Pat Cummins is steaming in at the SCG to Pakistan's star batter, Babar Azam.

He lets a ball rip that out of the hand is heading outside the off-stump, before it swerves dramatically in to trap Azam on his pads.

It's close to the perfect example of an inswinger by a bowler at the top of his game.

The mastery of swing with the new and old cricket ball is the ultimate weapon that separates the great fast bowlers from the rest.

It is one of the most devastating weapons a fast bowler can own — but the art of swing is steeped in mystery and myths that only science can explain.

Continue reading on ABC News for the full experience

How many times have you heard the ball swings when it's humid, when there's cloud cover, when it's dusk, or when one side of the ball is heavier than the other?

Some of these statements may be true, and some are blatantly false, but the scientists who have studied swing bowling say the aerodynamics that explain it is relatively straightforward despite the myths and mystery.

It all comes down to the world of fluid dynamics, and while it certainly isn't rocket science, it took a NASA scientist to explain the aerodynamics of swing and reverse-swing bowling.

Dr Rabindra Mehta is an expert in experimental fluid mechanics and aerodynamics at NASA's AMES research centre in California.

He's also a former club cricket fast bowler.

In the early 1980s, Dr Mehta's childhood friend, the former Pakistani captain and later Prime Minister, Imran Khan, told Dr Mehta that sometimes the ball would swing what he called "the wrong way".

He was describing his accidental discovery of what is now called reverse swing and is a staple of any swing bowler.

"At the time, I honestly did not believe that such a phenomenon could occur since I could not explain it scientifically," Dr Mehta wrote in Cricinfo.

Dr Mehta identified four types of swing bowling: conventional swing, reverse swing, and contrast swing, which can be further broken down into conventional contrast and reverse contrast swing.

Conventional swing

England great Jimmy Anderson unleashed a brilliant spell of conventional outswing bowling against New Zealand in 2008, taking 7-43.

Time and again he beat the batters, who would play across the ball and get bowled on the off stump, get hit LBW, or get a nick to the slips.

To understand how he did it, we need to think about how air travels over the surface of the ball.

A test cricket ball is made up of four pieces of leather. Initially, two pieces of leather are sewn together internally to make one half of the ball.

A central cork core is placed between the two halves of the ball, which are then squeezed together in a machine.

The central seam is then sewn together, followed by two parallel seams on each side to produce the classic six-stitcher.

That seam is slightly raised above the surface of the ball.

Now picture an Anderson outswinger to a right-handed batter. He angles the seam of the ball towards first slip about a 20 degree angle from the ball's trajectory.

The ball will tend to move away from the batter towards the slips – or if it's initially on a leg-stump or leg side trajectory it will straighten up towards the stumps.

But what makes it swing?

Here we have the ball travelling in the direction of the arrow, with the seam angled at 20 degrees.

As the air hits the side of the ball facing the batsman (which we can call the non-seam side), it travels smoothly over the ball in what’s known as laminar flow.

The air will travel over a portion of the ball until it starts to separate and leave the ball’s surface.

But on the other side of the ball – the one facing away from the batter (which we call the seam side) – the airflow is tripped by the ball's seam into a turbulent state.

The turbulent air sticks to the surface of the ball for longer and separates later, travelling faster than the smooth air on the other side.

According to the Bernoulli Principle, that faster travelling air results in a decrease of pressure, creating a pressure differential.

On the turbulent side of the ball – the one facing away from the batter – the airflow is travelling faster and has a lower pressure than the smooth flowing high-pressure airflow on the other side.

Therefore the ball will swing towards the low pressure side of the ball and away from the batter in the direction of the seam.

And the beauty of conventional swing is that you can do it with a new ball.

"It's the seam which is doing the work, tripping the air into a turbulent state," Dr Rene Ferdinands, a biomechanical engineer and lecturer at Sydney University's Faculty of Medicine and Health explains.

The former cricketer, who has worked around the world as a professional cricket coach specialising in fast and spin bowling, says bowlers "have worked out that if you had a small seam angle [you'd notice] 'hang on, it's not swinging as much.'"

An inswinger to a right-hander is bowled by angling the seam at 20 degrees in the other direction – so that it's now facing down the leg side towards about fine leg.

The ball will swing from outside the batter's off-stump in towards them – as was the case with the Cummins delivery.

With all swing bowling it's important for the seam to stay in the same relative position. Bowlers do this by applying some backspin when they release the ball. The gyroscopic effect of the ball rotating keeps it steady.

