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By Venu Palaparthi

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Previous scientific explanations for swing have sought to present humidity as a major factor, and most practitioners of swing bowling are absolutely convinced that their art is made possible by the swelling of the seam of the cricket ball as well as the increased lift force caused by humidity.  However, wind tunnel studies have not been very conclusive regarding the impact of humidity on ball geometry and in lateral deviation. 

Pic Right - Courtesy Erika Bird and Robin Searle - High altitude cricket match in progress in Dzongri (13,000 feet above sea level).  One word of advice - don't get your runs in singles.

A paper published on May 26, 2012 in Procedia Engineering sheds new light on the effect of atmospheric conditions on the swing of a cricket ball.  In the paper, the three researchers - David James and John Hart from from Britain's Sheffield Hallam University and Danielle MacDonald from Auckland University of Technology in New Zealand - concluded that humidity, while it caused a small lateral deviation, did not significantly affect the ability of the bowler to make the ball swing.  The same conclusion was drawn regarding temperature differences. 

Altitude, it was observed, had a larger effect.  The researchers also felt that cloud cover provided the ideal environment for swing bowling because of reduced turbulence, typically caused by air currents due to the heat of the harsh sun.  Calmer air under clouds made it easier for the ball to experience asymmetrical flight, the researchers observed. 

The physics of all of this is fairly simple.  Generally, a ball travels further in warm and humid air.  That is because humid air is less dense than dry air due to the lower molecular weight of water vapor.  Therefore, when temperature and pressure are held constant, humid air is lighter and more favorable to a traveling ball.   However, while a batsman can smash the ball farther, greater humidity can also make the ball juicier, harder and heavier shifting some of that advantage to bowlers.   According to swing bowlers, they also get a fair amount of assistance if the air was colder and humid because of greater lift force. 

The impact of temperature and pressure on air's density is even more straight-forward.  Greater temperature and lower pressure both result in lower air density, which in turn allows the ball to travel farther. 

The lower humidity and thinness of air at higher altitude also causes balls to shrink, renders them more elastic and a tad lighter. Ball trajectories become more predictable and this is clearly advantageous for the batsman. 

Pic Right - The walk-in humidor at the Coors Field in Denver where balls for the season are stored.  

In baseball, there is an abundance of evidence on effects of higher altitude and lower humidity.  At the Coors Field in the Mile-High city of Denver where the air is thin, and relative humidity is low at 30%, the balls were known to fly away from the bat.  "Coors Light" was a common pun back in the day.  A home run ball that would normally be expected to fly 400 feet, flew 420 feet.  

Pitchers also complained that the change in shape also made it harder to grip the balls.  20% fewer curve balls and more predictable trajectories also contributed to two runs greater per game at Coors than at other venues. 

In cricket, evidence is more anecdotal.  Somewhat famously, Wanderers Stadium in Johannesburg is located at an altitude of 6000 feet above sea level and this is where South Africa successfully chased down Australia's world record total of 434 for 4.

The Procedia Engineering paper is somewhat circumspect on the impact of humidity on the geometry of the ball.  The researchers observed that while cricket balls lost 0.16 grams (0.005 oz) of mass when the humidity was lowered to 25% and the ball was exposed for 2 hours to those conditions, the difference was not significant enough to impact the ball's swing or trajectory.  The researchers also observed that the primary seam did not swell or shrink significantly.  However, it appears that the study focused more on short-term atmospheric impact and less on the impact of prolonged exposure of balls to lower humidity.

It is quite possible that exposure to longer periods of low humidity may cause cricket balls to shrink by a greater amount and become lighter, producing outcomes that are more consistent with what was observed at the Coors Field prior to 2002. Baseball, by regulation, must weigh between 5 and 5.25 oz and measure 9 to 9.25 inches in circumference.  At the Coors Field in Denver, the ball circumference was known to have averaged lower than the regulation permitted and the lowest recorded circumference was 8.5 inches.  The mass dropped to below 5 oz going as low as 4.6 oz. 

In 2002, a Coors engineer came up with the idea of using a humidor for the balls used at the stadium, similar to what you might find at a cigar store.  That season, Coors Field acquired a humidor for the balls, which now maintains the balls at 50% humidity and 70 F.  This has helped in maintaining the weight and circumference of the balls.  The results on the field were as expected.  The number of home runs hit at Coors Field has dropped from 268 in 2001 to 185 in 2007.  Even Denver's players were surprised.  "I was hitting balls and they weren't going out of the yard," Rockies first baseman Todd Helton told a reporter. "I was wondering what the heck was going on."

Of course, ball tampering takes a whole new meaning when you start to control the conditions in which the ball is kept and then hand the key to the home team.  No surprises then that the Giants suspected that Denver was using juiced up humidified balls when pitching against opponents and non-humidified dry balls when they were batting.  This led to more independent oversight by umpires, who take the whole stock of balls now before the game starts.

Mike Selvey observed in a 2009 blog piece on cricket balls, "the authorities seem to like the consistency while having little regard for the overall performance."  He called for more research, paid for by ICC if necessary, to arrive at a ball with tighter winding and a leather that will suit all conditions.  Perhaps the ICC should also consider investing in humidors, especially at the higher altitude grounds of Dharamsala (4320 feet above sea level) or Wanderers (6000 feet above sea level).