the first innings of the 2023 Ashes. In the build up to this moment, there will be discussion from pundits about team selection, batting line-ups and, invariably, whether the ball will swing. Whichever captain wins the toss, they are likely to cite the conditions, with possible reference to the pitch and weather, notably the cloud cover, temperature or humidity on the day.
At 11am, the first ball will be bowled. The crowd watches with baited breath as the ball is delivered, and travels down the pitch. It flies dead straight, thudding into the middle of the openers bat. Somewhere in the crowd, friends turn to one another, looking up at the replay, and say “don’t worry, it’ll swing once the clouds come over”.
Hypotheticals aside, the discussion around why a ball swings, or doesn’t, is a staple of any test match coverage. The aim of this article is to cut through the noise you are going to hear over the next few weeks, and give cricket fans some science-backed factors to watch out for this summer. While not everyone will go as far as me in checking the barometer at the Birmingham University weather station, hopefully fans can become more aware of what really impacts swing.
“The adage at Headingley is to look up, not down, and though day one looks to be another scorcher, more cloud cover (and possible rain) is due to roll in over the weekend.” - Ali Martin, The Guardian, 3rd Test vs New Zealand, 2022
The following sections contain pointers for thinking about swing which help decipher what is happening in a Test match. There are a lot of simplistic ideas around swing which hold some truth, but these are not as universal as you might think, given the confidence with which they are often stated. Instead, the concepts in this article outline things to look for and consider as you watch the best bowlers in the world display their skills over the next six weeks.
No two deliveries are the same
Every delivery in cricket has three elements which affect how the ball travels through the air, and whether or not it swings: the bowler, the ball, and the atmospheric conditions. The bowler will release the ball at a certain speed, at a given orientation and with some form of spin. The ball itself may be brand new, have one side rough and the other buffed to a shine, or look like it has been chewed by a dog. The air through which the ball travels will be affected by the weather. It is these three components that set up the aerodynamics of a delivery.
The complexity of combining these elements leads to a key point, which is that swing is a statistical problem. Every delivery is unique, and so it is hard to discuss swing in terms of absolutes - what will and won’t happen for certain. Instead it is better to consider the probability of an outcome occurring, based on the different elements of a delivery, or number of deliveries.
My research involves identifying how these elements promote or reduce swing, though, for a given delivery it isn’t always possible to know the exact combination at play. There are, however, things you can look out for in video footage and weather forecasts that can help you understand why a ball behaved in a certain way, and whether that is likely to continue or not. But first, let’s look at why a ball swings in the first place.
The ball swings when there is different airflow on either side
The main aim for the bowler is to create different types of airflow on one side of the ball compared to the other. The air behind the ball, called the wake, is then deflected in one direction, and the ball moves in the other, thanks to Newton’s Third Law: Every action has an equal and opposite reaction.
To understand what causes the wake to be deflected, we need to get a bit more technical. For those of you who aren’t particularly interested in the precise science, apologies, but this bit is for the real cricket aerodynamics nerds.
On a brand new ball, both sides are smooth, and so the bowler uses the seam to change the airflow on one side of the ball. Anyone who has watched a slow motion video of Jimmy Anderson’s out-swinger has seen the seam angled towards the slips, and the ball rotating perfectly around the axis of the seam (go and watch the video in the first post again, you know you want to!). The angled seam creates two distinct sides of the ball, the seam side, and the non seam side. In the smoke flow picture above, the top half of the ball is the seam side, and the bottom half is the non seam side.
So what happens on either side of the ball? On the non seam side, the surface is smooth, and the airflow close to the ball is laminar. This clean air doesn’t stay attached to the ball very well and separates at the middle of the ball - you can see this at the bottom of the ball in the photo above where the smoke just starts to appear.
On the seam side, the air close to the ball interacts with the seam, creating turbulent flow just above the ball. When the air close to the ball separates now, it hits this turbulent flow and re-attaches, sticking to the ball’s surface for longer than on the non seam side. The difference between the separation position of the air on each side of the ball causes the wake deflection, creating the swing force towards the seam side. One common claim is that the seam acts a ‘rudder’ - this implies that the seam pushes the air to cause the deflection but this isn’t really what is going on so I avoid using that terminology.
So, to make the ball swing away to a right-handed batter, you angle the seam to Third Man, and to make it swing in, you angle it to Fine Leg. Easy, right? In practice, of course, the key question is often why is the ball not swinging? The next few sections will look at the three elements of swing I discussed earlier, bowler, ball and atmospheric conditions, and show why the scenario illustrated in the diagrams doesn’t always occur.
The skill of a swing bowler is in controlling the seam position
Now you understand the basics of how to generate swing, lets focus on the bowler skills. There are three things a bowler controls which are relevant for swing: the speed, the seam angle, and the rotation of the ball. All three of these play a role in swing, and influence the airflow around the ball.
