The effect of weather conditions on ball swing is perhaps the most discussed and most controversial topic in cricket. The one bit of advice that cricket “gurus” have consistently passed down over the years is that, a humid or damp and overcast day is conducive to swing bowling. The implication is that whenever there is moisture present in the atmosphere, it is easier for bowlers to swing a cricket ball. There is no shortage of discussion on this topic in any cricket series, regardless of the venue and weather conditions.

We shall first see if this is this a real effect or one of those folklores... Let me start off by declaring that I used to be a strong believer. There was no shortage of damp days when I played in England in the 1970s and I found that sometimes the ball would swing a lot more than usual. On one particular occasion, on a damp cold day in Worcester, I recall that both opening bowlers (Imran Khan and myself) were relieved of their duties rather abruptly and unceremoniously since the ball swung too much for it to be effective. However, after I brought this issue up at the England and Wales Cricket Board’s National Cricket Centre in Loughborough in 2005 and at Cricket Australia’s Centre of Excellence in Brisbane in 2007, I was quite surprised to see that there was no real consensus on this topic among professional coaches and cricket players. The overall feeling was that the ball did occasionally swing more, but this could not be exclusively attributed to weather conditions.

Let us first examine the science behind conventional cricket ball swing. As the ball is flying through the air with an angled seam, a thin layer of air called the ‘boundary layer’ forms along the ball’s surface. The smooth and steady ‘laminar’ boundary layer over the bottom surface leaves or separates from the surface at about the apex location. However, over the top surface, the laminar boundary layer is ‘tripped’ by the seam into an unsteady and chaotic ‘turbulent’ state. By virtue of its increased activity and energy, the turbulent boundary layer separates further back, which results in lower pressure on this side. This differential in pressures produces a side force which makes the ball swing in the upward direction.

A popular theory that has circulated for years suggests that in damp conditions, the primary seam swells by absorbing moisture, thus making it a more efficient boundary layer trip. We investigated this possibility in detail by first measuring profiles of the primary seam using a precision stylus device on a new ball before and after a few minutes soaking in water. A similar test was also performed on a used ball, where the varnish on the seam had worn-off. In both cases, no swelling of the seam was observed. These two balls were then tested in a wind tunnel to measure the side force. These tests showed conclusively that there was no increase in side force for the wet balls. Other investigators have also failed to find any positive effects of humid conditions on cricket ball swing in laboratory tests. Is it possible that changes in the atmospheric properties can cause the ball to swing more?

How many times have you heard cricket commentators refer to humid or damp conditions as constituting a ‘heavy’ atmosphere? This implies an increased air density and it turns out that the side force on a cricket ball is directly proportional to the air density. Well, the fact is that humid air is actually less dense than dry air, although the dependence is quite weak. There is in fact a more interesting effect that is caused by air temperature. The air density is higher on a cold day compared to that on a hot day. However, the dependence is not very strong with the air density being only about four per cent higher at 15 degrees centigrade compared to that at 25 degrees centigrade. This means that a ball which swings about two feet at 25 degrees centigrade will deviate about another inch at 15 degrees centigrade. This is obviously not enough to explain what is observed on a cricket ground, although it does illustrate why it is easier to hit a six on a hot day compared to a cold night (the drag on the ball, which slows it down, is also proportional to the air density). So is there anything else that can cause the ball to swing more in damp conditions?

A theory I proposed many years ago hints that damp conditions could affect the amount of spin imparted to the ball. The varnish applied to new balls reacts with moisture to produce a rather tacky surface. The tacky surface would ensure a better grip and thus result in more spin as the ball rolls off the fingers. An observation in our wind tunnel tests also proposes that an increase in backspin rate (at least up to about 11 revolutions/second) increases the side force. So, perhaps without actually realising it, the bowler may just be imparting more spin on a humid or damp day. This theory has yet to be tested, but I would not be too surprised if the theory is not confirmed. So is this whole issue just a myth?