The effect of weather conditions on 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. In particular, it is widely believed that the ball will swing more on a humid or damp day compared to a relatively dry day. There is no shortage of discussion on this topic in any cricket series, regardless of the venue and weather conditions.
First of all, is this a real effect or just folklore? 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, when I brought up this issue 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 amongst professional coaches and players. The overall feeling was that the ball did occasionally swing more, but this could not be exclusively attributed to the weather conditions.
Let us first examine the science behind conventional cricket ball swing (http://www.espncricinfo.com/ci/content/story/258645.html). As the ball is flying through the air with an angled seam (see smoke flow visualization photo below), a thin layer of air called the “boundary layer” forms along the ball’s surface. The smooth and steady “laminar” boundary layer over the top surface leaves or “separates” from the surface at about the apex location (note the smooth smoke edge in the photo). However, over the bottom surface, the laminar boundary layer is “tripped” by the seam into an unsteady and chaotic “turbulent” state (note the ragged smoke edge in the photo). By virtue of its increased activity and energy, the turbulent boundary layer separates further back, which results in a lower pressure on this side. This differential in pressures produces a side force which makes the ball swing in the downward direction.
A popular theory that has circulated for years is 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 in 1980 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. The more interesting effect is that due to 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 4% higher at 15° C compared to that at 25° C. This means that a ball which swings about 2 feet at 25° C will deviate about another inch at 15° C. 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 is 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, and as observed in our wind tunnel tests, an increase in backspin rate (at least up to about 11 revolutions/second) increases the side force. So, perhaps without actually realizing 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?
Well, I have come up with a possible explanation, based on some recent observations and conversations with players and coaches that I would like to propose here. It is my belief and observation (especially in the recent Cricket World Cup) that a true swing bowler will be able to swing just about any new cricket ball under any weather conditions, as long as it is released in the right attitude (the seam angled towards the slips or fine leg and the ball spinning steadily along the seam with minimum wobble). From what I have observed and been told by coaches, swing bowlers often go through patches when the ball will not swing for them. The reason for this is invariably that the ball is not released in the optimum attitude. This could be due to the bowlers trying too hard or perhaps just succumbing under extra pressure. So is it possible that when the bowlers show up at the cricket ground on a cold damp day, everyone expects the ball to swing and the bowlers subconsciously concentrate more on releasing the ball in the optimum condition and bowling at a good length for swing, rather than trying to bowl too fast or trying to get that extra bounce by bowling short?
There are also some secondary or indirect effects of the weather conditions that will affect the ball aerodynamics. For one thing, the general weather patterns in a region or country determine the condition of the pitches and outfields. For example, with the soft and grassy conditions in England, the new ball will retain its shine for a longer time, thus enabling the ball to swing (conventionally) deeper into the innings. In contrast, the hard and barren conditions generally encountered on the sub-continent result in the ball roughening up rather quickly which makes it easier to generate reverse and contrast swing. In the 2006 Champions Trophy played in India, it was interesting to note that the white ball was often replaced towards the latter part of an innings because it became dirty. In particular, when the dew set in at night, the dirt turned into mud which often got stuck to the ball surface and this makes for an interesting situation. Rather than wiping the whole ball, I would be tempted to leave the mud stuck to one side, especially on a newer ball so that, with this “naturally-added” roughness, reverse and contrast swing may be obtained more easily.
And finally, there are cricket grounds such as The Hove in England that have a reputation for being ideal for swing bowling. In these cases, it is often a predominant wind direction that comes into play. Imagine a strong wind coming off the sea from the fine leg position. In this case, the seam does not need to be angled for the ball to swing. Even a new ball can be bowled with the seam straight up, as in contrast swing mode, and the ball will swing away towards the slips because of the angled relative wind direction. Of course, the advantage here is that it is much easier to release the ball with the seam straight up rather than angled towards fine leg or the slips. Additionally, in windy conditions, medium pacers can benefit if they bowl into a near headwind. As far as the ball is concerned, its effective speed is now increased and this can result in more swing. Of course, bowlers are not always keen to bowl into the wind, but as the old saying goes: “NO PAIN – NO GAIN.”
Here are some questions that can help a teacher set off a discussion in class:
Why do fast bowlers constantly shine the ball?
Why don’t old balls swing (conventionally) as much as new balls?
Apart from temperature, what other air properties affect the density?
Why do golf balls have dimples?
Does negative roughness (scuff marks on the ball) have the same aerodynamic effect as positive roughness (mud stuck on the ball)?
The author is a Sports Aerodynamics Consultant and NASA Scientist based in California. He can be reached at firstname.lastname@example.org.