As the ball ages, the bowling team will polish the non-seam side of the ball to keep it as smooth as possible in order to maintain the laminar flow.

In the case of conventional swing, the ball will always swing away from the "shiny" side and in the same direction that the seam is pointing.

Speed is also important. In their chapter on cricket ball aerodynamics, Dr Mehta states that a new four-piece will swing at speeds from 47km/h to 128km/h. In an older ball, the upper range is reduced to around 115km/h.

Obviously, the flow of the air over the ball is also affected by the prevailing wind, be it behind, into or from the side of the bowler. A ball bowled at 120km/h into a 20km/h headwind, will have an airflow speed of 140km/h.

For that reason, it's more accurate to refer to the speed of the airflow over the ball, rather than just the ball's velocity.

One study has shown the ideal conditions for conventional swing is with a seam angled at 20 degrees, at a surprisingly low speed of 108km/h and with a backspin of 14 revolutions per second.

Reverse swing

Take a look at this magnificent ball from the former, England fast bowler, Simon Jones:

The seam is facing towards the slips and the shiny side is facing the batter.

Everything suggests the ball will swing away from the batter towards the slips, but instead it goes the other way, thumping into the stumps and prompting Richie Benaud in commentary to describe it as "a great piece of reverse swing".

Reverse swing is a phenomenon that came into vogue during the careers of the brilliant Pakistani bowlers Imran Khan, Wasim Akram, and Waqar Younis.

For years the art was also associated with ball-tampering — but you don't need to tamper with the ball to get it to reverse.

What you do need to be is fast.

"Reverse swing occurs at higher velocities than conventional swing," Dr Ferdinand says.

Traditionally, reverse swing also comes into play when the ball gets older and loses its natural shine.

As the ball ages and becomes more roughed-up, the fielding team will repeatedly polish the side facing the batter.

"And you want to keep that other side as smooth as possible," Dr Ferdinand, says.

The bowling team will allow the other side to roughen up as it's repeatedly bowled into the pitch or scarred as it hits the wicket square.

So why does the ball swing in when everything suggests that the ball is set up for a classic outswinger?

Despite the polished appearance of the "smooth" side the fielders are working on, it is still rougher than a new ball and will have minor abrasions and cuts in its surface, which all create turbulence.

This means the air hitting the side facing the batter becomes turbulent, rather than a laminar flow. And as we've seen, a turbulent airflow sticks to the ball for longer and travels faster, while the separation point moves towards the back.

On the rough seam side, the air hitting the ball will become turbulent immediately, and only increases when it encounters the seam.

As Dr Mehta explains in one of his papers: "In this case, the seam has a 'detrimental' effect whereby the boundary layer is thickened and weakened."

This thicker turbulence is more susceptible to shearing off earlier, moving the separation point towards the front of the ball.

So now there's a higher pressure on the rough side and a lower pressure on the polished side.

This pressure differential is the reverse of the conventional swing, which results in the ball swinging in the opposite direction of the seam inward, towards the batter.

That's why Simon Jones' ball — which looks like it should swing away from the batter, actually swings in.

A key factor in reverse swing is the speed of ball through the air.

"At a high enough velocity even relatively smooth surfaces will promote turbulent airflow," Dr Ferdinands says.

In theory, then, it's possible to bowl reverse swing with a new ball.

Dr Mehta says a new ball with quarter seams will start to reverse swing at speeds above 128km/h, and an old ball will start to reverse about 115km/h.

The greater the speed, the more pronounced the effect.

Recapping the swing

Conventional swing happens at airflow speeds up to about 128km/h, with a new, or relatively new ball, bowled with a still proud seam angled about 20 degrees from its trajectory, and the non-seam side kept as smooth as possible.

The pressure differential will cause the ball to swing in the same direction as the alignment of the seam.

Reverse swing occurs with older balls which have some abrasions and when the speed is increased.

Now, the pressure differential is reversed, and the ball will swing in the opposite direction to the alignment of the seam.

If the bowling speed is high enough, reverse swing can occur with a new ball.

In the graph below, the red dots represent the separation points on the ball.

It shows that there is a crossover point. At first the ball swings towards the rough side and then to the smooth side. But at a certain airflow velocity, the separation point and thus the pressure differential is the same on both sides of the ball, which means it will not swing at all.

We can't say what that particular velocity is, because it will vary according to the wind conditions and the condition of the ball.

