Humid balls help Rockies hit fewer home runs

2019-03-14 12:04:03

By Bob Holmes When the Colorado Rockies baseball team reached the World Series last October, many analysts gave part of the credit to the humid room where the team stores its baseballs. But a new analysis by a pair of physicists suggests that the humidor’s effect is not what baseball experts had thought. The Rockies’ home field in Denver is 1600 metres above sea level, far higher than any other team in the Major Leagues. Denver has always had a reputation as a hitter’s heaven and a pitcher’s hell, because batted balls travel further in the thin, dry air. As a result, hits that would fall harmlessly into a fielder’s glove at other parks are more likely to clear the fences for a home run, leading teams to average more than two extra runs per game compared to other cities. In an attempt to compensate for this, in 2002 the Rockies began storing baseballs in a humidified room before games. They reasoned that the balls would absorb more moisture and thus not fly as far. The strategy has worked, reducing the average runs per game from nearly seven per team to less than six. To find out precisely why, physicists Edmund Meyer and John Bohn of the University of Colorado at Boulder measured the size and weight of five baseballs stored at 30% humidity – roughly that of the atmosphere in Denver – and compared those with measurements of the same balls stored at the humidor’s 50% humidity. The balls in the humidor averaged 0.24% bigger and 1.6% heavier than the lower-humidity balls, they found. Then the researchers then calculated what effect this difference would have on the aerodynamic forces acting on the ball during its flight. A bigger ball would experience more drag, but a heavier ball would decelerate more slowly, they say. “The net effect is that the humid ball actually travels a bit further,” says Bohn, though the difference is only about half a metre. In short, the humidor does not reduce home runs through aerodynamic effects, the researchers conclude in an unpublished preprint paper available online (arXiv:0712.0380v1). Instead, Bohn suggests that the humidor may make the surface of the baseball less slick. Pitchers had noted that the dry balls in pre-humidor days were harder to get a grip on, so that their pitches had less spin and thus did not change direction as strongly. Such “hanging” pitches are easier for batters to hit. Since the introduction of the humidor, pitchers have felt they got a better grip on the ball and can spin it more sharply as they throw it. This would make their pitches change direction more strongly and thus make them harder for batters to hit squarely. “The pitchers are much happier now,” says Bohn. “If they need to put movement on a ball they can do it, and they couldn’t before.” Bohn’s conclusion makes sense, says Alan Nathan, a nuclear physicist at the University of Illinois at Urbana-Champaign, US, who also studies the physics of baseball. However, Nathan also notes two other possible explanations for the humidor effect: humid balls, being heavier, should accelerate off the bat more slowly; they may also be less springy,