Updated: July 12, 2019 10:04:21 am
It could be argued that the weather played a key role in the denial of a place to India in the World Cup final. The air was moist and there were clouds and rain in Manchester. Moisture content reduces cohesion and makes the pitch weak. A pitch with just 30-35 per cent clay, for example, becomes less cohesive when it rains, which favours swing and seam bowlers. In contrast, a pitch with more than 40 per cent clay dries out in the hot sun, making it stronger and bouncier and favouring pacers.
Under moist conditions, a batsman has to struggle with the effects of the damp ground, which slows down the ball. The bat absorbs moisture, which makes it harder to strike the ball deep. Moisture also reduces the grip between the feet and the ground due to lower cohesion, and a lot more effort is needed during batting. In dry weather, a batsman does not have to struggle that much to hit a four or a six.
Shortly after the match, captain Virat Kohli said, “We felt like we had the momentum… but the credit has to go to the New Zealand bowlers… and the kind of swing that they got on the surface, the help that they got from the surface.”
Bowlers can make the ball swing by releasing it at an angle. Turbulence, or a chaotic flow of air, is generated along one side of the ball, while a laminar flow exists along the other side, which causes a sudden pressure difference leading to deviation in the ball’s trajectory, known as swing.
Many scientists are of the view that moisture does not change the shape of the ball and does not directly affect swing. However, there are other factors that can influence swing. In a landmark work on swing in cricket balls, Mehta et al, writing in the journal Nature (1983), mentioned that maximum swing is found at a bowling speed of 70 miles per hour, a seam angle of 20° and a spin rate of 11.4 revolutions per second. At lower speeds, the spin rate should be raised to 14 revolutions per second for the same swing. However, under humid conditions, the bowler can impart a higher amount of spin even at lower speeds. In moist weather, the coefficient of friction on the ball’s surface increases and it gives a special advantage to the bowler. A 40 per cent change in humidity can change the coefficient of friction by around 10-15 per cent. Some researchers (James et al, Procedia Engg, 2012) have argued that cloud cover – which was present in Manchester – provides an ideal environment of swing as turbulence caused by hot air under sunlight is reduced and bowlers have stronger control.
A spinning ball during flight generates an asymmetry in streamlines of flow along its top and bottom surfaces, which causes a Magnus force (the turning ball drags some of the air around with it). Often, spin bowlers use this effect to create a drift in the ball’s trajectory when it touches the ground. However, the effect is also employed to transfer spin along a vertical axis, which generates a lateral swing in the air before the ball touches the ground. Such an effect can become more prominent on a colder day as air density is higher. For example, air density at 15°C is higher by around 4 per cent in comparison to air density at 25°C, which implies that a ball that would have swung around 2 feet at 25°C will swing an additional 1 inch at 15°C (Mehta, 19th Australasian Fluid Mechanics Conference, 2014). Such minor effects can significantly influence the prospects of hitting a four or a six at critical times.
To sum up, it is important for any team to train batsmen under conditions of rain and moisture at relatively lower temperatures.
(The author is a Postdoctoral Associate at Massachusetts Institute of Technology. He holds a doctorate in electrical engineering from the University of Cambridge, UK. He specialises in microsystems, sensors and antennas.)
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