Why the Ball Drifts and Dips

A batter shoulders arms — lifts his bat up out of the way, certain the ball is sliding harmlessly past off stump — and watches it curl back and clip the top of the stumps. It never bounced on a crack. It never gripped the pitch. It simply bent through the air on the way down, and he read it wrong.

We have been treating spin as something that happens at the bounce: the ball gripping and turning. But a spinning ball is already working on the batter long before it lands. It drifts sideways, and it dips — and both come from the same revolutions that will later turn it. This is a different force from swing, and grasping it is most of what separates watching a spinner from seeing what he is doing.

The deviation nobody credits#

First, clear up a confusion. A fast bowler can also curve the ball through the air — that is swing, and it comes from the seam steering the airflow around a ball that is barely spinning. What a spinner does is not swing. His ball is spinning fast, and it is the spin itself, not the angle of the seam, that bends the air. To the eye the result looks similar — a curve in flight — but the cause is completely different.

How spin bends the air#

Picture the ball spinning as it flies. It drags a thin skin of air round with it. On one side of the ball that dragged air is travelling the same way as the air rushing past, so it slides along cleanly and clings to the surface a long way round. On the other side the dragged air is fighting the oncoming air, and it breaks away early. The flow around the ball is now lopsided.

Air that clings longer on one side and breaks away early on the other leaves the ball shoved toward the clinging side. That sideways shove has a name — the Magnus effect — and it is the same force that curves a sliced tennis ball or bends a struck football. On a ball spun by a slow bowler it shows up as drift: the gentle sideways curve of the ball in the air, often a foot or more of it by the time the ball lands.

Dip, and its opposite#

Now tilt the axis of the spin. If the ball is rolling forwards — topspin, turning top-over-bottom in its direction of travel — the Magnus shove points straight down. The ball dips: it drops out of the air earlier and more steeply than the batter's eye predicts, landing shorter than he expected and kicking up higher off the pitch. A batter who has committed to a drive finds the dipping ball has fallen under his bat.

Spin it the other way — backspin, rolling backwards — and the shove points up. The ball floats, hanging in the air and carrying fuller than it should, drawing the batter forward onto a ball that never quite arrives where he reaches for it. One force, aimed three ways: sideways for drift, down for dip, up for float.

The ball that moves both ways#

Put the drift in the air together with the turn off the pitch and you get the single most dangerous thing a wrist-spinner does. A well-bowled leg-break — the wrist-spinner's stock ball, which turns away from a right-handed batter off the pitch — first drifts the opposite way in the air, in toward him. The batter follows the drift, planting his front foot across to cover the line the ball seems to be taking. Then it lands and turns back the other way, away from his groping bat, toward the edge or the stumps. The ball has moved both ways, and he has been wrong twice.

The most famous delivery in modern cricket is exactly this ball. In the 1993 Ashes, Shane Warne's first delivery in Test cricket in England drifted in toward Mike Gatting's pads and pitched outside leg stump — a line Gatting felt he could safely ignore — then spun back across him and clipped the top of off stump. Gatting stood and stared, unable to work out how a ball that pitched a foot outside his legs had hit the far side of his stumps. It had drifted, then turned.

We have watched the ball misbehave in the air. But the bigger of its two deviations is still the one at the bounce — the turn itself. And it hides a surprise: the bowler is only half in charge of it. The pitch holds the other half.