First, let’s consider the speed of the ball. It is a well known aerodynamic result that the faster you bowl, the harder it is to swing the ball conventionally. I won’t got into the full details - we don’t have all day to discuss Osborne Reynolds and Tollmein-Schlicting waves - but definitely have a look at the speed gun in the first few overs if you are expecting the ball to shape in the air.
Professional cricket happens to be played at a speed range that is very close to an aerodynamic ‘cliff-edge’. This is purely by luck, but it does mean that a ball can stop swinging simply because of a change in speed, where a small change in the airflow results in a big change in the force on the ball.
The role of the seam, as mentioned, is to change the airflow on one side of the ball. Unsurprisingly, this only works if the seam stays on one side of the ball as it travels down the pitch. You will have seen a wobble-seam ball, where the seam flips sides as it travels down the pitch and rotates, and subsequently doesn’t swing. This is because there is no clear seam and non seam sides and the wake is not consistently deflected one way or another.
The real skill of elite swing bowlers is keeping the seam stable, i.e. to minimise the seam wobble and maintain an angled seam. If a ball doesn’t swing, check the replay. You’ll often see that the seam is wobbling throughout the ball’s flight.
The shiny side is king for conventional swing
You may have noticed that I haven’t yet mentioned the rough and shiny sides of the ball. That’s because you don’t need a rough side to swing the ball. We have all seen a brand new ball swing, so why do people insist on saying that swing happens because of a rough and shiny side?
This is not to say that ball condition is not important; it is actually the main reason a ball stops swinging because you can only create laminar flow if the non seam side is smooth. No laminar flow, no difference to the seam side, and no swing. This is why teams polish the ball, you can’t have swing without a shiny side.
Remember that cliff-edge I mentioned earlier? It turns out that the shiny non seam side is susceptible to it as well. Damage to the ball and splits in the quarter-seams can all be enough to stop the ball swinging, especially at fast bowling speeds. If there is no swing, keep an eye out for a close up of the ball - have the team looked after it well enough?
The wind is more important than the clouds
Atmospheric conditions are the topic I am most frequently asked about, probably because they are the area with the least consensus between players, fans and pundits. The simplest way to consider the role of the weather is to think about how it affects the airflow around the ball. The only way this can happen is by changing the block of air over the pitch, which the ball travels through for a delivery.
The first way this happens is by changing the air properties, specifically the air density, which is the ‘weight’ of the air, and the air viscosity, which can be thought of as its ‘stickiness’. These factors are controlled by the temperature of the air and the atmospheric pressure. Warmer days reduce air density, which moves the cliff-edge to higher bowling speeds, and make it easier to swing the ball. The same goes for high altitude, where pressure is low, and the density reduced.
The wind also affects the air over the pitch, by causing it to move as the ball travels through it. This movement has a number of affects, but the one of the most prominent is increasing the atmospheric turbulence. Higher wind speeds increase the background unsteadiness in the air, making it harder to keep laminar flow on the shiny side. On some days, there is enough atmospheric turbulence to stop a new ball swinging altogether!
The two areas that are often mentioned in the same breath as swing are humidity and cloud cover. Humidity has been the focus of several scientific studies, with no evidence that it affects swing at all. Cloud cover is unlikely to directly change the air over the pitch, but may correlate with other factors. Days with cloud cover result in less heating of the ground, which on average may result in fewer wind currents. Along with this, days with low wind feel humid and sticky, as sweat is slow to evaporate. Such days with warm, still air are perfect for conventional swing.
Overall, however, the general influence of atmospheric conditions is likely over stated compared to the bowler and ball. The range of conditions found throughout this Ashes series will vary less than those seen playing on different continents. As well as this, differences in bowler technique and ball condition usually change at a much faster rate than the weather, and will likely have a bigger impact ball to ball.
Three things to watch for in the 2023 Ashes
I thought I would finish off the article with a few things to watch out for in the upcoming series. I’m not one for predictions, but there are a few storylines I expect to play out at some point.
A team will change the ball and it will start swinging
This happened in the 2019 Ashes, and shows the impact of small changes in the ball condition. Sometimes it isn’t the weather conditions that are the problem, teams realise this and try and get the ball changed!
Reverse swing will play a role at Old Trafford
I deliberately haven’t talked about reverse swing in this article as I am saving it as a topic for future posts, however, if the weather stays warm, I expect it to play a part in the series. Stay tuned for some more in-depth discussion on this as the series goes on, especially around the 4th Test at a venue known for reverse swing.
Michael Neser will be a danger with a swinging ball
This one is a bit left field, as he’s not in the initial squad, but he made some waves earlier in the year playing for Glamorgan, and took a superb hattrick with some incredible swing bowling: https://twitter.com/CountyChamp/status/1654450243524603904. I expect him to play a role if Australia look to exploit typical ‘English conditions’.
Thanks for reading, and I hope you’re as excited for the series as I am. I will try and post more throughout the Test matches, so subscribe to stay up to date. Go well!