The idea of using a humidor at Chase Field, as they do at the Rockies’ home park in Denver, is to make the park less of a hitter’s venue. This works, because the humidor - as its name suggests - keeps the balls used in the game in an environment of relatively high humidity, compared to the “outside” one. This has two effects. Firstly, it makes the balls less “bouncy” - or in more scientific terms, lowers their coefficient of restitution. This increased squishiness means that they don’t fly off the bat as fast, and consequently, as far.
Perhaps as important, baseball that are kept in a humidor are easier for pitchers to grip. This gives them a better opportunity at giving their pitches the necessary spin, which allows them to break and move on their way toward the hitter. Obviously, the more that happens, the harder batters will find it to hit, compared to relatively flat pitches - the dreaded “hovercraft slider” or “cement-mixer”, to borrow phrases from Mark Grace.
Installing one at Chase Field is not a new ideal. It was first suggested back in 2010, but nothing came of it. However, speaking on the Doug & Wolf Show on Arizona Sports 96.9 this morning, team president Derrick Hall announced the team will be proceeding with installation, and hopes to have it completed in about a month, ready for the low humidity months in Phoenix. [MLB appears to have no problem with a humidor being enabled during a season, as long as it remains in use for all games thereafter] The second reason above appears to be the driving factor: “We talked to former pitchers, whether it’s J.J. Putz or pitchers who have retired or pitchers we’ve traded, and said ‘what did you like, what didn’t you like?’ They all talk about the grip.”
What impact might it have? Back in 2010, the Republic analyzed the potential effects on hitters of a humidor at Chase. The conclusions reached were quite startling. They worked on the basis that balls had been stored for a fortnight at 50% humidity, which was estimated to decrease speed off the bat by 2.5 mph, translating to about 14 feet less in distance. Feeding that over to Greg Rybarczyk of HitTrackerOnline.com, he crunched the numbers:
"I took every home run hit at Chase Field from 2006 until about a week ago, which was 865 home runs. And I re-ranthem, reducing each one by 2.5 miles per hour. What I got was that 38 percent of those 865 home runs would not have made it over the fence. You're talking about a pretty drastic reduction.”
In fact, due to the humidity in Phoenix being significantly lower than Denver - indeed, it’s easily the lowest of any park - the impact of the humidor would be an even bigger reduction than happened at Coors Field. There, home-runs dropped by about 25% after the introduction of the humidor. Ryan Morrison also looked at the potential impact of a humidor, and concluded that in the driest month (June), the reduction could potentially be as much as a staggering 62% in home-runs. However, he also notes that the season after our Triple-A affiliate in Reno introduced a humidor, in a similarly dry environment, the number of home-runs actually INCREASED.