The crossover point may explain why some bowlers aren't able to swing the ball at certain times. It may be that at the speed they are bowling and for the given conditions of the ball, the pressure on each side is the same.

Contrast swing

Contrast swing can be divided into two categories: conventional and reverse contrast swing.

Both occur when the seam is upright and so it plays no part in the movement of the ball.

Rather, the pressure differential on either side of the ball is caused entirely by the contrasting state of the ball's surface — rough on one side and smoother on the other.

In conventional contrast swing at lower speeds, a rough surface will do the same job as the seam causing a turbulent airflow, while a smooth surface will result in a laminar flow.

As with conventional swing, the turbulent air will stick to the ball for longer and move faster, resulting in a lower pressure than the high pressure caused by the laminar flow on the other side.

But what happens when the speed of the ball through the air increases? It's the same as with conventional reverse swing.

Again the flow of air over the smoother side of the ball will become turbulent, faster and separate later, resulting in a low pressure on that side of the ball.

The flow of air over the rough side will become even more turbulent and weaker and will separate earlier resulting in a higher pressure. Again we have a pressure differential, only now the force is in the opposite direction.

"The difference between conventional contrast and reverse contrast is entirely due to the velocity of the ball," Dr Ferdinand says.

Mehta and others believe many commentators confuse reverse swing with reverse contrast swing — in true reverse swing, the seam will be angled, but in contrast swing, the seam is straight.

What about late swing?

We've all heard commentators refer to late swing – the idea that the ball moves more the further down the pitch it travels.

Mitch Starc's dismissal of Rory Burns off the first ball of the last Ashes series in Australia is a classic example.

"There's no swing," Shane Warne says in commentary.

"Yeah it swings back, late, I think," Mark Waugh offers in reply.

There are potentially two factors at play.

Dr Mehta says the trajectory of a ball follows a parabolic path.

The curve follows a relatively straight path before it suddenly bends around a fixed point known as a focus.

A cricket ball following this path will then move more as it reaches the focus, but it's not going to swing around and start travelling in the other direction.

"There are many theories for late swing, but it turns out that since the flight paths are parabolic, late swing is in fact 'built-in'," Dr Mehta wrote in a 2000 paper.

The other potential reason for late swing is the speed of the ball.

The ball that Starc bowled left his hand at 142km/h, which is too fast for conventional swing. But that ball is also constantly slowing from the moment it leaves Starc's hand due to wind resistance.

Starc's 142km/h thunderbolt may have slowed down to around 125km/h by the time it hits the ground, somewhere around the hapless Burns' crease.

By now, the ball is travelling slowly enough to swing in.

Starc himself says he was just bowling his stock ball.

"Running in I was just trying to hit the stumps, it was probably a bit fuller than I would have liked," he says.

Swinging the ball was an art Starc learned as a young man at the Australian Cricket Academy from coach Troy Cooley, who famously also coached England's star swing bowlers, including Anderson, Jones, Stuart Broad, and Andrew Flintoff.

Starc knows the basics of the science behind swing bowling, but he says making it happen is another skill altogether.

"Knowing what it does doesn't mean it always happens," he says.

As for contrast swing, he says the Australian bowlers call it 'power fade'.

"I find that at times it just happens for no reason, and it's at those higher speeds," he says.

"And we just say it's 'power fading'.

"It's just like it should swing one way, but … it's going in the other way. So it's like that reverse contrast swing at high pace, in a ball that shouldn't do that — in our minds anyway."

Starc also says conditions play a part. It is one of the long-running givens about swing bowling – that when it's humid, or there's cloud cover, that the ball will swing more.

"I certainly think it probably plays a bigger part in somewhere like England," Starc says.

"We often joke about it when James Anderson and Stuart Broad come on to bowl, the clouds come over and it swings around corners, and when we bowl the sun comes out.

"So I think it certainly has a bigger part to play in potentially the northern hemisphere or in England, I don't think it matters so much with the Kookaburra in Australia."

It is generally accepted that the Dukes branded ball used in England has a higher seam and tends to swing more than the Kookaburra created ball in Australia.

But the jury is out on humidity and cloud cover – at least in the scientific world.

"I can't see much evidence for that," Dr Ferdinands says.

"A number of tests have been done in wind tunnels and they've used water vapour, they've done all of those things — nothing has really been found with the humidity."

What is clear is that more work could be done in the real world than in wind tunnels to really understand what conditions affect swing bowling.