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When clouds roll in over Headingley, swing bowlers smile in expectation. The venue for the fourth npower Test has long been known for extravagant movement in the air and James Anderson, Graham Onions and Ben Hilfenhaus will be hoping for overcast conditions.

The notion that the ball swings when skies are grey is part of cricket folklore and players and commentators have invoked the weather to explain why the ball moved ferociously on the second and fourth mornings at Edgbaston, but not at other times.

Science, however, has a different view of swing. Its causes have been well established by experiments that explode the myth that cloud cover and humidity have an effect.

Rabindra Mehta, a Nasa scientist and schoolfriend of Imran Khan, the former Pakistan all-rounder, has studied the aerodynamics of cricket balls for three decades and has advised Troy Cooley, the Australia bowling coach. And he has been watching the Ashes on television from his California home in exasperation.

“What the commentators, cricketers I much admire, have been saying about swing is plain wrong,” he told The Times yesterday. “They’ve been talking about the clouds, how the new ball won’t swing until the lacquer has come off, and it’s just rubbish.”

Contrary to common belief, there are three types of swing bowling, not two, he said. Seam position and bowling speed are critical to achieving all of them, but overcast weather conditions are not.

As the ball moves through the air, a thin “boundary layer” of air hugs its surface before breaking away. If the boundary layer breaks later on one side of the ball than the other, pressure will be reduced, causing the ball to swing that way.

For conventional swing, the ball is gripped with the seam angled towards slip for an outswinger and fine leg for an inswinger. Part of the shiny side must face the batsman with the bowlers’ fingers resting next to the seam. As air catches the raised seam, it creates turbulence on that side only. The boundary layer breaks later on the other side of the seam, causing swing in that direction.

Reverse-swing works, as the name suggests, in reverse. The seam is pointed away from the direction in which the bowler wants swing and part of the rough side of the ball faces the batsman.

The roughness means the boundary layer starts off turbulent on both sides. The seam, however, weakens this turbulence on its side, causing the boundary layer to break earlier and sending the ball swinging in the other direction.

The key to normal and reverse-swing is to bowl the ball with backspin so that it revolves along the seam. A bowler such as Anderson, with a high action and strong wrist, will achieve this more reliably than one with a low arm such as Mitchell Johnson.

The condition of only one side of the ball is important: the shiny side for conventional and the rough side for reverse. “The new ball is perfectly capable of swinging this way: that the other side is shiny doesn’t matter,” Dr Mehta said.

Also critical is speed. For conventional swing, the optimum pace is about 70mph and balls of above 80mph will not move at all; very fast bowlers must rely on reverse-swing. Dr Mehta thinks the radar used in the present series is exaggerating the pace of all bowlers.

Reverse-swing works with the new ball only at high pace, of 90mph or more, but as one side roughens, the speed at which it will reverse drops.

The third type of movement, often confused with reverse-swing, is contrast swing. This is achieved with a vertical seam and works when one side of the ball is rough and one smooth, and speed of delivery determines which way the ball moves.

Slower balls move towards the rough side, because of greater turbulence on that side. Faster balls, however, will move towards the shiny side: as with reverse-swing, there is initially turbulence on both sides, but this weakens over the rough surface.

“Commentators often say it’s reversing when it’s actually contrast swing,” Dr Mehta said. “True reverse-swing occurs only when the ball moves in the opposite direction to the seam.”

Pitch conditions can help: a grassier wicket protects shine for longer, assisting conventional swing, while an abrasive one will cut it up, promoting reverse-swing. A prevailing wind can alter turbulence around the ball, which may explain why some grounds and ends are better for swing.

Dr Mehta says the reason why it is believed weather conditions matter is psychological. The idea that the ball swings under cloud is so ingrained that we remember occasions that confirm this impression and forget those that do not, and overcast conditions may also affect bowlers’ confidence.

“My latest theory is that when bowlers show up and the conditions look ideal, maybe they subconsciously orient their wrists better and bowl a bit fuller,” he said. “Perhaps they just bowl better.”